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Transistor was also a common name for a 1960s era handheld transistor radio.

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 it draws from a connected voltage source.

Contents 1 Introduction 2 History 3 Importance 4 Types 4.1 Bipolar junction transistor 4.2 Field-effect transistor 4.3 Other transistor types 4.4 Semiconductor material 5 Packaging 6 Usage 6.1 Commutation 6.2 Amplifiers 6.3 Computers 7 Advantages of transistors over vacuum tubes 8 Gallery 9 See also 10 External links and references

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; see the respective articles for further details.

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

See semiconductor device for more history.

The 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. It is not clear whether either design was ever built, however, and is generally considered unlikely.

In 1947 William Shockley, John Bardeen and Walter Brattain succeeded in building the first practical point-contact transistor at Bell Labs. This work followed from their war-time efforts to produce extremely pure germanium, used in radar units as a receiver element for microwaves. Early tube-based technology did not switch fast enough for this role, leading the Bell team to use solid state diodes instead. With this knowledge in hand they turned to the design of a triode, but found this was not at all easy. Bardeen eventually developed a new branch of surface physics to account for the "odd" behaviour they saw, and Bardeen and Brattain eventually succeeded in building a working device.

Bell Telephone Laboratories needed a generic name for the new invention: "Semiconductor Triode", "Solid Triode", "Surface States Triode", "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.

Bell put the transistor into production at Western Electric in Allentown, Pennsylvania. They also licensed it to a number of other electronics companies, including Texas Instruments who produced a limited run of transistor radios as a sales tool. Another company liked the idea and decided to also take out a license, introducing their own radio under the brand name Sony. Early transistors were "unstable" and useful for low-power needs only, but as construction techniques improved these problems were slowly overcome. 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.

Shockley, Bardeen and Brattain were honored with the Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect". Bardeen would win a second Nobel in physics, the only person to receive more than one, for his work on the exploration of superconductivity.

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

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_T, 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. It is commonly described as a current operated device because the the collector current is controlled by the current flowing between base and emitter terminals. 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:

The base internal current is mainly by diffusion and

Where

• I_E is the emitter current
• I_C is the collector current
• α_F is the common base forward short circuit current gain (0.98 to 0.998)
• I_ES is the reverse saturation current of the base-emitter diode (on the order of 1e-15 to 1e-12 Amperes)
• V_T is volt equivalent temperature (approximately 26 mV at room temperature ≈ 300K)
• V_BE is the base-emitter voltage
• W is the base width

The collector current is slightly less than the emitter current, since the value of α_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_FE. 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:

Emitter Efficiency:

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:

Where:

• I_d is the drain current
• I_dss is the drain current at zero gate-source voltage
• V_gs is the gate-source voltage
• V_p 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 [1] 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

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.

Although the physical characteristics of the packages are standardized, the assignment of a transistor's terminals to the packages pins is not: different transistor types assign different terminals to the package's pins.

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.

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, though this is usually a desirable part of the sound of guitar amplifiers)

" 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.)

• 2N3904/2N3906, BC182/BC212 and BC546/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.

• LM394: So-called 'supermatch pair', with two NPN BJTs on a single substrate.

• 2N2219A/2N2905A: BJT, general purpose, medium power, complementary pair. With metal cases they are rated at about one watt.

• 2N3055/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[2].

• BU508: NPN, 1500V power BJT. Designed for television horizontal deflection, its high voltage capability also finds use in ignition systems.

• MJ11012/MJ11015: 30A, 120V, 200W, high power Darlington complementary pair BJTs. Used in audio amplifiers and control and power switching.

• 2N5457/2N5460: 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.

• IRF3710/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
• Vacuum tube

External links and references

• U.S. Patent 2524035 -- J. Bardeen et. al.
• U.S. Patent 2569347 -- W. Shockley
• AudioUK's Milestones. Photograph of first working transistor
• Transistorized. Historical and technical information from the Public Broadcasting Service (PBS) web site
• The Transistor Legacy Then and Now. From Lucent Technologies (Bell Telephone Laboratories) (AT&T)
• This Month in Physics History: November 17 to December 23, 1947: Invention of the First Transistor. From the American Physical Society (APS)
• 50 Years of the Transistor. From Science Friday, December 12, 1997
• The CK722 Museum. Website devoted to the "classic" hobbyist germanium transistor
• Bob's Virtual Transistor Museum & History. Treasure trove of transistor history
• 1954 to 2004, the TR-1's Golden Anniversary. In depth coverage of Regency radio.
• Jerry Russell's Transistor Cross Reference Database.
• Riordan, Michael & Hoddeson, Lillian (1998). Crystal Fire, W.W Norton & Company Limited. ISBN 0393318516. The invention of the transistor & the birth of the information age
• Amos S W & James M R (1999). Principals of Transistor Circuits, Butterworth-Heinemann. ISBN 0750644273.
• Horowitz, Paul & Hill, Winfield (1989). The Art of Electronics, Cambridge University Press. ISBN 0521370957.
• Warnes, Lionel (1998). Analogue and Digital Electronics, Macmillan Press Ltd. ISBN 0-333-65820-5.
• Williams, Tim (2001). EMC for Product Designers, 3rd edition, Butterworth Heinemann. ISBN 0-7506-4930-5.
• Michael Riordan (November 2005). How Europe Missed the Transistor. IEEE Spectrum 42 (11): 52–57. ISSN 0018-9235.
• Armand Van Dormael. ({{{Year}}}). "The French Transistor". Proceedings of the 2004 IEEE Conference on the History of Electronics, Bletchly Park, June 2004, {{{Pages}}}.
• “Herbert F. Mataré, An Inventor of the Transistor has his moment”, The New York Times, 24 February 2003.

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