transistor

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transistor,

three-terminal, solid-state electronic device used for amplification and switching. It is the solid-state analog to the triode electron tubeelectron tube,
device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube).
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; the transistor has replaced the electron tube for virtually all common applications.

Types of Transistors

The transistor is an arrangement of semiconductorsemiconductor,
solid material whose electrical conductivity at room temperature is between that of a conductor and that of an insulator (see conduction; insulation). At high temperatures its conductivity approaches that of a metal, and at low temperatures it acts as an insulator.
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 materials that share common physical boundaries. Materials most commonly used are silicon, gallium-arsenide, and germanium, into which impurities have been introduced by a process called "doping." In n-type semiconductors the impurities or dopants result in an excess of electrons, or negative charges; in p-type semiconductors the dopants lead to a deficiency of electrons and therefore an excess of positive charge carriers or "holes."

The Junction Transistor

The n-p-n junction transistor consists of two n-type semiconductors (called the emitter and collector) separated by a thin layer of p-type semiconductor (called the base). The transistor action is such that if the electric potentials on the segments are properly determined, a small current between the base and emitter connections results in a large current between the emitter and collector connections, thus producing current amplification. Some circuits are designed to use the transistor as a switching device; current in the base-emitter junction creates a low-resistance path between the collector and emitter. The p-n-p junction transistor, consisting of a thin layer of n-type semiconductor lying between two p-type semiconductors, works in the same manner, except that all polarities are reversed.

The Field-Effect Transistor

A very important type of transistor developed after the junction transistor is the field-effect transistor (FET). It draws virtually no power from an input signal, overcoming a major disadvantage of the junction transistor. An n-channel FET consists of a bar (channel) of n-type semiconductor material that passes between and makes contact with two small regions of p-type material near its center. The terminals attached to the ends of the channel are called the source and the drain; those attached to the two p-type regions are called gates. A voltage applied to the gates is directed so that no current exists across the junctions between the p- and n-type materials; for this reason it is called a reverse voltage. Variations of the magnitude of the reverse voltage cause variations in the resistance of the channel, enabling the reverse voltage to control the current in the channel. A p-channel device works the same way but with all polarities reversed.

The metal-oxide semiconductor field-effect transistor (MOSFET) is a variant in which a single gate is separated from the channel by a layer of metal oxide, which acts as an insulator, or dielectric. The electric field of the gate extends through the dielectric and controls the resistance of the channel. In this device the input signal, which is applied to the gate, can increase the current through the channel as well as decrease it.

Invention and Uses of the Transistor

The invention of the transistor by American physicists John Bardeen, Walter H. Brattain, and William Shockley, later jointly awarded a Nobel Prize, was announced by the Bell Telephone Laboratories in 1948; it was also independently developed nearly simultaneously by Herbert Mataré and Heinrich Welker, German physicists working at Westinghouse Laboratory in Paris. Since then many types have been designed. Transistors are very durable, are very small, have a high resistance to physical shock, and are very inexpensive. At one time, only discrete devices existed; they were usually sealed in ceramic, with a wire extending from each segment to the outside, where it could be connected to an electric circuit. The vast majority of transistors now are built as parts of integrated circuitsintegrated circuit
(IC), electronic circuit built on a semiconductor substrate, usually one of single-crystal silicon. The circuit, often called a chip, is packaged in a hermetically sealed case or a nonhermetic plastic capsule, with leads extending from it for input, output,
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. Transistors are used in virtually all electronic devices, including radio and television receivers, computers, and space vehicles and guided missiles.

See microelectronicsmicroelectronics,
branch of electronic technology devoted to the design and development of extremely small electronic devices that consume very little electric power. Although the term is sometimes used to describe discrete electronic components assembled in an extremely small
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.

Transistor

A solid-state device involved in amplifying small electrical signals and in processing of digital information. Transistors act as the key element in amplification, detection, and switching of electrical voltages and currents. They are the active electronic component in all electronic systems which convert battery power to signal power. Almost every type of transistor is produced in some form of semiconductor, often single-crystal materials, with silicon being the most prevalent. There are several different types of transistors, classified by how the internal mobile charges (electrons and holes) function. The main categories are bipolar junction transistors (BJTs) and field-effect transistors (FETs).

Single-crystal semiconductors, such as silicon from column 14 of the periodic table of chemical elements, can be produced with two different conduction species, majority and minority carriers. When made with, for example, 1 part per million of phosphorus (from column 15), the silicon is called n-type because it adds conduction electrons (negative charge) to form the majority carrier. When doped with boron (from column 13), it is called p-type because it has added positive mobile carriers called holes. For n-type doping, electrons are the majority carrier while holes become the minority carrier. For p-type doping holes are in larger numbers, hence they are the majority carriers, while electrons are the minority carriers. All transistors are made up of regions of n-type and p-type semiconducting material. See Semiconductor, Single crystal

The bipolar transistor has two conducting species, electrons and holes. Field-effect transistors can be called unipolar because their main conduction is by one carrier type, the majority carrier. Therefore, field-effect transistors are either n-channel (majority electrons) or p-channel (majority holes). For the bipolar transistor, there are two forms, n+pn and p+np, depending on which carrier is majority and which is the minority in a given region. As a result the bipolar transistor conducts by majority as well as by minority carriers. The n+pn version is by far the most used as it has several distinct performance advantages, as does the n-channel for the field-effect transistors. (The n+ indicates that the region is more heavily doped than the other two regions.)

Bipolar transistors

Bipolar transistors have additional categories: the homojunction for one type of semiconductor (all silicon), and heterojunction for more than one (particularly silicon and silicon-germanium, Si/Si1-xGex/Si). At present the silicon homojunction, usually called the BJT, is by far the most common. However, the highest performance (frequency and speed) is a result of the heterojunction bipolar transistor (HBT).

Bipolar transistors are manufactured in several different forms, each appropriate for a particular application. They are used at high frequencies, for switching circuits, in high-power applications, and under extreme environmental stress. The bipolar junction transistor may appear in discrete form as an individually encapsulated component, in monolithic form (made in and from a common material) in integrated circuits, or as a so-called chip in a thick-film or thin-film hybrid integrated circuit. In the pn-junction isolated integrated-circuit n+pn bipolar transistor, an n+ subcollector, or buried layer, serves as a low-resistance contact which is made on the top surface (Fig. 1).

Isolated n + pn bipolar junction transistor for integrated-circuit operationenlarge picture
Isolated n+pn bipolar junction transistor for integrated-circuit operation

Field-effect transistors

Majority-carrier field-effect transistors are classified as metal-oxide-semiconductor field-effect transistor (MOSFET), junction “gate” field-effect transistor (JFET), and metal “gate” on semiconductor field-effect transistor (MESFET) devices. MOSFETs are the most used in almost all computers and system applications. However, the MESFET has high-frequency applications in gallium arsenide (GaAs), and the silicon JFET has low-electrical noise performance for audio components and instruments. In general, the n-channel field-effect transistors are preferred because of larger electron mobilities, which translate into higher speed and frequency of operation.

An n-channel MOSFET (Fig. 2) has a so-called source, which supplies electrons to the channel. These electrons travel through the channel and are removed by a drain electrode into the external circuit. A gate electrode is used to produce the channel or to remove the channel; hence it acts like a gate for the electrons, either providing a channel for them to flow from the source to the drain or blocking their flow (no channel). With a large enough voltage on the gate, the channel is formed, while at a low gate voltage it is not formed and blocks the electron flow to the drain. This type of MOSFET is called enhancement mode because the gate must have sufficiently large voltages to create a channel through which the electrons can flow. Another way of saying the same idea is that the device is normally “off” in an nonconducting state until the gate enhances the channel.

An n -channel enhancement-mode metal-oxide-semiconductor field-effect transistor (MOSFET)enlarge picture
An n-channel enhancement-mode metal-oxide-semiconductor field-effect transistor (MOSFET)

In the JFET (Fig. 3), a conducting majority-carrier n channel exists between the source and drain. When a negative voltage is applied to the p+ gate, the depletion regions widen with reverse bias and begin to restrict the flow of electrons between the source and drain. At a large enough negative gate voltage (symbolized VP), the channel pinches off.

An n -channel junction field-effect transistor (JFET)enlarge picture
An n-channel junction field-effect transistor (JFET)

The MESFET is quite similar to the JFET in its mode of operation. A conduction channel is reduced and finally pinched off by a metal Schottky barrier placed directly on the semiconductor. Metal on gallium arsenide is extensively used for high-frequency communications because of the large mobility of electrons, good gain, and low noise characteristics. Its cross section is similar to that of the JFET (Fig. 3), with a metal used as the gate.

Transistor

 

an electronic device that is based on a semiconductor crystal, has three or more electrodes, and is used to generate or convert electrical oscillations. The transistor was invented in 1948 by W. Shockley, W. Brattain, and J. Bardeen (Nobel Prize winners, 1956). There are two major classes of transistors: unipolar and bipolar.

In unipolar transistors the flow of current through the crystal is due only to charge carriers of one polarity, either electrons or holes (seeSEMICONDUCTOR). Such transistors are discussed in FIELD-EFFECT TRANSISTOR.

In bipolar transistors the current flowing through the crystal is due to the motion of charge carriers of both polarities. Such a transistor is (Figure 1) a single-crystal semiconductor wafer in which three regions having either hole (p) or electron (n) conductivity are produced by means of special fabrication processes. According to the order in which the regions alternate, we distinguish between p-n-p and n-p-n transistors. The middle region, which is generally made very thin (of the order of several micrometers), is called the base; the other two regions are known as the emitter and the collector. The base is separated from the emitter and the collector by p-n junctions: the emitter-base junction (EJ) and the collector-base junction (CJ). Metallic leads are connected to the base, the emitter, and the collector.

Let us consider the physical processes that occur, for example, in an n-p-n transistor (Figure l,a). A voltage Ube is applied to the emitter junction. This voltage lowers the potential barrier of the junction and thus reduces the junction’s electrical resistance; that is, the EJ is biased in the low-resistance, or forward, direction. A voltage Ucb also is applied to the collector junction. This voltage raises the potential barrier of that junction and increases the junction’s resistance; that is, the CJ is biased in the high-resistance, or reverse, direction. The voltage Ube causes a current ie to flow through the EJ. This current results mainly from the transport, or injection, of electrons from the emitter to the base. Upon penetrating the base and reaching the CJ region, the electrons are captured by the CJ field and drawn into the collector. A collector current ic then flows through the CJ.

Not all the injected electrons, however, reach the CJ; some of them recombine along the way with the majority carriers in the base, that is, with the holes (the smaller the thickness of the base and the hole concentration in the base, the smaller the number of recombined electrons). Since under steady-state conditions the number of holes in the base is constant, the recombination means that some of the electrons migrate from the base to the EJ circuit, thereby producing a base current ib. Thus, ie = ic + ib. Usually ibic, so that icie and Δic ≈ Δie. The quantity α = Δicie is called the current gain, or current transfer ratio, and is a function of the thickness of the base and the parameters of the semiconductor material of the base. For most transistors the current gain is close to unity. Any change in Ube produces a change in ie—in conformity with the current-voltage characteristic of the p-n junction—and, consequently, a change in ic. The resistance of the CJ is high; therefore, the load resistance RL in the CJ circuit can be made sufficiently high so that Δic causes a substantial change in the collector voltage. As a result, electric signals with a power many times greater than the power consumed in the EJ circuit can be obtained at RL. Similar physical processes occur in a p-n-p transistor (Figure l,b), but the electrons and holes in this case switch roles, and the polarities of the applied voltages must be reversed. In symmetrical transistors the emitter can be made to function as the collector and vice versa merely by changing the polarity of the corresponding voltages.

According to the mechanism by which the minority carriers are transported across the base, a distinction is made between diffusion transistors and drift transistors. In diffusion transistors there is no accelerating electric field in the base, and charges are transported from the emitter to the collector by diffusion; in drift transistors two charge-transport mechanisms—diffusion and drift in an electric field—operate simultaneously in the base. According to their electrical characteristics and areas of application, transistors are classified as low-power low-noise transistors (used in the input circuits of radio amplifiers), pulse transistors (used in electronic pulse-forming systems), power transistors (used in radio transmitters), switching transistors (used as electronic switches in automatic control systems), phototransistors (used in devices for converting light signals into amplified electric signals), and special-purpose transistors. A distinction also is made among low-frequency

Figure 1. Schematics of amplifier circuits using n-p-n and p-n-p transistors and the schematic symbols for the two transistors: (a) schematic of an amplifier circuit using an n-p-n transistor, (b) schematic of an amplifier circuit using a p-n-p transistor, (c) schematic symbol for an n-p-n transistor, (d) schematic symbol for a p-n-p transistor; (E) emitter, (B) base, (C) collector, (RL) load, (U) supply source voltage, (i) current. The arrows indicate the direction of the electron motion, which is opposite to the direction of the current.

transistors (mainly for operation in the acoustic and ultrasonic frequency ranges), high-frequency transistors (up to 300 megahertz), and microwave transistors (above 300 megahertz).

Germanium and silicon are mainly used as the semiconductor materials for the fabrication of transistors. According to the technology used to obtain regions with different types of conductivity in the crystal (seeSEMICONDUCTOR ELECTRONICS), the following types of transistors are distinguished: alloy, diffused, inversion, diffused-alloy, mesa, epitaxial, planar (seePLANAR PROCESS), and planar-epitaxial. Transistors are also subdivided into those having hermetically sealed metal-glass, cermet, or plastic casings and those without casings; casingless transistors have a temporary shield (for example, a thin layer of lacquer, resin, or low-melting glass) to protect the crystal from environmental effects, and the device that contains the transistors is hermetically sealed. Silicon planar and planar-epitaxial transistors have become the most widely used types.

The invention of the transistor made possible the miniaturization of electronic equipment on the basis of the advances in rapidly developing semiconductor electronics. Compared with first-generation electronic equipment, which is based on electron tubes, second-generation electronic equipment for the same purposes, which is based on such semiconductor devices as transistors, is one-tenth to one-hundredth the size and weight, has better reliability, and requires considerably less electric power. The semiconductor component in a present-day transistor is extremely small; even in the most powerful transistors the area of the crystal is not more than several square millimeters. The operational reliability of a transistor, which is determined from the statistical mean time to failure, is typically ~105 hours and reaches 106 hours in certain cases. Unlike electron tubes, transistors can operate with low-voltage power supplies (down to several tenths of a volt); the current required in this case may be as small as a few microamperes. High-power transistors operate at voltages of 10–30 volts and currents of up to several tens of amperes and deliver up to 100 or more watts to a load.

Transistors can be used to amplify signals with frequencies of up to 10 gigahertz; this upper limit corresponds to a wavelength of 3 cm. With respect to noise characteristics in the low-frequency range, transistors compete successfully with low-noise electrometer tubes. In the frequency range up to 1 gigahertz, transistors have a noise factor of not more than 1.5–3.0 decibels. At higher frequencies the noise factor increases, reaching 6–10 decibels at frequencies of 6–10 gigahertz.

The transistor is the fundamental element in present-day microelectronic devices. Advances in the planar process have made it possible to produce, on a single semiconductor crystal with an area of 30–35 mm2, electronic devices in which there are up to several tens of thousands of transistors. Such devices, which are called integrated circuits (IC’s), form the basis of third-generation electronic equipment. Examples of such equipment include electronic wristwatches, which contain 600 to 1,500 transistors, and pocket calculators, which contain several thousand transistors. The shift to the use of IC’s has established a new trend in the design and production of small-sized and reliable electronic equipment: microelectronics. The advantages of transistors, together with advances in production technology, have made it possible to develop computers containing up to several hundred thousand elements, to install complex electronic equipment in aircraft and space vehicles, and to fabricate miniaturized electronic equipment for use, for example, in diverse fields of industry, in medicine, and in the home. As with other semiconductor devices, transistors have both advantages and a number of disadvantages; the primary drawback of transistors is their limited range of operating temperatures. Thus, germanium transistors operate at temperatures not higher than 100°C, and silicon transistors are restricted to temperatures not higher than 200°C. Other shortcomings of transistors include substantial variations in their parameters with variations in operating temperature and a fairly high sensitivity to ionizing radiation. (See alsoDRIFT TRANSISTOR, PULSE TRANSISTOR, INVERSION TRANSISTOR, and AVALANCHE TRANSISTOR.)

REFERENCES

Fedotov, la. A. Osnovy fiziki poluprovodnikovykh priborov [2nd ed.]. Moscow, 1970.
Kremnievye planarnye tranzistory. Edited by la. A. Fedotov. Moscow, 1973.
Sze, S. M. Fizika poluprovodnikovykh priborov. Moscow, 1973. (Translated from English.)

IA. A. FEDOTOV

transistor

[tran′zis·tər]
(electronics)
An active component of an electronic circuit consisting of a small block of semiconducting material to which at least three electrical contacts are made, usually two closely spaced rectifying contacts and one ohmic (nonrectifying) contact; it may be used as an amplifier, detector, or switch.

transistor

a semiconductor device, having three or more terminals attached to electrode regions, in which current flowing between two electrodes is controlled by a voltage or current applied to one or more specified electrodes. The device is capable of amplification, etc., and has replaced the valve in most circuits since it is much smaller, more robust, and works at a much lower voltage

transistor

(electronics)
A three terminal semiconductor amplifying device, the fundamental component of most active electronic circuits, including digital electronics. The transistor was invented on 1947-12-23 at Bell Labs.

There are two kinds, the bipolar transistor (also called the junction transistor), and the field effect transistor (FET).

Transistors and other components are interconnected to make complex integrated circuits such as logic gates, microprocessors and memory.

transistor

In the analog world of continuously varying signals, a transistor is a device used to amplify its electrical input. In the digital world, a transistor is a binary switch and the fundamental building block of computer circuitry. Like a light switch on the wall, the transistor either prevents or allows current to flow through.

Made of Semiconductor Material
The active part of the transistor is made of silicon or some other semiconductor material that can change its electrical state when pulsed. In its normal state, the material may be nonconductive or conductive, either impeding or letting current flow. When voltage is applied to the gate, the transistor changes its state. To learn more about the transistor, see transistor concept and chip. See active area, phototransistor and High-K/Metal Gate.


From Transistors to Systems
Transistors, along with resistors, capacitors and diodes, are wired in patterns that make up logic gates. Logic gates wired in patterns make up circuits, and circuits wired in patterns make up electronic systems. See Boolean logic.







Conceptual View of a Transistor
In a digital circuit, a transistor is an on/off switch becoming conductive for a moment when it is pulsed with electricity. However, it can also function like a relay to amplify analog signals such as from an audio source, transferring a low voltage at the gate to a larger voltage at the drain.







The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)


The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)




The First Silicon Transistor
The first transistors had five times the mass of an entire chip today. In 1954, Texas Instruments pioneered the commercial production of silicon transistors. See transistor concept. (Image courtesy of Texas Instruments, Inc.)







Transistors Get Smaller
When IBM announced the System/360 in 1964, its Solid Logic technology tied three transistors together. With the top of the module removed, you can see the three silvery transistors. This half square inch of space today can hold more than a billion transistors. See active area. (Image courtesy of IBM.)
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