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Transistor

Transistor

The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. It acts as a variable valve which, based on its input voltage, controls the current drawn by it from a connected voltage supply.

Introduction

Transistors are divided into two main categories: Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs). Transistors have three terminals where, in simplified terms, the application of voltage to the input terminal increases the conductivity between the other two terminals and hence controls current flow through those terminals. The physics of this "transistor action" are quite different between the BJT and FET. In analog circuits, transistors are used in amplifiers, audio amplifiers, radio frequency amplifiers, regulated power supplies, and in computer PSUs, especially in switching power supplies. Transistors are also used in digital circuits where they function similarly to electrical switches. Digital circuits include logic gates, RAM (random access memory) and microprocessors.

History

First patents for the transistor-principle were registered in 1928 by Julius Edgar Lilienfeld in Germany. Then in 1934 the German physicist Dr. Oskar Heil patented the field-effect transistor. In 1947 William Shockley, John Bardeen and Walter Brattain succeeded in building the first practical bipolar transistor at Bell Labs and put it into production at Western Electric, Allentown, Pennsylvania. They were honored with the Nobel Prize in Physics for the investigations which led to the invention of the bipolar transistor. Over the next two decades, transistors replaced earlier vacuum tube technology in most electronics and later made possible many new devices such as integrated circuits and personal computers. Bell Telephone Laboratories needed a generic name for the new invention: "Semiconductor Triode", "Solid Triode", "Surface States Triode" [sic], "Crystal Triode" and "Iotatron" were all considered, but "transistor", coined by John R. Pierce, won an internal ballot. The rationale for the name is described in the following extract from the company's Technical Memoranda (May 28, 1948) [26] calling for votes:
Transistor. This is an abbreviated combination of the words "transconductance" or "transfer", and "resistor". The device logically belongs in the varistor family, and has the transconductance or transfer impedance of a device having gain, so that this combination is descriptive.
In August 1948 German physicists Herbert F. Mataré (1912– ) and Heinrich Walker (ca. 1912–1981), working at Compagnie des Freins et Signaux Westinghouse in Paris, France applied for a patent on an amplifier based on the minority carrier injection process which they called the "transistron". Since Bell Labs did not make a public announcement of the transistor until June 1948, the transistron was considered to be independently developed. Mataré had first observed transconductance effects during the manufacture of germanium duodiodes for German radar equipment during WWII. Transistrons were commercially manufactured for the French telephone company and military, and in 1953 a solid-state radio receiver with four transistrons was demonstrated at the Düsseldorf Radio Fair.

Importance

The transistor is considered by many to be one of the greatest inventions in modern history, ranking in importance with inventions such as the printing press, the automobile and the telephone. It is the key active component in practically all modern electronics. Its importance in today's society rests on its ability to be mass produced using a highly automated process (fabrication) that achieves vanishingly low per-transistor costs. Although millions of individual (known as discrete) transistors are still used, the vast majority of transistors are fabricated into integrated circuits (also called microchips or simply chips) along with diodes, resistors, capacitors and other components to produce complete electronic circuits. A logic gate comprises about twenty transistors whereas an advanced microprocessor, as of 2005, can use as many as 289 million transistors. The transistor's low cost, flexibility and reliability have made it an almost universal device for non-mechanical tasks, such as digital computing. Transistorized circuits are replacing electromechanical devices for control of appliances and machinery as well, because it is often less expensive and more effective to simply use a standard microcontroller and write a computer program to carry out the same mechanical task using electronic control than to design an equivalent control function mechanically. Because of the low cost of transistors and hence digital computers, there has come the trend to digitize information. With digital computers offering the ability to quickly find, sort and process digital information, more and more effort has been put into making information digital. Much media today is delivered in digital form, finally being converted and presented in analog form by computers. Areas influenced by the Digital Revolution are television, radio and newspapers.

Types

|- align = "center" | Image:BJT_symbol_PNP.png || PNP || Image:JFET_symbol_P.png || P-channel |- align = "center" | Image:BJT_symbol_NPN.png || NPN || Image:JFET_symbol_N.png || N-channel |- align = "center" | BJT || || JFET || Transistors are categorized by:
- Semiconductor material: germanium, silicon, gallium arsenide
- Type: BJT, JFET, IGFET (MOSFET), "other types"
- Polarity: NPN, PNP, N-channel, P-channel
- Maximum power rating: low, medium, high
- Maximum operating frequency: low, medium, high, radio frequency (RF), microwave
- Application: switch, general purpose, audio, high voltage, super-beta, matched pair
- Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array Thus, a particular transistor may be described as: silicon, surface mount, BJT, NPN, low power, high frequency switch. The maximum effective frequency of a transistor is denoted by the term f_\mathrm, an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain.

Bipolar junction transistor

The bipolar junction transistor (BJT) was the first type of transistor to be commercially mass-produced. Bipolar transistors are so named because the main conduction channel uses both electrons and holes to carry the main electric current. The terminals are named emitter, base and collector. Two p-n junctions exist inside the BJT, collector-base junction and base-emitter junction. Although commonly described as a current operated device, the collector current is actually controlled by the voltage difference between base and emitter terminals. The BJT is commonly used with voltage feedback to control the base voltage, but sometimes the base is driven by a current input. In contrast to the FET, the BJT is a low input-impedance device when used without voltage feedback. The BJT achieves higher transconductance compared with the FET, so it is preferred for linear amplification. Bipolar transistors can be turned on with light as well as electricity. Devices designed for this purpose are called phototransistors. The emitter and collector currents in normal operation is given by the Ebers-Moll model: : I_\mathrm = I_\mathrm (e^ - 1) : I_\mathrm = \alpha_F I_\mathrm (e^ - 1) The base internal current is mainly by diffusion and : J_p(Base) = \frac \left[exp \left(\frac\right)\right] Where
- I_\mathrm is the emitter current
- I_\mathrm is the collector current
- \alpha_F is the common base forward short circuit current gain (0.98 to 0.998)
- I_\mathrm is the reverse saturation current of the base-emitter diode (on the order of 1e-15 to 1e-12 Amperes)
- V_\mathrm is volt equivalent temperature (approximately 26 mV at room temperature ≈ 300K)
- V_\mathrm is the base-emitter voltage
- W is the base width The collector current is slightly less than the emitter current, since the value of \alpha_F is very close to 1.0. In the BJT a small amount of base-emitter current causes a larger amount of collector-emitter current. The ratio of the allowed collector-emitter current to the base-emitter current is called current gain, β or h_\mathrm. A β value of 100 is typical for small bipolar transistors. In a typical configuration, a very small signal current flows through the base-emitter junction to control the emitter-collector current. β is related to α through the following relations: : \alpha_F = \frac : \beta_F = \frac : \beta_F = \frac Emitter Efficiency: \eta = \frac

Field-effect transistor

Field-effect transistors (occasionally called unipolar transistors) use only one of the two carrier types (either electrons or holes, depending on the subtype). The terminals of the FET are named source, gate and drain. In the FET a small amount of voltage is applied to the gate in order to control current flowing between the source and drain. In FETs the main current appears in a narrow conducting channel formed near the gate. This channel connects the source terminal to the drain terminal. The channel conductivity can be altered by varying the voltage applied to the gate terminal, enlarging or constricting the channel and thereby controlling the main current. The drain current is given by: : I_\mathrm = I_\mathrm \left [ 1 - \frac \right ]^2 Where:
- I_\mathrm is the drain current
- I_\mathrm is the drain current at zero gate-source voltage
- V_\mathrm is the gate-source voltage
- V_\mathrm is the pinch off voltage FETs are divided into two families: junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more commonly known as metal oxide semiconductor FET (MOSFET). Unlike MOSFETs, the JFET gate terminal forms a diode with the channel which lies between the source and drain. Functionally, this makes the N-channel JFET the solid state equivalent of the vacuum tube triode which, similarly, forms a diode between its grid and cathode. Also, both devices operate in the depletion mode, they both have a high input impedance, and they both conduct current under the control of an input voltage. FETs are further divided into enhancement mode and depletion mode types. Mode refers to the polarity of the gate voltage with respect to the source when the device is conducting. For an N-channel FET: in depletion mode the gate is negative with respect to the source while in enhancement mode the gate is positive. For both modes, if the gate voltage is made more positive the source/drain current will increase. For P-channel devices the polarities are reversed. Most IGFETs are enhancement mode types and nearly all JFETs are depletion mode types.

Other transistor types


- Unijunction transistors can be used as simple pulse generators. They comprise a main body of either P- or N-type semiconductor with ohmic contacts at each end (terminals Base1 and Base2). A junction with the opposite semiconductor type is formed at a point along the length of the body for the third terminal (Emitter).
- Dual gate FETs have a single channel with two gates in cascode; a configuration that is optimized for high frequency amplifiers, mixers, and oscillators.
- Transistor arrays are used for general purpose applications, function generation and low-level, low-noise amplifiers. They include two or more transistors on a common substrate to ensure close parameter matching and thermal tracking, characteristics that are especially important for long tailed pair amplifiers.
- Darlington transistors comprise a medium power BJT connected to a power BJT. This provides a high current gain equal to the product of the current gains of the two transistors. Power diodes are often connected between certain terminals depending on specific use.
- Insulated gate bipolar transistors (IGBTs) use a medium power IGFET, similarly connected to a power BJT, to give a high input impedance. Power diodes are often connected between certain terminals depending on specific use. IGBTs are particularly suitable for heavy-duty industrial applications. The Asea Brown Boveri (ABB) 5SNA2400E170100 [http://library.abb.com/GLOBAL/SCOT/scot256.nsf/VerityDisplay/71B8625C035676C2C1256F9000471D3C/$File/5SNA%202400E170100_5SYA%201555-02Aug%2004.pdf] illustrates just how far power semiconductor technology has advanced. Intended for three-phase power supplies, this device houses multiple NPN IGBT chips connected in parallel in a case measuring 38 by 140 by 190 mm and massing 1.5kg. The module is rated at 1,700 volts and can handle 2,400 amperes.

Semiconductor material

The first BJTs were made from germanium (Ge) and some high power types still are. Silicon (Si) types currently predominate but certain advanced microwave and high performance versions now employ the compound semiconductor material gallium arsenide (GaAs) and the semiconductor alloy silicon germanium (SiGe). Germanium was largely replaced by silicon because silicon semiconductor behavior is stable at higher relative temperatures. Single element semiconductor material (Ge and Si) is described as elemental. Characteristics of the most common semiconductor materials used to make transistors are given in the table below:
Semiconductor material characteristics
Semiconductor
material 		Junction forward
voltage
V @ 25°C		 Electron mobility
m/s @ 25°C		 Hole mobility
m/s @ 25°C		 Max. junction temp.
°C
Ge 0.27 0.39 0.19 70 to 100
Si 0.71 0.14 0.05 150 to 200
GaAs 1.03 0.85 0.05 150 to 200
Al-Si junction 0.3 – – 150 to 200
The junction forward voltage is the voltage applied to the emitter-base junction of a BJT in order to make the base conduct a specified current. The values given in the table are typical for a current of 1 mA (the same values apply to semiconductor diodes). The lower the junction forward voltage the better, as this means that less power is required to "drive" the transistor. The junction forward voltage for a given current decreases with temperature. For a typical silicon junction the change is approximately −2.1 mV/°C. The electron mobility and hole mobility columns show the average speed that electrons and holes diffuse through the semiconductor material with an electric field of 1 Volt per meter applied across the material. In general, the higher the electron mobility the faster the transistor. The table indicates that Ge is a better material than Si in this respect. However, Ge has four major shortcomings compared to silicon and gallium arsenide: its maximum temperature is limited, it has relatively high leakage current, it cannot withstand high voltages and it is less suitable for fabricating integrated circuits. Because the electron mobility is higher than the hole mobility for all semiconductor materials, a given bipolar NPN transistor tends to be faster than an equivalent PNP transistor type. GaAs has the fastest electron mobility of the three semiconductors. It is for this reason that GaAs is used in high frequency applications. A relatively recent FET development, the high electron mobility transistor (HEMT), has a heterostructure (junction between different semiconductor materials) of aluminium gallium arsenide (AlGaAs)-gallium arsenide (GaAs) which has double the electron mobility of a GaAs-metal barrier junction. Because of their high speed and low noise, HEMTs are used in satellite receivers working at a frequency around 12 GHz. Max. junction temperature values represent a cross section taken from various manufacturers' data sheets. This temperature should not be exceeded or the transistor may be destroyed. Al-Si junction refers to the high-speed (aluminum-silicon) semiconductor-metal barrier diode, commonly known as a Schottky diode. This is included in the table because most silicon power IGFETs have a parasitic reverse Schottky diode formed between the source and drain as part of the fabrication process.

Packaging

Schottky diode Transistors come in many different chip carriers (see images), both through-hole (or leaded) such as metal canister and dual in-line package (DIP); and surface-mount, also known as surface mount device (SMD). The ball grid array (BGA) is the latest surface mount package (currently only for large transistor arrays and digital functions). It has solder 'balls' on the underside in place of leads. Because they are smaller and have shorter interconnections, SMDs have higher frequency characteristics but lower power rating. Often several package options are offered for a given transistor. Transistor arrays are sold in the same chip carriers as integrated circuits, often in DIP or SMD packages. A transistor array consists of multiple transistors built onto a single die for use as several individual transistors. Transistor packages are made of glass, metal, ceramic or plastic. The power rating and frequency of operation often dictates the type of packaging used. Power transistors have large packages that can be clamped to a heat sink for enhanced cooling. Additionally, most power transistors have the collector or drain physically connected to the metal can/metal plate. At the other size extreme, some surface-mount microwave transistors resemble grains of sand.

Usage

In the early days of transistor circuit design, the bipolar junction transistor, or BJT, was the most commonly used transistor. Even after MOSFETs became available, the BJT remained the transistor of choice for digital and analog circuits because of their ease of manufacture and ruggedness. However, the MOSFET has several desirable properties for digital circuits, and since major advancements in digital circuits have pushed MOSFET design to state-of-the-art, MOSFETs are now commonly used for both analog and digital purposes. MOSFET MOSFET

Commutation

MOSFET transistors are commonly used as electronic switches, for both high power applications in switched-mode power supplies and low power applications such as logic gates.

Amplifiers

From mobile phones to televisions, vast numbers of products include amplifiers in audio, RF, and active filters. The first discrete transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved. Transistors are commonly used in modern musical instrument amplifiers, where circuits up to a few hundred watts are common and relatively cheap. Transistors have largely replaced valves in instrument amplifiers. Some musical instrument amplifier manufacturers mix transistors and vacuum tubes in the same circuit, to utilize the inherent benefits of both devices.

Computers

The "first generation" of electronic computers used vacuum tubes, which generated large amounts of heat and were bulky, fragile, and unreliable. The development of the transistor was key to computer miniaturization. The "second generation" of computers, through the late 1950s and 1960s featured boards filled with individual transistors and magnetic cores. Subsequently, transistors, other components, and their necessary wiring were integrated into a single, mass-manufactured component: the integrated circuit. Transistors incorporated into an integrated circuit has taken the place of most discrete transistor design applications in modern digital electronics.

Advantages of transistors over vacuum tubes

Before the development of transistors, vacuum tubes (or in the UK thermionic valves or just valves) were the main active components in electronic equipment. The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are:
- Smaller size (despite continuing miniaturization of vacuum tubes)
- Highly automated manufacture
- Lower cost (in volume production)
- Lower possible operating voltages
- Operation without a warm-up period (most vacuum tubes need 10 to 60 seconds to "warm up")
- Lower power dissipation (no heater power, very low saturation voltage)
- Higher reliability and greater ruggedness to physical shocks (although vacuum tubes are more resistant to nuclear electromagnetic pulses (NEMP) and electrostatic discharge (ESD) )
- Much longer lifetime (vacuum tube cathodes are eventually exhausted)
- Complementary devices available (allowing circuits with complementary symmetry–complementary vacuum tubes are not available)
- Ability to control large currents (power transistors are available to control hundreds of amperes, vacuum tubes to control even one ampere are large and costly)
- Non-microphonic (vibration can modulate vacuum tube characteristics) " Nature abhors a vacuum tube " John R. Pierce, Bell Telephone Laboratories, circa 1948.

Gallery

A wide range of transistors has been available since the 1960s and manufacturers continually introduce improved types. A few examples from the main families are noted below. Unless otherwise stated, all types are made from silicon semiconductor. Complementary pairs are shown as NPN/PNP or N/P channel. Links go to manufacturer datasheets, which are in PDF format. (On some datasheets the accuracy of the stated transistor category is a matter of debate.)
- [http://www.onsemi.com/pub/Collateral/2N3903-D.PDF 2N3904]/[http://www.onsemi.com/pub/Collateral/2N3906-D.PDF 2N3906], [http://www.onsemi.com/pub/Collateral/BC182-D.PDF BC182]/[http://www.onsemi.com/pub/Collateral/BC212-D.PDF BC212] and [http://www.onsemi.com/pub/Collateral/BC546-D.PDF BC546]/[http://www.onsemi.com/pub/Collateral/BC556B-D.PDF BC556]: BJT, general-purpose, low-power, complementary pairs. They have plastic cases and cost roughly ten cents U.S. in small quantities, making them popular with hobbyists, and nearly ubiquitous.
- BFP183: Low power, 8 GHz microwave NPN BJT.
- [http://www.national.com/ds/LM/LM194.pdf LM394]: So-called 'supermatch pair', with two NPN BJTs on a single substrate.
- [http://www.st.com/stonline/books/pdf/docs/9288.pdf 2N2219A]/[http://www.st.com/stonline/books/pdf/docs/9037.pdf 2N2905A]: BJT, general purpose, medium power, complementary pair. With metal cases they are rated at about one watt.
- [http://www.onsemi.com/pub/Collateral/2N3055-D.PDF 2N3055]/[http://www.onsemi.com/pub/Collateral/2N3055-D.PDF MJ2955]: For years, the venerable NPN 2N3055 has been the standard 'power transistor'. Its complement, the PNP MJ2955 arrived later. These 1 MHz, 15 A, 60 V, 115 W BJTs are used in audio power amplifiers, power supplies and control.
- 2SC3281/2SA1302: Made by Toshiba to have low-distortion characteristics, these are used in high-power audio amplifiers. They have been widely counterfeited[http://sound.westhost.com/counterfeit.htm].
- [http://www.st.com/stonline/books/pdf/docs/4491.pdf BU508]: NPN, 1500V power BJT. Designed for television horizontal deflection, its high voltage capability also finds use in ignition systems.
- [http://www.onsemi.com/pub/Collateral/MJ11012-D.PDF MJ11012/MJ11015]: 30A, 120V, 200W, high power Darlington complementary pair BJTs. Used in audio amplifiers and control and power switching.
- [http://www.fairchildsemi.com/ds/2N%2F2N5457.pdf 2N5457]/[http://www.fairchildsemi.com/ds/2N%2F2N5460.pdf 2N5460]: JFET, general purpose, low power, complementary pair.
- BSP296/BSP171: IGFET, medium power, near complementary pair. Used for logic level conversion and driving power transistors in amplifiers.
- [http://www.irf.com/product-info/datasheets/data/irf3710.pdf IRF3710]/[http://www.irf.com/product-info/datasheets/data/irf5210.pdf IRF5210] IGFET (enhancement mode), 40 A, 100 V, 200 W, near complementary pair. For high-power amplifiers and power switches, especially in automobiles.

See also


- Avalanche transistor
- Band gap
- Bipolar junction transistor
- Compound transistor
- Darlington transistor
- Field effect transistor
- IGBT
- NPN
- PNP
- Semiconductor
- Transconductance
- Transresistance
- Transistor Models
- Wikibooks: Transistors
- Vacuum tube

External links and references


- -- J. Bardeen et. al.
- -- W. Shockley
- [http://www.audiouk.com/info/transistor.htm AudioUK's Milestones]. Photograph of first working transistor
- [http://www.pbs.org/transistor/ Transistorized]. Historical and technical information from the Public Broadcasting Service (PBS) web site
- [http://www.lucent.com/minds/transistor/ The Transistor Legacy Then and Now]. From Lucent Technologies (Bell Telephone Laboratories) (AT&T)
- [http://www.aps.org/apsnews/1100/110004.cfm This Month in Physics History: November 17 to December 23, 1947: Invention of the First Transistor]. From the American Physical Society (APS)
- [http://www.sciencefriday.com/pages/1997/Dec/hour1_121297.html 50 Years of the Transistor]. From Science Friday, December 12, 1997
- [http://www.ck722museum.com/ The CK722 Museum]. Website devoted to the "classic" hobbyist germanium transistor
- [http://users.arczip.com/rmcgarra2/index.html Bob's Virtual Transistor Museum & History]. Treasure trove of transistor history
- [http://people.msoe.edu/~reyer/regency/ 1954 to 2004, the TR-1's Golden Anniversary]. In depth coverage of Regency radio.
- [http://www.ee.washington.edu/circuit_archive/parts/cross.html Jerry Russell's Transistor Cross Reference Database].
- The invention of the transistor & the birth of the information age
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- Michael Riordan, How Europe Missed the Transistor, IEEE Spectrum, November 2005 (NA Edition)
- Armand Van Dormael, "The French Transistor", Proceedings of the 2004 IEEE Conference on the History of Electronics, Bletchly Park, England June 2004. Available on the Web at http://www.ieee.org/organizations/history_center/Che2004/VanDormael.pdf
- [http://www.mindfully.org/Technology/2003/Transistor-Matare-Inventor24feb03.htm "Herbert F. Mataré, An Inventor of the Transistor has his moment" The New York Times 24 February 2003] Category:Transistors Category:Electronics Category:Electronic engineering ko:트랜지스터 ja:トランジスタ

Semiconductor device

Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide. Semiconductor devices have replaced thermionic devices (vacuum tubes) in most applications. They use electronic conduction in the solid state as opposed to the gaseous state or thermionic emission in a high vacuum. Semiconductor devices are manufactured as single discrete devices or integrated circuits (ICs), which consist of a number—from a few devices to millions—of devices manufactured onto a single semiconductor substrate.

Semiconductor device fundamentals

The main reason semiconductor materials are so useful is that the behavior of a semiconductor can be easily manipulated by the addition of impurities, known as doping. Semiconductor conductivity can be controlled by introduction of an electric field, by exposure to light, and even pressure and heat; thus, semiconductors can make excellent sensors. Current conduction in a semiconductor occurs via mobile or "free" electrons and holes (collectively known as charge carriers). Doping a semiconductor such as silicon with a small amount of impurity atoms, such as phosphorus or boron, greatly increases the number of free electrons or holes within the semiconductor. When a doped semiconductor contains excess holes it is called "p-type", and when it contains excess free electrons it is known as "n-type". The semiconductor material used in devices is doped under highly controlled conditions in a fabrication facility, or fab, to precisely control the location and concentration of p- and n-type dopants. The junctions which form where n-type and p-type semiconductors join together are called p-n junctions.

Diode

The p-n junction diode is a device made from a p-n junction. At the junction of a p-type and an n-type semiconductor there forms a region called the depletion zone which blocks current conduction from the n-type region to the p-type region, but allows current to conduct from the p-type region to the n-type region. Thus when the device is forward biased, with the p-side at higher electric potential, the diode conducts current easily; but the current is very small when the diode is reverse biased. Exposing a semiconductor to light can generate electron-hole pairs, which increases the number of free carriers and its conductivity. Diodes optimized to take advantage of this phenomenom are known as photodiodes. Compound semiconductor diodes can also be used to generate light, as in light-emitting diodes and laser diodes.

Transistor

Bipolar junction transistors are formed from two p-n junctions, in either n-p-n or p-n-p configuration. The middle, or base, region between the junctions is typically very narrow. The other regions, and their associated terminals, are known as the emitter and the collector. A small current injected through the junction between the base and the emitter changes the properties of the base-collector junction so that it can conduct current even though it is reverse biased. This creates a much larger current between the collector and emitter, controlled by the base-emitter current. Another type of transistor, the field effect transistor operates on the principle that semiconductor conductivity can be increased or decreased by the presence of an electric field. An electric field can increase the number of free electrons and holes in a semiconductor, thereby changing its conductivity. The field may be applied by a reverse-biased p-n junction, forming a junction field effect transistor, or JFET; or by an electrode isolated from the bulk material by an oxide layer, forming a metal-oxide-semiconductor field effect transistor, or MOSFET. The MOSFET is the most used semiconductor device today. The gate electrode, is charged to produce an electric field that controls the conductivity of a "channel" between two terminals, called the source and drain. Depending on the type of carrier in the channel, the device may be an n-channel (for electrons) or a p-channel (for holes) MOSFET. Although the MOSFET is named in part for its "metal" gate, in modern devices polysilicon is typically used instead.

Semiconductor device materials

By far, silicon (Si) is the most widely used material in semiconductor devices. Its combination of low raw material cost, relatively simple processing, and a useful temperature range make it currently the best compromise among the various competing materials. Silicon used in semiconductor device manufacturing is currently fabricated into boules that are large enough in diameter to allow the production of 300 mm (12 in.) wafers. Germanium (Ge) was a widely used early semiconductor material but its lower melting point makes it less useful than silicon. Today, germanium is often alloyed with silicon for use in very-high-speed SiGe devices; IBM is a major producer of such devices. Gallium arsenide (GaAs) is also widely used in high-speed devices but so far, it has been difficult to form large-diameter boules of this material, limiting the wafer diameter to sizes significantly smaller than silicon wafers thus making mass production of GaAs devices significantly more expensive than silicon. Other less common materials are also in use or under investigation. Silicon carbide (SiC) has found some application as the raw material for blue light emitting diodes (LEDs) and is being investigated for use in semiconductor devices that could withstand very high operating temperatures and environments with the presence of significant levels of ionizing radiation. IMPATT diodes have also been fabricated from SiC. Various indium compounds (indium arsenide, indium antimonide, and indium phosphide) are also being used in LEDs and solid state laser diodes. Selenium sulfide is being studied in the manufacture of photovoltaic solar cells.

List of common semiconductor devices

Two-terminal devices:
- Avalanche diode (avalanche breakdown diode)
- DIAC
- Diode (rectifier diode)
- Gunn diode
- IMPATT diode
- Laser diode
- Light-emitting diode (LED)
- Photocell
- PIN diode
- Schottky diode
- Solar cell
- Tunnel diode
- VCSEL
- VECSEL
- Zener diode Three-terminal devices:
- Bipolar transistor
- Darlington transistor
- Field effect transistor
- IGBT (Insulated Gate Bipolar Transistor)
- SCR (Silicon Controlled Rectifier)
- Thyristor
- Triac
- Unijunction transistor Four-terminal devices:
- Hall effect sensor (magnetic field sensor) Multi-terminal devices:
- Charge-coupled device (CCD)
- Microprocessor
- Random Access Memory (RAM)
- Read-only memory (ROM)

Semiconductor device applications

All transistor types can be used as the building blocks of logic gates, which are fundamental in the design of digital circuits. In digital circuits like microprocessors, transistors act as on-off switches; in the MOSFET, for instance, the voltage applied to the gate determines whether the switch is on or off. Transistors used for analog circuits do not act as on-off switches; rather, they respond to a continuous range of inputs with a continuous range of outputs. Common analog circuits include amplifiers and oscillators. Circuits that interface or translate between digital circuits and analog circuits are known as mixed-signal circuits. power semiconductor devices are discrete devices or integrated circuits intended for high current or high voltage applications. Power integrated circuits combine IC technology with power semiconductor technology, these are sometimes referred to as "smart" power devices. Several companies specialize in manufacturing power semiconductors.

Component identifiers

The type designators of semiconductor devices are often manufacturer specific. Nevertheless, there have been attempts at creating standards for type codes, and a subset of devices follow those. For discrete devices, for example, there are three standards: JEDEC JESD370B in USA, Pro Electron in Europe and JIS in Japan.

History of semiconductor device development

1900s

Semiconductors had been used in the electronics field for some time before the invention of the transistor. Around the turn of the 20th century they were quite common as detectors in radios, used in a device called a "cat's whisker". These detectors were somewhat troublesome, however, requiring the operator to move a small tungsten filament (the whisker) around the surface of a carborundum (silicon carbide) crystal until it suddenly started working. Then, over a period of a few hours or days, the cat's whisker would slowly stop working and the process would have to be repeated. At the time their operation was completely mysterious. After the introduction of the more reliable and amplified vacuum tube based radios, the cat's whisker systems quickly disappeared. The "cat's whisker" is a primitive example of a special type of diode still popular today, called a Schottky diode.

World War II

During World War II, radar research quickly pushed radar receivers to operate at ever higher frequencies and the traditional tube based radio receivers no longer worked well. On a whim, Russell Ohl of Bell Laboratories decided to try a cat's whisker. After hunting one down at a used radio store in Manhattan, he found that it worked much better than tube-based systems. Ohl investigated why the cat's whisker functioned so well. He spent most of 1939 trying to grow more pure versions of the crystals. He soon found that with higher quality crystals their finicky behaviour went away, but so did their ability to operate as a radio detector. One day he found one of his purest crystals nevertheless worked well, and interestingly, it had a clearly visible crack near the middle. However as he moved about the room trying to test it, the detector would mysteriously work, and then stop again. After some study he found that the behaviour was controlled by the light in the room–more light caused more conductance in the crystal. He invited several other people to see this crystal, and Walter Brattain immediately realized there was some sort of junction at the crack. Further research cleared up the remaining mystery. The crystal had cracked because either side contained very slightly different amounts of the impurities Ohl could not remove–about 0.2%. One side of the crystal had impurities that added extra electrons (the carriers of electrical current) and made it a "conductor". The other had impurities that wanted to bind to these electrons, making it (what he called) an "insulator". Because the two parts of the crystal were in contact with each other, the electrons could be pushed out of the conductive side which had extra electrons (soon to be known as the emitter) and replaced by new ones being provided (from a battery, for instance) where they would flow into the insulating portion and be collected by the whisker filament (named the collector). However, when the voltage was reversed the electrons being pushed into the collector would quickly fill up the "holes", and conduction would stop almost instantly. This junction of the two crystals (or parts of one crystal) created a solid-state diode, and the concept soon became known as semiconduction. Anode and cathode are the terms used to denote the two terminals of a diode. The mechanism of action when the diode is off has to do with the separation of charge carriers around the junction. This is called a "depletion region".

Development of the diode

Armed with the knowledge of how these new diodes worked, a vigorous effort began in order to learn how to build them on demand. Teams at Purdue University, Bell Labs, MIT, and the University of Chicago all joined forces to build better crystals. Within a year germanium production had been perfected to the point where military-grade diodes were being used in most radar sets.

Development of the transistor

The key to the development of the transistor was the further understanding of the process of the electron mobility in a semiconductor. It was realized that if there was some way to control the flow of the electrons from the emitter to the collector of this newly discovered diode, one could build an amplifier. For instance, if you placed contacts on either side of a single type of crystal the current would not flow through it. However if a third contact could then "inject" electrons or holes into the material, the current would flow. Actually doing this appeared to be very difficult. If the crystal were of any reasonable size, the number of electrons (or holes) required to be injected would have to be very large–making it less than useful as an amplifier because it would require a large injection current to start with. That said, the whole idea of the crystal diode was that the crystal itself could provide the electrons over a very small distance. The key appeared to be to place the input and output contacts very close together on the surface of the crystal. Brattain started working on building such a device, and tantalizing hints of amplification continued to appear as the team worked on the problem. Sometimes the system would work but then stop working unexpectedly. In one instance a non-working system started working when placed in water. Ohl and Brattain eventually developed a new branch of quantum mechanics known as surface physics to account for the behaviour. The electrons in any one piece of the crystal would migrate about due to nearby charges. Electrons in the emitters, or the "holes" in the collectors, would cluster at the surface of the crystal where they could find their opposite charge "floating around" in the air (or water). Yet they could be pushed away from the surface with the application of a small amount of charge from any other location on the crystal. Instead of needing a large supply of injected electrons, a very small number in the right place on the crystal would accomplish the same thing. Their understanding solved the problem of needing a very small control area to some degree. Instead of needing two separate semiconductors connected by a common, but tiny, region, a single larger surface would serve. The emitter and collector leads would both be placed very close together on one side, with the control lead placed on the base of the crystal. When current was applied to the "base" lead, the electrons or holes would be pushed out, across the block of semiconductor, and collect on the far surface. As long as the emitter and collector were very close together, this should allow enough electrons or holes between them to allow conduction to start.

The first transistor

The point-contact transistor was invented at Bell Telephone Laboratories in December 1947 (first demonstrated on 23 December) by John Bardeen, Walter Houser Brattain, and William Bradford Shockley, who were awarded the Nobel Prize in physics in 1956. Ironically, they had set out to manufacture a field-effect transistor (FET) predicted by Julius Edgar Lilienfeld as early as 1925 but eventually discovered current amplification in the point-contact transistor. They made many attempts to build such a system with various tools, but generally failed. Setups where the contacts were close enough were invariably as fragile as the original cat's whisker detectors had been, and would work briefly, if at all. Eventually they had a practical breakthrough. A piece of gold foil was glued to the edge of a plastic wedge, and then the foil was sliced with a razor at the tip of the triangle. The result was two very closely spaced contacts of gold. When the plastic was pushed down onto the surface of a crystal and voltage applied to the other side (on the base of the crystal), current started to flow from one contact to the other as the base voltage pushed the electrons away from the base towards the other side near the contacts. The point-contact transistor had been invented. While the device was constructed a week earlier, Brattain's notes describe the first demonstration to higher-ups at Bell Labs on the afternoon of 23 December 1947, often given as the birthdate of the transistor. The "PNP point-contact germanium transistor" operated as a speech amplifier with a power gain of 18 in that trial.

Improvements in transistor design

Shockley was upset about the device being credited to Brattain and Bardeen, who he felt had built it "behind his back" to take the glory. Matters became worse when Bell Labs lawyers found that some of Shockley's own writings on the transistor were close enough to those of an earlier patent that they thought it best that his name be left off the patent application. Shockley was incensed, and decided to demonstrate who was the brains of the operation. Only a few months later he invented an entirely new type of transistor with a layer or 'sandwich' structure. This new form was considerably more robust than the fragile point-contact system, and would go on to be used for the vast majority of all transistors into the 1960s. It would evolve into the bipolar junction transistor. With the fragility problems solved, a remaining problem was purity. Making germanium of the required purity was proving to be a serious problem, and limited the number of transistors that actually worked from a given batch of material. Germanium's sensitivity to temperature also limited its usefulness. Scientists theorized that silicon would be easier to fabricate, but few bothered to investigate this possibility. Gordon Teal was the first to develop a working silicon transistor, and his company, the nascent Texas Instruments, profited from its technological edge. Germanium disappeared from most transistors by the late 1960s. Within a few years, transistor-based products, most notably radios, were appearing on the market. A major improvement in manufacturing yield came when a chemist advised the companies fabricating semiconductors to use distilled water rather than tap water: calcium ions were the cause of the poor yields. "Zone melting", a technique using a moving band of molten material through the crystal, further increased the purity of the available crystals.

Origin of the term "transistor"

Bell Telephone Laboratories needed a generic name for their new invention: "Semiconductor Triode", "Solid Triode", "Surface States Triode" [sic], "Crystal Triode" and "Iotatron" were all considered, but "transistor", coined by John R. Pierce, won an internal ballot. The rationale for the name is described in the following extract from the company's Technical Memoranda (May 28, 1948) [26] calling for votes:
Transistor. This is an abbreviated combination of the words "transconductance" or "transfer", and "varistor". The device logically belongs in the varistor family, and has the transconductance or transfer impedance of a device having gain, so that this combination is descriptive.

See also


- Integrated circuit
- VLSI
- :Category:Semiconductor device fabrication

References


-
- Category:Semiconductors ja:半導体素子

Switch

: This article is about electrical switches. For other meanings of the word "switch", see Switch (disambiguation). Switch (disambiguation) A switch is a device for making or breaking an electric circuit, or for selecting between multiple circuits. In the simplest case, a switch has two pieces of metal called contacts that touch to make a circuit, and separate to break the circuit. The contact material is chosen for its resistance to corrosion, because most metals form insulating oxides that would prevent the switch from working. Sometimes the contacts are plated with noble metals. They may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used. The moving part that applies the operating force to the contacts is called the actuator, and may be a toggle or dolly, a rocker, a push-button or any type of mechanical linkage (see photo).

Contact arrangements

actuator A pair of contacts is said to be 'closed' when there is no space between them, allowing electricity to flow from one to the other. When the contacts are separated by a space, they are said to be 'open', and no electricity can flow. Switches can be classified according to the arrangement of their contacts. Some contacts are normally open until closed by operation of the switch, while others are normally closed and opened by the switch action. A switch with both types of contact is called a changeover switch. The terms pole and throw are used to describe switch contacts. A pole is a set of contacts that belong to a single circuit. A throw is one of two or more positions that the switch can adopt. These terms give rise to abbreviations for the types of switch which are used in the electronics industry. In mains wiring names generally involving the word way are used; however, these terms differ between British and American English and the terms two way and three way are used in both with different meanings.
Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number or in some cases the letter T (for triple). In the rest of this article the terms SPST SPDT and intermediate will be used to avoid the ambiguity in the use of the word "way".

Make-before-break, break-before-make

In a multi-throw switch, there are two possible transient behaviors as you move from one postion to another. In some switch designs, the new contact is made before the old contact is broken. This is known as make-before-break, and ensures that the moving contact never sees an open circuit. The alternative is break-before-make, where the old contact is broken before the new one is made. This ensures that the two contacts are never shorted to each other. Both types of design are in common use, for different applications.

Biased switches

A biased switch is one containing a spring that returns the actuator to a certain position. The "on-off" notation can be modified by placing parentheses around all positions other than the resting position. For example, an (on)-off-(on) switch can be switched on by moving the actuator in either direction away from the centre, but returns to the central off position when the actuator is released. The momentary push-button switch is a type of biased switch. The most common type is a push-to-make switch, which makes contact when the button is pressed and breaks when the button is released. A push-to-break switch, on the other hand, breaks contact when the button is pressed and makes contact when it is released. An example of a push-to-break switch is a button used to release a door held open by an electromagnet. Changeover push button switches do exist but are even less common.

Special types

Switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (the float switch), the turning of a key (key switch), linear or rotary movement (the limit switch or microswitch), or presence of a magnetic field (the reed switch). The mercury switch consists of a blob of mercury inside a glass bulb. The two contacts pass through the glass, and are shorted together when the bulb is tilted to make the mercury roll on to them. The advantage of this type of switch is that the liquid metal flows around particles of dirt and debris that might otherwise prevent the contacts of a conventional switch from closing. Other types of switch include:
- centrifugal switch
- DIP switch
- Hall-effect switch
- toggle switch
- Transfer switch

Intermediate switch

A DPDT switch has six connections, but since polarity reversal is a very common usage of DPDT switches, some variations of the DPDT switch are internally wired specifically for polarity reversal. They only have four terminals rather than six. Two of the terminals are inputs and two are outputs. When connected to a battery or other DC source, the 4-way switch selects from either normal or reversed polarity. Intermediate switches are also an important part of multiway switching systems with more than two switches (see next section).

Multiway switching

Multiway switching is a method of connecting switches in groups so that any switch can be used to connect or disconnect the load. This is most commonly done with lighting.

Two locations

Transfer switch Switching a load on or off from two locations (for instance, turning a light on or off from either end of a flight of stairs) requires two SPDT switches. There are two basic methods of wiring to achieve this. In the first method, mains is fed into the common terminal of one of the switches; the switches are then connected through the L1 and L2 terminals (swapping the L1 and L2 terminals will just make the switches work the other way round), and finally a feed to the light is taken from the common of the second switch. A connects to B or C, D connects to B or C; the light is on if A connects to D, i.e. if A and D both connect to B or both connect to C. The second method is to join the three terminals of one switch to the corresponding terminals on the other switch and take the incoming supply and the wire out to the light to the L1 and L2 terminals. Through one switch A connects to B or C, through the other also to B or C; the light is on if B connects to C, i.e. if A connects to B with one switch and to C with the other. Wiring needed in addition to the mains network (not including protective earths): First method:
- double wire between both switches
- single wire from one switch to the mains
- single wire from the other switch to the load
- single wire from the load to the mains Second method:
- triple wire between both switches
- single wire from any position between the two switches, to the mains
- single wire from any position between the two switches, to the load
- single wire from the load to the mains If the mains and the load are connected to the system of switches at one of them, then in both methods we need three wires between the two switches. In the first method one of the three wires just has to pass through the switch, which tends to be less convenient than being connected. When multiple wires come to a terminal they can often all be put directly in the terminal. When wires need to be joined without going to a terminal a crimped joint, piece of terminal block, wirenut or similar device must be used and the bulk of this may require use of a deeper backbox.

More than two locations

Transfer switch For more than two locations, the two cores connecting the L1 and L2 of the switches must be passed through an intermediate switch (as explained above) wired to swap them over. Any number of intermediate switches can be inserted, allowing for any number of locations. Wiring needed in addition to the mains network (not including protective earths):
- first method
  - double wire along the sequence of switches
  - single wire from the first switch to mains
  - single wire from the last switch to the load
  - single wire (neutral) from load to mains
- second method
  - double wire along the sequence of switches
  - single wire from first switch to last switch
  - single wire from anywhere between two of the switches to the mains
  - single wire from anywhere between the same two switches to the load
  - single wire (neutral) from load to mains

Contact bounce

Contact bounce (also called chatter) is a common problem with mechanical switches and relays. Switch and relay contacts are usually made of springy metals that are forced into contact by an actuator. When the contacts strike together, their momentum and elasticity act together to cause bounce. The result is a rapidly pulsed electrical current instead of a clean transition from zero to full current. The waveform is then further modified by the parasitic inductances and capacitances in the switch and wiring, resulting in a series of damped sinusoidal oscillations. This effect is usually unnoticeable in AC mains circuits, where the bounce happens too quickly to affect most equipment, but causes problems in some analogue and logic circuits that are not designed to cope with oscillating voltages. Sequential digital logic circuits are particularly vulnerable to contact bounce. The voltage waveform produced by switch bounce usually violates the amplitude and timing specifications of the logic circuit. The result is that the circuit may fail, due to problems such as metastability, race conditions, runt pulses and glitches.

Hardware debouncing

Special circuits called "debouncing circuits" are often used to process the voltage from a switch or relay before it is applied to the input of a sensitive circuit.
- A simple analogue debouncing circuit consists of an RC (resistor-capacitor) filter that removes fast oscillations from the signal. However, the slowly changing edge that this produces is unsuitable for triggering high-speed logic circuits, where it could cause metastability. This issue can be resolved by feeding the signal through a Schmitt trigger.
- A debouncing circuit suitable for logic circuits consists of a monostable multivibrator, a circuit that registers the first voltage pulse, produces an output pulse of fixed width, then ignores any further switch pulses until the output pulse has terminated. The circuit designer sets the monostable's pulse width to exceed the bounce time.
- If the switch or relay has changeover (also called double-throw or SPDT) contacts, then a debouncing circuit can be made by adding an SR flip-flop. One contact of the switch drives a pulse into the set input of the flip-flop, and the other contact drives a pulse into the reset input. The output of the flip-flop is a clean pulse that goes high when the switch is pushed away from its rest position, and then low when the switch is released, with no bounce.

Software debouncing

If the switch voltage is fed directly to the input of a microprocessor, then the software might become confused by the rapid sequence of high and low logic levels when it is expecting only a single, stable transition between "on" and "off". If debouncing circuits have not been provided, then there are software remedies that can be used. A simple algorithm is to wait for the first transition (say, 0 to 1), then ignore the input for a fixed time before sampling it again. The time delay is selected so that the switch has stopped bouncing before it is sampled again.

Reference


- Walker, PMB, Chambers Science and Technology Dictionary, Edinburgh, 1988, ISBN 0-85296-151-1 (definition of contact bounce) ---- SWITCH (slang context) In many urban settings, the words "sweet" and "bitch" are homgonenized to the single word "switch". Thus, referring to an attractive female will warrant the using of the term "switch".

See also


- Circuit breaker
- Contactor
- Relay
- Analogue switch
- UL, CSA, VDA

External links


- [http://www.indepthinfo.com/wire-switch/index.shtml How to Wire a Switch] (very U.S. specific)
- [http://www.engineersedge.com/instrumentation/membrane_switch_construction.htm Membrane Switch Construction] ja:開閉器 Category:Switches

Field effect transistor

The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a "channel" in a semiconductor material. FETs are sometimes used as voltage-controlled resistors. The terminals in FET are called gate, drain and source. (Compare these to the terminology used for BJTs: base, collector and emitter.) The voltage applied between the gate and source terminals modulates the current between the source and drain terminals. Most FETs are made with conventional bulk semiconductor processing techniques, using the single crystal semiconductor wafer as the active region, or channel.

Types of field-effect transistors

The FET is simpler in concept than the bipolar transistor and can be constructed from a wide range of materials. The different types of field-effect transistors can be distinguished by the method of isolation between channel and gate:
- The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) utilizes an isolator (typically SiO2).
  - Power MOSFETs become less conductive with increasing temperature and can therefore be thought of as n-channel devices by default. Silicon devices that use electrons, rather than holes, as the majority carriers are slightly faster and can carry more current than their P-type counterparts. The same is true in GaAs devices.
- The JFET (Junction Field-Effect Transistor) uses a p-n junction as the gate.
- The MESFET (Metal-Semiconductor Field-Effect Transistor) substitutes the p-n-junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.
- Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High Electron Mobility Transistor), also named an HFET (heterostructure FET). The fully depleted wide-band-gap material forms the isolation.
- TFTs (thin-film transistor) use amorphous silicon, polycrystalline silicon or other amorphous semiconductors as body material.
- A subgroup of TFTs are organic field effect transistors that are based on organic semiconductors and often apply organic gate insulators and electrodes.
- The channel region of any FET is either doped to produce n-type semiconductor, giving an "N-channel" device, or with p-type to give a "P-channel" device. The doping determines the polarity of gate operation.

FET Operation

The shape of the conducting channel in a FET is altered when a potential voltage is applied to the gate terminal (potential relative to either source or drain.) In an n-channel device, a negative gate voltage causes a depletion region to expand in size and encroach on the channel from the side, narrowing the channel. If the depletion region completely closes the channel, the resistance of the channel becomes very large, and the FET is effectively turned off. Positive gate voltage attracts electrons from the surrounding semiconductor next to the gate, forming a conductive channel. At low source-to-drain voltages, small changes to the gate voltage will alter the channel resistance. In this mode the FET operates like a variable resistor. This mode is not employed when amplification is needed. If a larger potential difference is applied between the source and drain terminals, this creates a significant current in the channel and produces a gradient of voltage potential from source to drain. This also causes the shape of the depletion region to become asymmetrical–one end of the channel becomes narrow. If the potential difference is large enough, the depletion region begins to close the channel. The FET is said to be in saturation. Rather than entirely blocking the electrons from flowing from source to drain, electrons flow through the depletion region in a controlled manner. Any attempted increase of the drain-to-source voltage will lengthen the depletion region, increasing the channel resistance proportionally with the applied drain-to-source voltage which causes the value of drain current to remain relatively fixed. This mode of operation is called pinch-off. In this mode, the FET behaves as a constant-current source rather than as a resistor and can be used as a voltage amplifier. The value of gate voltage determines the value of the constant current in the channel.

Uses

The most commonly used FET is the MOSFET. The CMOS (complementary metallic oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where p-channel MOSFET and n-channel MOSFET (n and p being complementary MOSFET types) are connected in series such that when one is on, the other is off. In CMOS logic devices, the p-channel device pulls up the output and the n-channel device pulls down the output. The great advantage of CMOS circuits is that they allow no current to flow (ideally), except during the transition from one state to the other, which is very short. The gates are capacitive, and the charging and discharging of the gates each time a transistor switches states is the primary source of power usage in fast CMOS logic circuits. The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable to electrostatic damage during handling. This is not usually a problem after the device has been installed. FETs can switch signals of either polarity on the source or drain terminals, if their amplitude is significantly less than the gate swing, as the devices are typically symmetrical. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example. The power MOSFET has a reverse-biased 'parasitic diode' shunting the conduction channel that has half the current capacity of the conduction channel. Sometimes this diode is used when driving inductive circuits, but in other cases it causes problems. A more recent device for power control is the insulated-gate bipolar transistor, or IGBT. This has a control structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These have become quite popular.

See also

External links


- [http://www.pbs.org/transistor/science/info/transmodern.html PBS The Field Effect Transistor]
- [http://www.onr.navy.mil/sci_tech/information/312_electronics/ncsr/devices/jfet.asp Junction Field Effect Transistor] Category:Transistors ---- ja:電界効果トランジスタ

Analog circuit

An analog circuit (or analogue circuit) is an electric circuit that operates on analog signals. While operating on an analog signal, an analog circuit changes the signal in some manner or manners. It may be designed to amplify, attenuate, provide isolation, distort, or modify the signal in some other way. It can be used to convert the signal into some other format such as a digital signal. Analog circuits also modify signals in unintended ways such as adding noise or distortion. Passive analog circuits consume no external electrical power while active analog circuits use an electrical power source to achieve the designer's goals. An example of a passive analog circuit is a passive filter that limits the amplitude at some frequencies vs. others. A similar example of an active analog circuit is an active filter. It does a similar job only it uses an amplifier to accomplish a similar task. Advantages of a passive analog circuit are it requires no power source, gives off less heat, and may produce less noise. Advantages of an active analog circuit is it can load the signal less, amplify as well as attenuate the signal and by using capacitors in combination with amplifiers it can simulate an inductor. Simulation of inductions has the advantage of reducing weight and cost.

Analog integrated circuit

Active or passive analog electronic circuits can be fabricated directly onto semiconductor substrates, such as silicon. Such circuits are called analog integrated circuits. They may occur as sub-systems of other digital systems (e.g., an analog comparator in a microcontroller.) Analog integrated circuit design is a highly specialized area. Category:Electronics

Linear regulator

In electronics, a linear regulator is a voltage regulator based on a transistor operating in its "linear region". That transistor acts like a variable resistor. (In contrast, a switching regulator is based on a transistor forced to act as an on/off switch). The transistor is used as one half of a potential divider to control the output voltage, and a feedback circuit compares the output voltage to a reference voltage in order to adjust the input to the transistor, thus keeping the output voltage reasonably constant. This is inefficient: since the transistor is acting like a resistor, it will dissipate heat. In fact, the power loss due to heating in the transistor is the current times the voltage dropped across the transistor. The same function can be performed more efficiently by a switched-mode power supply (SMPS), but it is more complex and the switching currents in it tend to produce electromagnetic interference. All said, most modern SMPS's have a performance that can be as good as or exceed that of linear regulators. Most computers have a SMPS to power them. A SMPS can easily provide more than 30A of current at voltages as low as 3V, while for the same voltage and current, a linear regulator would be very bulky and heavy. Linear regulators exist in two basic forms: series regulators and shunt regulators.
- Series regulators are the more common form. The series regulator works by providing a path from the supply voltage to the load through a variable resistance (the main transistor is in the "top half" of the voltage divider). The power dissipated by the regulating device is equal to the power supply output current times the voltage drop in the regulating device.
- The shunt regulator works by providing a path from the supply voltage to ground through a variable resistance (the main transistor is in the "bottom half" of the voltage divider). The current through the shunt regulator is diverted away from the load and flows uselessly to ground, making this form even less efficient than the series regulator. It is, however, simpler, sometimes consisting of just a voltage-reference diode, and is used in very low-powered circuits where the wasted current is too small to be of concern. This form is very common for voltage reference circuits. All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the drop-out voltage. For example, a common regulator such as the 7805 has an output voltage of 5V, but can only maintain this if the input voltage remains above about 7V. Its drop-out voltage is therefore 7V - 5V = 2V. When the supply voltage is less than about 2V above the desired output voltage, as is the case in low-voltage microprocessor power supplies, so-called low dropout regulators (LDOs) must be used. When one wants a voltage higher than the available input voltage, no linear regulator will work (not even an LDO). In this situation, a switching regulator must be used.

Simple zener regulator

switching regulator The image shows a simple zener voltage regulator. It is a shunt regulator and operates by way of the zener diode's action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the zener breakdown region. The resistor R1 supplies the zener current IZ as well as the load current IR2 (R2 is the load). R1 can be calculated as - R1 = \frac where, VZ is the zener voltage, and IR2 is the required load current. This regulator is used for very simple low power applications where the currents involved are very small and the load is permanently connected across the zener diode (such as voltage reference or voltage source circuits). Once R1 has been calculated, removing R2 will cause the full load current (plus the zener current) to flow through the diode and may exceed the diode's maximum current rating thereby damaging it. The regulation of this circuit is also not very good due to the fact that the zener current (and hence the zener voltage) will vary depending on the load and VS. To make this circuit work with better regulation, a transistor buffer is connected between the zener and the output.

Simple series regulator

voltage source The simple zener regulator, when buffered using a transistor (Q) as shown forms a simple series voltage regulator. Here, the load current IR2 is supplied by the transistor whose base is now connected to the zener diode. Thus the transistor's base current (IB) forms the load current for the zener diode and is much smaller than the current through R2. This regulator is classified as "series" because the regulating element, viz., the transistor, appears in series with the load. R1 sets the zener current (IZ) and is determined as - R1 = \frac where, VZ is the zener voltage, IB is the transistor's base current and K = 1.2 to 2 (to ensure that R1 is low enough for adequate IB). I_ = \frac where, IR2 is the required load current and is also the transistor's emitter current (assumed to be equal to the collector current) and hFE(min) is the minimum acceptable DC current gain for the transistor. This circuit has much better regulation than the simple zener regulator, since the base current of the transistor forms a very light load on the zener, thereby minimising variation in zener voltage due to variation in the load. Note that the output voltage will always be about 0.65V less than the zener due to the transistor's VBE drop. Although this circuit has good regulation, it is still sensitive to the load and supply variation. It also does not have the capability to be adjustable. Both these issues can be resolved by incorporating negative feedback circuitry into it. This regulator is often used as a "pre-regulator" in more advanced series voltage regulator circuits. See also: Capacitance multiplier

Using a linear regulator

Linear regulators can be discrete as well as in integrated circuit form. The most common linear regulators are three-terminal integrated circuits in P1d packages/TO-220 package. (The TO-220 package is the same kind that many medium-power transistors commonly come in: three legs in a straight line protruding from a black plastic molded case with a metal backplate which has a hole for bolting to a heatsink). After one connects the appropriate pins to 0v and incoming power, the regulated output voltage appears on the output pin. Common solid-state series voltage regulators are the LM78xx (for positive voltages) and LM79xx (for negative voltages), and common fixed voltages are 5 V (for transistor-transistor logic circuits) and 12 V (for communications circuits and peripheral devices such as disk drives). In fixed voltage regulators the reference pin is tied to ground, whereas in variable regulators the reference pin is connected to the centre point of a fixed or variable voltage divider fed by the regulator's output. A variable voltage divider (such as a potentiometer) allows the user to adjust the regulated voltage.

Fixed regulators

"Fixed" three-terminal linear regulators are commonly available to generate fixed voltages of plus 3 V, and plus or minus 5 V, 9 V, 12 V, or 15 V when the load is less than about 7 amperes. The "78" series (7805, 7812, etc.) regulate positive voltages while the "79" series (7905, 7912, etc.) regulate negative voltages. Often, the last two digits of the device number are the output voltage; eg, a 7805 is a +5 V regulator, while a 7915 is a -15 V regulator.

Adjustable regulators

For output voltages not provided by standard fixed regulators and load currents of less than 7 amperes, commonly-available "adjustable" three-terminal linear regulators may be used. An adjustable regulator generates a fixed low nominal voltage between its output and its 'adjust' terminal (equivalent to the ground terminal in a fixed regulator). The "317" series (+1.2V) regulates positive voltages while the "337" series (-1.2V) regulates negative voltages. The adjustment is performed by constructing a potential divider with its ends between the regulator output and ground, and its centre-tap connected to the 'adjust' terminal of the regulator. The ratio of resistances determines the output voltage using the same feedback mechanisms described earlier.

Other devices

More complex regulators are available in packages with more than three pins, including dual in-line packages. ---- Category:Power supplies

Switched-mode power supply

A switched-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator — an internal control circuit that switches the load current rapidly on and off in order to stabilise the output voltage. Switching regulators are used as replacements for simpler linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated and more expensive, their switching currents can cause noise problems if not carefully suppressed, and simple designs can have a poor power factor. SMPS can also be classified into four types according to the input and output waveforms, as follows.
- AC in, DC out: rectifier, off-line converter
- DC in, DC out: voltage converter, or current converter, or DC to DC converter
- AC in, AC out: frequency changer, cycloconverter
- DC in, AC out: inverter AC and DC are abbreviations for alternating current and direct current. Switched-mode PSUs in domestic products such as personal computers often have universal inputs, meaning that they can accept power from most mains supplies throughout the world, with frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V (although a manual voltage "range" switch may be required). In the case of TV sets, for example, one can test the excellent regulation of the power supply by using a variac. For example, in some models made by Philips, the power supply starts when the voltage reaches around 90 volts. From then, one can change the voltage with the variac, and go as low as 40 volts and as high as 260, and the image will show absolutely no alterations.

SMPS compared with linear PSUs

Two main types of regulated power supplies are available: SMPS and linear. The reasons for choosing one type or the other can be summarized as follows.
- Size and weight. Linear power supplies use a mains-transformer operating at the mains frequency of 50/60 Hz and line-frequency smoothing filters. These components are larger and heavier than the corresponding parts of a SMPS, which operate at higher frequencies.
- Efficiency. Linear power supplies regulate their output by generating a higher voltage than needed at the output, then reducing it by converting some of the electrical power to heat. This loss is a necessary part of the operation of the circuit, and cannot be eliminated by improving the design. SMPS generate no more voltage than they require, and only a small amount of energy is wasted.
- Heat output or power dissipation. This is determined by the efficiency, above. Linear PSUs produce much more heat than SMPS.
- Complexity. Linear PSUs are simple enough to be designed by beginners. SMPS are complicated, difficult to design well, and the higher number of components makes them more expensive to assemble and to repair.
- Noise. The switching currents in a SMPS contain much more energy at high frequencies than those in a linear PSU. This high-frequency energy can be transmitted by electromagnetic induction to other nearby equipment, or as radio waves over long distances, causing interference. Care is therefore needed to eliminate this energy at source, or to contain it by screening.
- Power factor. If the current drawn by a load such as a SMPS from an AC supply is nonsinusoidal and/or out of phase with the supply voltage waveform, the power factor will be less than unity and the efficiency, capacity and reliability of generating plants and the transmission grid can be substantially decreased. The simplest and most common SMPS designs have a power factor of about 0.6, and their use in increasingly popular personal computers and compact fluorescent lamps presents a growing problem. Power factor correction (PFC) circuits can reduce this problem, and are required in some countries by regulation. Notably, power factor correction is not yet widely required or used in North America.

How an SMPS works

Power factor correction

Rectifier stage

Power factor correction If the SMPS has an AC input, then its first job is to convert the input to DC. This is called rectification. The rectifier circuit is often the same as that in a linear power supply, and produces an unregulated DC voltage which is then smoothed by a filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high frequency energy which reduces the power factor. Special control techniques can be employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage thus correcting the power factor. A SMPS with a DC input does not require this stage. A SMPS designed for AC input can often be run from a DC supply, as the DC passes through the rectifier stage unchanged. (The user should check the manual before trying this!) If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage (~120 VAC) range and as a straight rectifier when operating on the high voltage (~240 VAC) range. If an input range switch is not used, then a full-wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of dc voltages that will be produced by the rectifier stage. In higher-power SMPSs, some form of automatic range switching may be used.

Inverter stage

The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by switching it on and off ('chopping') at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The switching is done by MOSFETs, which are a type of transistor with a low on-resistance and a high current-handling capacity. This section refers to the block marked "Chopper" in the block diagram.

Voltage converter

If the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer. This converts the voltage up or down to the required output level on its secondary winding. The output transformer in the block diagram serves this purpose. If a DC output is required, the AC output from the transformer is rectified and smoothed by a filter consisting of inductors and capacitors. The higher the switching frequency, the smaller these components can be made. Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the so called "buck-boost converter". These belong to the simplest class of single input, single output converters which utilise one inductor and one active switch (MOSFET). The buck converter reduces the input voltage, in direct proportion, to the ratio of the active switch "on" time to the total switching period, called the Duty Ratio. For example an ideal buck converter with a 10V input operating at a duty ratio of 50% will produce an average output voltage of 5V. A feedback control loop is usually employed to maintain (regulate) the output voltage by varing the duty ratio to compensate for variations in input voltage. The output voltage of a boost converter is always greater than the input voltage and the buck-boost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage. There are many variations and extensions to this class of converters but these three form the bases of almost all isolated and non-isolated DC to DC conveters. By adding a second inductor the Cuk and SEPIC converters can be implemented or by adding additional active switches various bridge converters can be realised. Other types of SMPS use a capacitor-diode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents.

Regulation

A feedback circuit monitors the output voltage and compares it with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage, the feedback circuit compensates by adjusting the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator. The "Chopper controller" shown in the block diagram serves this purpose. Depending on design/safety requirements, the controller may or may not contain an isolation mechanism (such as opto-couplers) to isolate it from the DC output. Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct.

Power factor

Unlike most other appliances, switched mode power supplies tend to be constant power devices, drawing more current as the line voltage reduces. Also, in common with many static rectifiers, maximum current draw occurs at the peaks of the waveform cycle. This means that basic switched mode power supplies tend to produce more harmonics in the mains power line and have a worse power factor than other types of appliances. This may cause stability problems in some situations such as emergency generator systems or for very heavy loads on ordinary power mains (as it can lead to increased neutral current and increased heating of the utility transformers. However, higher-quality switched-mode power supplies with power factor correction (PFC) are available; these are designed to present a near resistive load to the mains. European regulatory standards are now beginning to require power factor correction and harmonic reduction.

Types

Switched-mode power supplies can classified according to the circuit topology.
- buck regulator (single inductor; output voltage < input voltage)
- boost regulator (single inductor; output voltage > input voltage)
- buck-boost regulator (single inductor; output voltage can be more or less than the input voltage)
- flyback regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
- forward regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
- Ćuk converter (uses a capacitor for energy storage; produces negative voltage for positive input)
- Inverting charge-pump (Modified Ćuk with single inductor; output voltage negative and higher-magnitude than positive input voltage)
- SEPIC converter (two inductors; output voltage can be higher or lower than input voltage)

External articles


- [http://www.smpstech.com/tutorial/t00con.htm Switching-Mode Power Supply Design]
- [http://www.smps.us/Unitrode.html Unitrode Power Supply Design Seminar Books Online]
- [http://www.hills2.u-net.com/electron/smps.htm Switched Mode Power Supplies]. A fairly detailed discussion of converter types and control schemes. Does not cover modern switcher ICs.
- Watkins, Steve, "[http://www.steve-w.dircon.co.uk/fleadh/mphil/history.htm History and development of switched-mode power supplies pre 1987]". 1998 (ed. the bibliography is [http://www.steve-w.dircon.co.uk/fleadh/mphil/references.htm here].)
- [http://www.powerdesigners.com/InfoWeb/design_center/articles/DC-DC/converter.shtm]. A general description of DC-DC converters.

Book References


- Ned Mohan, Tore M. Undeland, William P. Robbins (2002). Power Electronics : Converters, Applications, and Design. Wiley. ISBN 0-471-22693-9.
- Muhammad H. Rashid (2003). Power Power electronics : circuits, devices, and applications. Prentice Hall. ISBN 0-131-22815-3.
- Fang Lin Luo, Hong Ye (2004). Advanced DC/DC Converters. CRC Press. ISBN 0-849-31956-0.
- Mingliang Liu (2006). Demystifying Switched-Capacitor Circuits. Elsevier. ISBN 0-750-67907-7.
- Fang Lin Luo, Hong Ye, Muhammad H. Rashid (2005). Power Digital Power Electronics and Applications. Elsevier. ISBN 0-120-88757-6.
- Abraham I. Pressman (1997). Switching Power Supply Design. McGraw-Hill. ISBN 0-070-52236-7. Category:Power supplies



Random access memory

:RAM redirects here. For other meanings of the word ram see Ram (disambiguation). For the Hindu God see Rama. Random access memory (sometimes random-access memory), commonly known by its acronym RAM, is a type of computer storage (in practice only computer chips) whose contents can be accessed in any (i.e., random) order. This is in contrast to sequential memory devices such as magnetic tapes, discs and drums, in which the mechanical movement of the storage medium forces the computer to access data in a fixed order. magnetic tapes are located in the rectangular areas to the left and right.]] RAM is typically used for primary storage (main memory) in computers to hold actively used and actively changing information, although some devices use certain types of RAM to provide long-term secondary storage. Because RAM chips can be both written to and read from, the term RAM is often used to mean read-write memory, and thus taken to be the opposite of read-only memory (ROM). However, RAM refers to the way memory is accessed in a chip so ROM in the form of a chip is also RAM. Also, DVD-RAM is a misnomer because a disk like a DVD (or CD or hard disk) is physically unfit for random access.

Overview

Computers use RAM to hold the program code and data during execution. One defining characteristic of RAM is that its accesses to different memory locations are almost always completed at about the same speed, in contrast to some other technologies that required a certain delay time for a bit or byte to “come around”. Early vacuum tube-based systems behaved much like modern RAM, even though the devices failed much more frequently. Core memory, which used wires attached to small ferrite electromagnetic cores, also had roughly equal access time (the term “core” is still used by some programmers to describe the RAM at the heart of a computer). The basic ideas behind tube and core memory are still used in modern RAM implemented with integrated circuits. Alternative primary storage mechanisms usually involved a non-uniform delay for memory access. Delay line memory used a sequence of sound wave pulses in mercury-filled tubes to hold a series of bits. Drum memory acted much like the modern hard disk, storing data magnetically in continuous circular bands. (See primary storage for a greater discussion of these alternatives and others.) Many types of RAM are volatile, which means that unlike some other forms of computer storage such as disk storage and tape storage, they lose their data when the computer is powered down. Modern RAM generally stores a bit of data as either a charge in a capacitor, as in dynamic RAM, or the state of a flip-flop, as in static RAM. Currently, there are several types of non-volatile RAM under development, which will preserve data while powered down. Technologies that are being used include carbon nanotube technology and magnetic tunnel effect. In the summer of 2003, a 128KB Magnetic RAM chip was introduced, which was manufactured with 0.18µm technology. The core technology of MRAM is based on the magnetic tunnel effect. In June of 2004, Infineon Technologies unveiled a 16MB prototype based on 0.18µm technology once again. As for carbon nanotube memory, a high-tech startup [http://www.nantero.com/ Nantero] has built a functioning prototype 10GB array in 2004. In computers, RAM can sometimes be allocated as a partition, allowing it to effectively act as a hard drive, only much faster. This is usually referred to as a RAM disk. Some types of RAM can detect or correct random unintentional changes to RAM contents, known as memory errors. See RAM parity.

The memory wall

In today's computers, memory access is becoming very slow when compared to CPU cycles since most computers use cheap, but comparatively slow, DRAM for the main memory. It is slow due to the fact that it has only one 64 bit (8 byte) data transfer while other chips have double that. Hence, the memory access, like hard disk access, might become the term that bounds computation speed. This is another important boundary for fast computations.

Shadow RAM

Shadow RAM is the part of RAM with its contents copied from ROMs from where it will run much faster [http://hardwarehell.com/articles/shadowram.htm]. (ROM is in general slower than RAM.) The original ROM is disabled and the new location on the RAM is write protected. This process is called shadowing.

Types of RAM

Common


- Static RAM (SRAM)
- Non-Volatile RAM (NV-RAM)
- Dynamic RAM (DRAM)
  - Fast Page Mode DRAM
  - EDO RAM or Extended Data Out DRAM
  - XDR DRAM
  - SDRAM or Synchronous DRAM
    - DDR SDRAM or Double Data Rate Synchronous DRAM; now being replaced by DDR2
    - RDRAM or Rambus DRAM

Uncommon


-