Size comparison of BJT transistor packages, from left to right: SOT-23, TO-92, TO-126, TO-3
Metal-oxide-semiconductor field-effect transistor (MOSFET), showing gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).

A transistor is a semiconductor device used to amplify or switch electrical signals and power. The transistor is one of the basic building blocks of modern electronics.[1] It is composed of semiconductor material, usually with at least three terminals for connection to an electronic circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Some transistors are packaged individually, but many more are found embedded in integrated circuits.

Austro-Hungarian physicist Julius Edgar Lilienfeld proposed the concept of a field-effect transistor in 1926, but it was not possible to actually construct a working device at that time.[2] The first working device to be built was a point-contact transistor invented in 1947 by American physicists John Bardeen and Walter Brattain while working under William Shockley at Bell Labs. The three shared the 1956 Nobel Prize in Physics for their achievement.[3] The most widely used type of transistor is the metal–oxide–semiconductor field-effect transistor (MOSFET), which was invented by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959.[4][5][6] Transistors revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, among other things.

Most transistors are made from very pure silicon, and some from germanium, but certain other semiconductor materials are sometimes used including gallium nitride and silicon carbide. A transistor may have only one kind of charge carrier, in a field-effect transistor, or may have two kinds of charge carriers in bipolar junction transistor devices. Compared with the vacuum tube, transistors are generally smaller and require less power to operate. Certain vacuum tubes have advantages over transistors at very high operating frequencies or high operating voltages. Many types of transistors are made to standardized specifications by multiple manufacturers.

## History

 Main article: History of the transistor
Julius Edgar Lilienfeld proposed the concept of a field-effect transistor in 1925.

The thermionic triode, a vacuum tube invented in 1907, enabled amplified radio technology and long-distance telephony. The triode, however, was a fragile device that consumed a substantial amount of power. In 1909, physicist William Eccles discovered the crystal diode oscillator.[7] Austro-Hungarian physicist Julius Edgar Lilienfeld filed a patent for a field-effect transistor (FET) in Canada in 1925,[8] which was intended to be a solid-state replacement for the triode.[9][10] Lilienfeld also filed identical patents in the United States in 1926[11] and 1928.[12][13] However, Lilienfeld did not publish any research articles about his devices nor did his patents cite any specific examples of a working prototype. Because the production of high-quality semiconductor materials was still decades away, Lilienfeld's solid-state amplifier ideas would not have found practical use in the 1920s and 1930s, even if such a device had been built.[14] In 1934, German inventor Oskar Heil patented a similar device in Europe.[15]

### Bipolar transistors

John Bardeen, William Shockley and Walter Brattain at Bell Labs in 1948. Bardeen and Brattain invented the point-contact transistor in 1947 and Shockley the bipolar junction transistor in 1948.
A replica of the first working transistor, a point-contact transistor invented in 1947
 Further information: Point-contact transistor and Bipolar junction transistor

From November 17, 1947, to December 23, 1947, John Bardeen and Walter Brattain at AT&T's Bell Labs in Murray Hill, New Jersey, performed experiments and observed that when two gold point contacts were applied to a crystal of germanium, a signal was produced with the output power greater than the input.[16] Solid State Physics Group leader William Shockley saw the potential in this, and over the next few months worked to greatly expand the knowledge of semiconductors. The term transistor was coined by John R. Pierce as a contraction of the term transresistance.[17][18][19] According to Lillian Hoddeson and Vicki Daitch, Shockley had proposed that Bell Labs' first patent for a transistor should be based on the field-effect and that he be named as the inventor. Having unearthed Lilienfeld's patents that went into obscurity years earlier, lawyers at Bell Labs advised against Shockley's proposal because the idea of a field-effect transistor that used an electric field as a "grid" was not new. Instead, what Bardeen, Brattain, and Shockley invented in 1947 was the first point-contact transistor.[14] In acknowledgement of this accomplishment, Shockley, Bardeen, and Brattain were jointly awarded the 1956 Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect".[20][21]

Shockley's research team initially attempted to build a field-effect transistor (FET), by trying to modulate the conductivity of a semiconductor, but was unsuccessful, mainly due to problems with the surface states, the dangling bond, and the germanium and copper compound materials. In the course of trying to understand the mysterious reasons behind their failure to build a working FET, this led them instead to invent the bipolar point-contact and junction transistors.[22][23]

Herbert Mataré in 1950. He independently invented a point-contact transistor in June 1948.

In 1948, the point-contact transistor was independently invented by German physicists Herbert Mataré and Heinrich Welker while working at the Compagnie des Freins et Signaux Westinghouse, a Westinghouse subsidiary located in Paris. Mataré had previous experience in developing crystal rectifiers from silicon and germanium in the German radar effort during World War II. Using this knowledge, he began researching the phenomenon of "interference" in 1947. By June 1948, witnessing currents flowing through point-contacts, Mataré produced consistent results using samples of germanium produced by Welker, similar to what Bardeen and Brattain had accomplished earlier in December 1947. Realizing that Bell Labs' scientists had already invented the transistor before them, the company rushed to get its "transition" into production for amplified use in France's telephone network and filed his first transistor patent application on August 13, 1948.[24][25][26]

The first bipolar junction transistors were invented by Bell Labs' William Shockley, which applied for patent (2,569,347) on June 26, 1948. On April 12, 1950, Bell Labs chemists Gordon Teal and Morgan Sparks had successfully produced a working bipolar NPN junction amplifying germanium transistor. Bell Labs had announced the discovery of this new "sandwich" transistor in a press release on July 4, 1951.[27][28]

Philco surface-barrier transistor developed and produced in 1953

The first high-frequency transistor was the surface-barrier germanium transistor developed by Philco in 1953, capable of operating at frequencies up to 60 MHz.[29] These were made by etching depressions into an n-type germanium base from both sides with jets of Indium(III) sulfate until it was a few ten-thousandths of an inch thick. Indium electroplated into the depressions formed the collector and emitter.[30][31]

AT&T first used transistors in telecommunications equipment in circuits of the No. 4A Toll Crossbar Switching System in 1953 for selecting trunk circuits from routing information encoded on translator cards.[32] The Western Electric No. 3A phototransistor read the mechanical encoding from punched metal cards.

The first "prototype" pocket transistor radio was shown by INTERMETALL (a company founded by Herbert Mataré in 1952) at the Internationale Funkausstellung Düsseldorf between August 29, 1953 and September 6, 1953.[33][34] The first "production" pocket transistor radio was the Regency TR-1, released in October 1954.[21] Produced as a joint venture between the Regency Division of Industrial Development Engineering Associates, I.D.E.A. and Texas Instruments of Dallas Texas, the TR-1 was manufactured in Indianapolis, Indiana. It was a near pocket-sized radio featuring 4 transistors and one germanium diode. The industrial design was outsourced to the Chicago firm of Painter, Teague and Petertil. It was initially released in one of six different colours: black, ivory, mandarin red, cloud grey, mahogany and olive green. Other colours were to shortly follow.[35][36][37]

The first "production" all-transistor car radio was developed by Chrysler and Philco corporations and it was announced in the April 28, 1955 edition of the Wall Street Journal. Chrysler had made the all-transistor car radio, Mopar model 914HR, available as an option starting in fall 1955 for its new line of 1956 Chrysler and Imperial cars which first hit the dealership showroom floors on October 21, 1955.[38][39][40]

The Sony TR-63, released in 1957, was the first mass-produced transistor radio, leading to the mass-market penetration of transistor radios.[41] The TR-63 went on to sell seven million units worldwide by the mid-1960s.[42] Sony's success with transistor radios led to transistors replacing vacuum tubes as the dominant electronic technology in the late 1950s.[43]

The first working silicon transistor was developed at Bell Labs on January 26, 1954, by Morris Tanenbaum. The first commercial silicon transistor was produced by Texas Instruments in 1954. This was the work of Gordon Teal, an expert in growing crystals of high purity, who had previously worked at Bell Labs.[44][45][46]

### Field effect transistors

 Main article: Field-effect transistor

The basic principle of the field-effect transistor (FET) was first proposed by Austrian physicist Julius Edgar Lilienfeld in 1926, when he filed a patent for a device similar to MESFET and in 1928 when he filed a patent for an insulated-gate field-effect transistor.[47][48] The FET concept was later also theorized by German engineer Oskar Heil in the 1930s and by William Shockley in the 1940s.

In 1945 JFET was patented by Heinrich Welker.[49] Following Shockley's theoretical treatment on JFET in 1952, a working practical JFET was made in 1953 by George C. Dacey and Ian M. Ross.[50]

In 1948 Bardeen patented the progenitor of MOSFET, an insulated-gate FET (IGFET) with an inversion layer. Bardeen's patent as well as the concept of an inversion layer forms the basis of CMOS technology today.[51]

### MOSFET (MOS transistor)

 Main article: MOSFET
Mohamed Atalla (left) and Dawon Kahng (right) invented the MOSFET (MOS transistor) at Bell Labs in 1959.

Semiconductor companies initially focused on junction transistors in the early years of the semiconductor industry. The junction transistor was a relatively bulky device that was difficult to mass-produce, which limited it to several specialized applications. Field-effect transistors (FETs) were theorized as potential alternatives to junction transistors, but researchers initially could not get FETs to work properly, largely due to the troublesome surface state barrier that prevented the external electric field from penetrating the material.[52]

In the 1950s, Egyptian engineer Mohamed Atalla investigated the surface properties of silicon semiconductors at Bell Labs, where he proposed a new method of semiconductor device fabrication, coating a silicon wafer with an insulating layer of silicon oxide so that electricity could reliably penetrate to the conducting silicon below, overcoming the surface states that prevented electricity from reaching the semiconducting layer. This is known as surface passivation, a method that became critical to the semiconductor industry as it later made possible the mass-production of silicon integrated circuits.[53][54] He presented his findings in 1957.[55] Building on his surface passivation method, he developed the metal–oxide–semiconductor (MOS) process.[53] He proposed the MOS process could be used to build the first working silicon FET, which he began working on building with the help of his Korean colleague Dawon Kahng.[53]

The metal–oxide–semiconductor field-effect transistor (MOSFET), or MOS transistor, was invented by Mohamed Atalla and Dawon Kahng in 1959.[4][5] The MOSFET was the first truly compact transistor that could be miniaturized and mass-produced for a wide range of uses.[52] In a self-aligned CMOS process, a transistor is formed wherever the gate layer (polysilicon or metal) crosses a diffusion layer.[56]: p.1 (see Fig. 1.1)  With its high scalability,[57] and much lower power consumption and higher density than bipolar junction transistors,[58] the MOSFET made it possible to build high-density integrated circuits,[6] allowing the integration of more than 10,000 transistors in a single IC.[59]

CMOS (complementary MOS) was invented by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963.[60] The first report of a floating-gate MOSFET was made by Dawon Kahng and Simon Sze in 1967.[61] A double-gate MOSFET was first demonstrated in 1984 by Electrotechnical Laboratory researchers Toshihiro Sekigawa and Yutaka Hayashi.[62][63] FinFET (fin field-effect transistor), a type of 3D non-planar multi-gate MOSFET, originated from the research of Digh Hisamoto and his team at Hitachi Central Research Laboratory in 1989.[64][65]

### GaN FET (GaN transistor)

 Main article: GaN FET

Gallium nitride (GaN) is a wide bandgap semiconductor used for high-efficiency field effect transistors. By growing a thin layer of aluminum gallium nitride (AlGaN) on top of a GaN crystal, a strain is created at the interface that induces a compensating two-dimensional electron gas[66] (2DEG) This 2DEG is used to efficiently conduct electrons when an electric field is applied across it. This 2DEG is highly conductive, in part due to the confinement of the electrons to a very small region at the interface. This confinement increases the mobility of electrons from about 1000 cm2/V·s in unstrained GaN to between 1500 and 2000 cm2/V·s in the 2DEG region. This high mobility produces GaN FETs that feature higher breakdown| strength, faster switching speed, higher thermal conductivity, and lower on-resistance than comparable silicon solutions[67].

## Importance

Transistors are the key active components in practically all modern electronics. Many thus consider the transistor to be one of the greatest inventions of the 20th century.[68]

The invention of the first transistor at Bell Labs was named an IEEE Milestone in 2009.[69] The list of IEEE Milestones also includes the inventions of the junction transistor in 1948 and the MOSFET in 1959.[70]

The MOSFET (metal–oxide–semiconductor field-effect transistor), also known as the MOS transistor, is by far the most widely used transistor, used in applications ranging from computers and electronics[54] to communications technology such as smartphones.[71] The MOSFET has been considered to be the most important transistor,[72] possibly the most important invention in electronics,[73] and the birth of modern electronics.[74] The MOS transistor has been the fundamental building block of modern digital electronics since the late 20th century, paving the way for the digital age.[75] The US Patent and Trademark Office calls it a "groundbreaking invention that transformed life and culture around the world".[71] Its importance in today's society rests on its ability to be mass-produced using a highly automated process (semiconductor device fabrication) that achieves astonishingly low per-transistor costs. MOSFETs are the most numerously produced artificial objects ever with more than 13 sextillion manufactured by 2018.[76]

Although several companies each produce over a billion individually packaged (known as discrete) MOS transistors every year,[77] the vast majority of transistors are now produced in integrated circuits (often shortened to IC, microchips or simply chips), along with diodes, resistors, capacitors and other electronic components, to produce complete electronic circuits. A logic gate consists of up to about twenty transistors whereas an advanced microprocessor, as of 2021, can use as many as 39 billion transistors (MOSFETs).[78]

The transistor's low cost, flexibility, and reliability have made it a ubiquitous device. Transistorized mechatronic circuits have replaced electromechanical devices in controlling appliances and machinery. It is often easier and cheaper to use a standard microcontroller and write a computer program to carry out a control function than to design an equivalent mechanical system to control that same function.

## Simplified operation

A Darlington transistor opened up so the actual transistor chip (the small square) can be seen inside. A Darlington transistor is effectively two transistors on the same chip. One transistor is much larger than the other, but both are large in comparison to transistors in large-scale integration because this particular example is intended for power applications.
A simple circuit diagram to show the labels of an n–p–n bipolar transistor

A transistor can use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals. This property is called gain. It can produce a stronger output signal, a voltage or current, which is proportional to a weaker input signal and thus, it can act as an amplifier. Alternatively, the transistor can be used to turn current on or off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements.[79]

There are two types of transistors, which have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing between the base and the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain.[80]

The image represents a typical bipolar transistor in a circuit. A charge will flow between emitter and collector terminals depending on the current in the base. Because internally the base and emitter connections behave like a semiconductor diode, a voltage drop develops between base and emitter while the base current exists. The amount of this voltage depends on the material the transistor is made from and is referred to as VBE.[80]

### Transistor as a switch

BJT used as an electronic switch, in grounded-emitter configuration

Transistors are commonly used in digital circuits as electronic switches which can be either in an "on" or "off" state, both for high-power applications such as switched-mode power supplies and for low-power applications such as logic gates. Important parameters for this application include the current switched, the voltage handled, and the switching speed, characterized by the rise and fall times.[80]

In a switching circuit, the goal is to simulate, as near as possible, the ideal switch having the properties of an open circuit when off, the short circuit when on, and an instantaneous transition between the two states. Parameters are chosen such that the "off" output is limited to leakage currents too small to affect connected circuitry, the resistance of the transistor in the "on" state is too small to affect circuitry, and the transition between the two states is fast enough not to have a detrimental effect.[80]

In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises, the emitter and collector currents rise exponentially. The collector voltage drops because of reduced resistance from the collector to the emitter. If the voltage difference between the collector and emitter were zero (or near zero), the collector current would be limited only by the load resistance (light bulb) and the supply voltage. This is called saturation because the current is flowing from collector to emitter freely. When saturated, the switch is said to be on.[81]

The use of bipolar transistors for switching applications requires biasing the transistor so that it operates between its cut-off region in the off-state and the saturation region (on). This requires sufficient base drive current. As the transistor provides current gain, it facilitates the switching of a relatively large current in the collector by a much smaller current into the base terminal. The ratio of these currents varies depending on the type of transistor, and even for a particular type, varies depending on the collector current. In the example of a light-switch circuit, as shown, the resistor is chosen to provide enough base current to ensure the transistor is saturated.[80] The base resistor value is calculated from the supply voltage, transistor C-E junction voltage drop, collector current, and amplification factor beta.[82]

### Transistor as an amplifier

Amplifier circuit, common-emitter configuration with a voltage-divider bias circuit

The common-emitter amplifier is designed so that a small change in voltage (Vin) changes the small current through the base of the transistor whose current amplification combined with the properties of the circuit means that small swings in Vin produce large changes in Vout.[80]

Various configurations of single transistor amplifiers are possible, with some providing current gain, some voltage gain, and some both.

From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing. 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.[80]

Modern transistor audio amplifiers of up to a few hundred watts are common and relatively inexpensive.

## Comparison with vacuum tubes

Before transistors were developed, vacuum (electron) 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 vacuum tubes in most applications are

• No cathode heater (which produces the characteristic orange glow of tubes), reducing power consumption, eliminating delay as tube heaters warm-up, and immune from cathode poisoning and depletion.
• Very small size and weight, reducing equipment size.
• Large numbers of extremely small transistors can be manufactured as a single integrated circuit.
• Low operating voltages compatible with batteries of only a few cells.
• Circuits with greater energy efficiency are usually possible. For low-power applications (for example, voltage amplification) in particular, energy consumption can be very much less than for tubes.
• Complementary devices available, providing design flexibility including complementary-symmetry circuits, not possible with vacuum tubes.
• Very low sensitivity to mechanical shock and vibration, providing physical ruggedness and virtually eliminating shock-induced spurious signals (for example, microphonics in audio applications).
• Not susceptible to breakage of a glass envelope, leakage, outgassing, and other physical damage.

### Limitations

Transistors may have the following limitations:

• They lack the higher electron mobility afforded by the vacuum of vacuum tubes, which is desirable for high-power, high-frequency operation — such as that used in some over-the-air television transmitters and in travelling wave tubes used as amplifiers in some satellites
• Transistors and other solid-state devices are susceptible to damage from very brief electrical and thermal events, including electrostatic discharge in handling. Vacuum tubes are electrically much more rugged.
• They are sensitive to radiation and cosmic rays (special radiation-hardened chips are used for spacecraft devices).
• In audio applications, transistors lack the lower-harmonic distortion — the so-called tube sound — which is characteristic of vacuum tubes, and is preferred by some.[83]

## Types

### Classification

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 PNP P-channel NPN N-channel BJT JFET
BJT and JFET symbols
 P-channel N-channel MOSFET enh MOSFET dep
MOSFET symbols

Transistors are categorized by

Hence, a particular transistor may be described as silicon, surface-mount, BJT, NPN, low-power, high-frequency switch.

### Mnemonics

Convenient mnemonic to remember the type of transistor (represented by a electrical symbol) involves the direction of the arrow. For the BJT, on an n-p-n transistor symbol, the arrow will "Not Point iN". On a p-n-p transistor symbol, the arrow "Points iN Proudly". This however does not apply to MOSFET-based transistor symbols as the arrow is typically reversed (i.e. the arrow for the n-p-n points inside).

### Field-effect transistor (FET)

 Main article: Field-effect transistor
Operation of a FET and its Id-Vg curve. At first, when no gate voltage is applied, there are no inversion electrons in the channel, so the device is turned off. As gate voltage increases, the inversion electron density in the channel increases, current increases, and thus the device turns on.

The field-effect transistor, sometimes called a unipolar transistor, uses either electrons (in n-channel FET) or holes (in p-channel FET) for conduction. The four terminals of the FET are named source, gate, drain, and body (substrate). On most FETs, the body is connected to the source inside the package, and this will be assumed for the following description.

In a FET, the drain-to-source current flows via a conducting channel that connects the source region to the drain region. The conductivity is varied by the electric field that is produced when a voltage is applied between the gate and source terminals, hence the current flowing between the drain and source is controlled by the voltage applied between the gate and source. As the gate–source voltage (VGS) is increased, the drain–source current (IDS) increases exponentially for VGS below threshold, and then at a roughly quadratic rate: (IDS ∝ (VGSVT)2, where VT is the threshold voltage at which drain current begins)[86] in the "space-charge-limited" region above threshold. A quadratic behavior is not observed in modern devices, for example, at the 65 nm technology node.[87]

For low noise at narrow bandwidth, the higher input resistance of the FET is advantageous.

FETs are divided into two families: junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more commonly known as a metal–oxide–semiconductor FET (MOSFET), reflecting its original construction from layers of metal (the gate), oxide (the insulation), and semiconductor. Unlike IGFETs, the JFET gate forms a p–n diode with the channel which lies between the source and drains. 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.

Metal–semiconductor FETs (MESFETs) are JFETs in which the reverse biased p–n junction is replaced by a metal–semiconductor junction. These, and the HEMTs (high-electron-mobility transistors, or HFETs), in which a two-dimensional electron gas with very high carrier mobility is used for charge transport, are especially suitable for use at very high frequencies (several GHz).

FETs are further divided into depletion-mode and enhancement-mode types, depending on whether the channel is turned on or off with zero gate-to-source voltage. For enhancement mode, the channel is off at zero bias, and a gate potential can "enhance" the conduction. For the depletion mode, the channel is on at zero bias, and a gate potential (of the opposite polarity) can "deplete" the channel, reducing conduction. For either mode, a more positive gate voltage corresponds to a higher current for n-channel devices and a lower current for p-channel devices. Nearly all JFETs are depletion-mode because the diode junctions would forward bias and conduct if they were enhancement-mode devices, while most IGFETs are enhancement-mode types.

#### Metal–oxide–semiconductor FET (MOSFET)

 Main article: MOSFET

The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS),[6] is a type of field-effect transistor that is fabricated by the controlled oxidation of a semiconductor, typically silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The MOSFET is by far the most common transistor, and the basic building block of most modern electronics.[75] The MOSFET accounts for 99.9% of all transistors in the world.[88]

### Bipolar junction transistor (BJT)

 Main article: Bipolar junction transistor

Bipolar transistors are so named because they conduct by using both majority and minority carriers. The bipolar junction transistor, the first type of transistor to be mass-produced, is a combination of two junction diodes and is formed of either a thin layer of p-type semiconductor sandwiched between two n-type semiconductors (an n–p–n transistor), or a thin layer of n-type semiconductor sandwiched between two p-type semiconductors (a p–n–p transistor). This construction produces two p–n junctions: a base-emitter junction and a base-collector junction, separated by a thin region of semiconductor known as the base region. (Two junction diodes wired together without sharing an intervening semiconducting region will not make a transistor).

BJTs have three terminals, corresponding to the three layers of semiconductor—an emitter, a base, and a collector. They are useful in amplifiers because the currents at the emitter and collector are controllable by a relatively small base current.[89] In an n–p–n transistor operating in the active region, the emitter-base junction is forward biased (electrons and holes recombine at the junction), and the base-collector junction is reverse biased (electrons and holes are formed at, and move away from the junction), and electrons are injected into the base region. Because the base is narrow, most of these electrons will diffuse into the reverse-biased base-collector junction and be swept into the collector; perhaps one-hundredth of the electrons will recombine in the base, which is the dominant mechanism in the base current. As well, as the base is lightly doped (in comparison to the emitter and collector regions), recombination rates are low, permitting more carriers to diffuse across the base region. By controlling the number of electrons that can leave the base, the number of electrons entering the collector can be controlled.[89] Collector current is approximately β (common-emitter current gain) times the base current. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications.

Unlike the field-effect transistor (see below), the BJT is a low-input-impedance device. Also, as the base-emitter voltage (VBE) is increased the base-emitter current and hence the collector-emitter current (ICE) increase exponentially according to the Shockley diode model and the Ebers-Moll model. Because of this exponential relationship, the BJT has a higher transconductance than the FET.

Bipolar transistors can be made to conduct by exposure to light because the absorption of photons in the base region generates a photocurrent that acts as a base current; the collector current is approximately β times the photocurrent. Devices designed for this purpose have a transparent window in the package and are called phototransistors.

### Usage of MOSFETs and BJTs

The MOSFET is by far the most widely used transistor for both digital circuits as well as analog circuits,[90] accounting for 99.9% of all transistors in the world.[88] The bipolar junction transistor (BJT) was previously the most commonly used transistor during the 1950s to 1960s. Even after MOSFETs became widely available in the 1970s, the BJT remained the transistor of choice for many analog circuits such as amplifiers because of their greater linearity, up until MOSFET devices (such as power MOSFETs, LDMOS and RF CMOS) replaced them for most power electronic applications in the 1980s. In integrated circuits, the desirable properties of MOSFETs allowed them to capture nearly all market share for digital circuits in the 1970s. Discrete MOSFETs (typically power MOSFETs) can be applied in transistor applications, including analog circuits, voltage regulators, amplifiers, power transmitters, and motor drivers.

### Other transistor types

Transistor symbol created on Portuguese pavement in the University of Aveiro
 For early bipolar transistors, see Bipolar junction transistor § Bipolar transistors.

## Device identification

Three major identification standards are used for designating transistor devices. In each, the alphanumeric prefix provides clues to the type of the device.

### Joint Electron Device Engineering Council (JEDEC)

The JEDEC part numbering scheme evolved in the 1960s in the United States. The JEDEC EIA-370 transistor device numbers usually start with 2N, indicating a three-terminal device. Dual-gate field-effect transistors are four-terminal devices, and begin with 3N. The prefix is followed by a two-, three- or four-digit number with no significance as to device properties, although early devices with low numbers tend to be germanium devices. For example, 2N3055 is a silicon n–p–n power transistor, 2N1301 is a p–n–p germanium switching transistor. A letter suffix, such as "A", is sometimes used to indicate a newer variant, but rarely gain groupings.

JEDEC prefix table
Prefix Type and usage
1N two-terminal device, such as diodes
2N three-terminal device, such as transistors or single-gate field-effect transistors
3N four-terminal device, such as dual-gate field-effect transistors

### Japanese Industrial Standard (JIS)

In Japan, the JIS semiconductor designation (|JIS-C-7012), labels transistor devices starting with 2S,[98] e.g., 2SD965, but sometimes the "2S" prefix is not marked on the package–a 2SD965 might only be marked D965 and a 2SC1815 might be listed by a supplier as simply C1815. This series sometimes has suffixes, such as R, O, BL, standing for red, orange, blue, etc., to denote variants, such as tighter hFE (gain) groupings.

JIS transistor prefix table
Prefix Type and usage
2SA high-frequency p–n–p BJT
2SB audio-frequency p–n–p BJT
2SC high-frequency n–p–n BJT
2SD audio-frequency n–p–n BJT
2SJ P-channel FET (both JFET and MOSFET)
2SK N-channel FET (both JFET and MOSFET)

### European Electronic Component Manufacturers Association (EECA)

The European Electronic Component Manufacturers Association (EECA) uses a numbering scheme that was inherited from Pro Electron when it merged with EECA in 1983. This scheme begins with two letters: the first gives the semiconductor type (A for germanium, B for silicon, and C for materials like GaAs); the second letter denotes the intended use (A for diode, C for general-purpose transistor, etc.). A three-digit sequence number (or one letter and two digits, for industrial types) follows. With early devices this indicated the case type. Suffixes may be used, with a letter (e.g. "C" often means high hFE, such as in: BC549C[99]) or other codes may follow to show gain (e.g. BC327-25) or voltage rating (e.g. BUK854-800A[100]). The more common prefixes are:

EECA transistor prefix table
Prefix Type and usage Example Equivalent Reference
AC Germanium, small-signal AF transistor AC126 NTE102A
AF Germanium, small-signal RF transistor AF117 NTE160
AL Germanium, RF power transistor ALZ10 NTE100
AS Germanium, switching transistor ASY28 NTE101
AU Germanium, power switching transistor AU103 NTE127
BC Silicon, small-signal transistor ("general purpose") BC548 2N3904 Datasheet
BD Silicon, power transistor BD139 NTE375 Datasheet
BF Silicon, RF (high frequency) BJT or FET BF245 NTE133 Datasheet
BS Silicon, switching transistor (BJT or MOSFET) BS170 2N7000 Datasheet
BL Silicon, high frequency, high power (for transmitters) BLW60 NTE325 Datasheet
BU Silicon, high voltage (for CRT horizontal deflection circuits) BU2520A NTE2354 Datasheet
CF Gallium arsenide, small-signal microwave transistor (MESFET CF739 Datasheet
CL Gallium arsenide, microwave power transistor (FET) CLY10 Datasheet

### Proprietary

Manufacturers of devices may have their proprietary numbering system, for example CK722. Since devices are second-sourced, a manufacturer's prefix (like "MPF" in MPF102, which originally would denote a Motorola FET) now is an unreliable indicator of who made the device. Some proprietary naming schemes adopt parts of other naming schemes, for example, a PN2222A is a (possibly Fairchild Semiconductor) 2N2222A in a plastic case (but a PN108 is a plastic version of a BC108, not a 2N108, while the PN100 is unrelated to other xx100 devices).

Military part numbers sometimes are assigned their codes, such as the British Military CV Naming System.

Manufacturers buying large numbers of similar parts may have them supplied with "house numbers", identifying a particular purchasing specification and not necessarily a device with a standardized registered number. For example, an HP part 1854,0053 is a (JEDEC) 2N2218 transistor[101][102] which is also assigned the CV number: CV7763[103]

### Naming problems

With so many independent naming schemes, and the abbreviation of part numbers when printed on the devices, ambiguity sometimes occurs. For example, two different devices may be marked "J176" (one the J176 low-power JFET, the other the higher-powered MOSFET 2SJ176).

As older "through-hole" transistors are given surface-mount packaged counterparts, they tend to be assigned many different part numbers because manufacturers have their systems to cope with the variety in pinout arrangements and options for dual or matched n–p–n + p–n–p devices in one pack. So even when the original device (such as a 2N3904) may have been assigned by a standards authority, and well known by engineers over the years, the new versions are far from standardized in their naming.

## Construction

### Semiconductor material

Semiconductor material characteristics
Semiconductor
material
Junction forward
voltage @ 25 °C, V
Electron mobility
@ 25 °C, m2/(V·s)
Hole mobility
@ 25 °C, m2/(V·s)
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 first BJTs were made from germanium (Ge). 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). Single element semiconductor material (Ge and Si) is described as elemental.

Rough parameters for the most common semiconductor materials used to make transistors are given in the adjacent table. These parameters will vary with an increase in temperature, electric field, impurity level, strain, and sundry other factors.

The junction forward voltage is the voltage applied to the emitter-base junction of a BJT to make the base conduct a specified current. The current increases exponentially as the junction forward voltage is increased. 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 an increase in temperature. For a typical silicon junction, the change is −2.1 mV/°C.[104] In some circuits special compensating elements (sensistors) must be used to compensate for such changes.

The density of mobile carriers in the channel of a MOSFET is a function of the electric field forming the channel and of various other phenomena such as the impurity level in the channel. Some impurities, called dopants, are introduced deliberately in making a MOSFET, to control the MOSFET electrical behavior.

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 can operate. 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:

1. Its maximum temperature is limited.
2. It has relatively high leakage current.
3. It cannot withstand high voltages.
4. 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 n–p–n transistor tends to be swifter than an equivalent p–n–p transistor. GaAs has the highest electron mobility of the three semiconductors. It is for this reason that GaAs is used in high-frequency applications. A relatively recent[when?] 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 twice 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 frequencies around 12 GHz. HEMTs based on gallium nitride and aluminum gallium nitride (AlGaN/GaN HEMTs) provide still higher electron mobility and are being developed for various applications.

Maximum junction temperature values represent a cross-section taken from various manufacturers' datasheets. This temperature should not be exceeded or the transistor may be damaged.

Al–Si junction refers to the high-speed (aluminum-silicon) metal–semiconductor barrier diode, commonly known as a Schottky diode. This is included in the table because some silicon power IGFETs have a parasitic reverse Schottky diode formed between the source and drain as part of the fabrication process. This diode can be a nuisance, but sometimes it is used in the circuit.

Material properties of GaN, 4H-SiC, and Si[105] This table identifies five key electrical properties of three semiconductor materials contending for the power management market.
Parameter Silicon Gallium Nitride (GaN) Silicon Carbide (SiC)
Band Gap Eg eV 1.12 3.39 3.26
Critical Field Ecrit MV/cm 0.23 3.3 2.2
Electron Mobility µn cm2/V•s 1400 1500 950
Permittivity ${\displaystyle \varepsilon }$r 11.8 9 9.7
Thermal Conductivity ${\displaystyle \lambda }$ W/cm•K 1.5 1.3 3.8

Band Gap (Eg)

The band gap of a semiconductor is related to the strength of the chemical bonds between the atoms in the lattice. These stronger bonds mean that it is harder for an electron to jump from one site to the next. Among the many consequences are lower intrinsic leakage currents and higher operating temperatures for higher band gap semiconductors. Based on the data in Table 1.1, GaN and SiC both have higher band gaps than silicon.

Critical Field (Ecrit)

The stronger chemical bonds that cause the wider band gap also result in a higher critical electric field needed to initiate impact ionization, which results in avalanche breakdown. The voltage at which a device breaks down can be approximated with the formula:

${\displaystyle V_{BR}={\frac {1}{2))w_{drift}\cdot E_{crit))$

(1.1)

The breakdown voltage of a device (VBR), therefore, is proportional to the width of the drift region (wdrift). In the case of SiC and GaN, the drift region can be 10 times smaller than in silicon for the same breakdown voltage. In order to support this electric field, there need to be carriers in the drift region that are depleted away at the point where the device reaches the critical field. This is where there is a huge gain in devices with high critical fields. The number of electrons (assuming an N-type semiconductor) between two terminals can be calculated using Poison’s equation:

${\displaystyle q\cdot N_{D}=\varepsilon _{0}\cdot \varepsilon _{\mathrm {r} }\cdot E_{crit}/w_{drift))$

(1.2)

In this equation q is the charge of the electron (1.6 · 10-19 Coulombs), ND is the total number of electrons in the volume, ɛo is the permittivity of a vacuum measured in Farads per meter (8.854· 10- 12 F/m). ɛr is the relative permittivity of the crystal compared to a vacuum. In its simplest form under DC conditions, permittivity is the dielectric constant of the crystal. Referring to equation (1.2), it can be seen that if the critical field of the crystal is 10 times higher, and from equation (1.1), the electrical terminals can be 10 times closer together. Therefore, the number of electrons, ND, in the drift region can be 100 times greater, but only have one-tenth the distance to travel. This is the basis for the ability of GaN and SiC to outperform silicon in power conversion.

On-Resistance (RDS(on))

Theoretical on-resistance for a one square millimeter device vs. blocking voltage capability for Si, SiC, and GaN based power devices

The theoretical on-resistance of a one square millimeter majority-carrier device (measured in ohms (Ω-mm2)) is therefore

${\displaystyle R_{DS(on)}=w_{drift}/q\cdot \mu _{n}\cdot N_{D))$

(1.3)

Where µn is the mobility of electrons. Combining equations (1), (2), and (3) produces the following relationship between breakdown voltage and on-resistance:

${\displaystyle R_{DS(on)}=4\cdot V_{BR}^{2}/\varepsilon _{0}\cdot \varepsilon _{\mathrm {r} }\mu _{\mathrm {r} }\cdot E_{crit}^{3))$

(1.4)

This equation can now be plotted as shown in the figure below for Si, SiC, and GaN. This plot is for an ideal structure. Real semiconductors are not always ideal structures and, therefore, it is always a challenge to achieve the theoretical limit. In the case of silicon MOSFETs, it took 30 years.

### Packaging

Assorted discrete transistors
Soviet KT315b transistors

Discrete transistors can be individually packaged transistors or unpackaged transistor chips (dies).

Transistors come in many different semiconductor packages (see image). The two main categories are through-hole (or leaded), and surface-mount, also known as surface-mount device (SMD). The ball grid array (BGA) is the latest surface-mount package. It has solder "balls" on the underside in place of leads. Because they are smaller and have shorter interconnections, SMDs have better high-frequency characteristics but lower power ratings.

Transistor packages are made of glass, metal, ceramic, or plastic. The package often dictates the power rating and frequency characteristics. Power transistors have larger packages that can be clamped to heat sinks for enhanced cooling. Additionally, most power transistors have the collector or drain physically connected to the metal enclosure. At the other extreme, some surface-mount microwave transistors are as small as grains of sand.

Often a given transistor type is available in several packages. Transistor packages are mainly standardized, but the assignment of a transistor's functions to the terminals is not: other transistor types can assign other functions to the package's terminals. Even for the same transistor type the terminal assignment can vary (normally indicated by a suffix letter to the part number, q.e. BC212L and BC212K).

Nowadays most transistors come in a wide range of SMT packages, in comparison, the list of available through-hole packages is relatively small, here is a shortlist of the most common through-hole transistors packages in alphabetical order: ATV, E-line, MRT, HRT, SC-43, SC-72, TO-3, TO-18, TO-39, TO-92, TO-126, TO220, TO247, TO251, TO262, ZTX851.

Unpackaged transistor chips (die) may be assembled into hybrid devices.[106] The IBM SLT module of the 1960s is one example of such a hybrid circuit module using glass passivated transistor (and diode) die. Other packaging techniques for discrete transistors as chips include direct chip attach (DCA) and chip-on-board (COB).[106]

#### Flexible transistors

Researchers have made several kinds of flexible transistors, including organic field-effect transistors.[107][108][109] Flexible transistors are useful in some kinds of flexible displays and other flexible electronics.

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