transistor (redirected from Transistors)

transistor, three-terminal, solid-state electronic device used for amplification and switching. It is the solid-state analog to the triode electron tube; the transistor has replaced the electron tube for virtually all common applications.

Types of Transistors

The transistor is an arrangement of semiconductor 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. Although discrete transistors are still used significantly, the vast majority of transistors are built as parts of integrated circuits. Transistors are used in virtually all electronic devices, including radio receivers, computers, and space vehicles and guided missiles.

See microelectronics.

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transistor

A transistor is a sandwich of dissimilar semiconductors to which are attached three electrodes. …

(credit: © Merriam-Webster Inc.)

Solid-state semiconductor device for amplifying, controlling, and generating electrical signals. Invented at Bell Labs (1947) by John Bardeen, Walter H. Brattain, and William B. Shockley, it displaced the vacuum tube in many applications. Transistors consist of layers of different semiconductors produced by addition of impurities (such as arsenic or boron) to silicon. These impurities affect the way electric current moves through the silicon. Transistors were pivotal in the advancement of electronics because of their small size, low power requirements, low heat generation, modest cost, reliability, and speed of operation. Single transistors were superseded in the 1960s and '70s by integrated circuits; present-day computer chips contain millions of transistors. Today transistors perform many different functions in nearly every type of electronic equipment.

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transistor

In the analog world of continuously varying signals, a transistor is a device used to amplify its electrical input. In the digital world of computing, a transistor is mostly a binary switch and the fundamental building block of digital circuitry. Like a light switch on the wall, the transistor acts as a simple on/off switch, either preventing or allowing 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 transistor's gate, it changes its state.

Transistors to Gates to Circuits to Systems
Transistors, as well as 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. To learn more about the transistor, see chip. See phototransistor and High-K/Metal Gate.

Conceptual View of a Transistor

In an "enhancement mode" transistor, the semiconductor material normally acts as an insulator. In a digital circuit, it works like 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. (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-inch square space today can hold millions of transistors. (Image courtesy of International Business Machines Corporation. Unauthorized use not permitted.)

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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

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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/Si_1-xGe_x/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 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)

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 V_P), the channel pinches off.

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.

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(electronics)

transistor - 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.