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Transistor

Transistor

A transistor is a solid-state electronic device used to control the flow of an electric current. The term solid-state refers to devices that take advantage of special properties of solids. (It usually refers to devices made of semiconducting materials.) Since they were invented in the 1940s, transistors have come to revolutionize modern communications. They are found in an enormous variety of electrical devices, ranging from popular consumer items such as home computer games, pocket calculators, and portable stereos to the complex electronic systems used by business and industry.

Until World War II (193945), most systems of communication used vacuum tubes for the amplification and control of electrical current. However, vacuum tubes have a number of serious disadvantages. They are bulky and fragile, they consume a lot of power, and they have a tendency to overheat. The demands of radar in particular during the war encouraged scientists to look for another method of amplifying and controlling electric current in communication devices.

Semiconductors

The discovery of the transistor was announced in 1948 by three scientists from the Bell Telephone Laboratories: William Shockley (19101989), John Bardeen (19081991), and Walter Brattain (19021987). The key to this discovery is a class of materials known as semiconductors. Semiconductors are substances that conduct an electric current only very poorly. They fall somewhere between true conductors (such as silver, aluminum, and copper) and nonconductors (such as wool, cotton, paper, air, wood, and most plastics). The two most commonly used semiconducting elements are silicon and germanium. Some important semiconducting compounds include cadmium selenide, cadmium telluride, and gallium arsenide.

Semiconductors fall into one of two general categories: n-type semiconductors and p-type semiconductors. The former class consists of materials that have a slight excess of electrons, while those in the latter class have a slight deficiency of electrons.

Words to Know

Amplification: Increasing the strength of some signal such as the amount of electrical current passing through a transistor.

Base: The middle slice of a transistor.

Chip: The piece of semiconducting material on which integrated circuits are etched.

Collector: One of the outer slices of a transistor.

Dopant: An impurity added to a semiconducting material.

Doping: The act of adding impurities to change semiconductor properties.

Emitter: One of the outer slices of a transistor.

Integrated circuit (IC): An electronic device that contains thousands or millions of microscopic-sized transistors etched on a single piece (chip) of material.

N-type semiconductor: An element or compound that has a slight excess of electrons.

P-type semiconductor: An element or compound that has a slight deficiency of electrons.

Resistor: Component used to introduce resistance.

Semiconductor: A substance that conducts an electric currentbut only very poorly.

Solid-state: A term used for electronic devices that take advantage of special properties of solids. It usually refers to devices made of semiconducting materials.

The conductivity of both n-type and p-type semiconductors can be enhanced greatly by adding very small amounts of impurities. This process is known as doping and involves the addition of roughly one atom of dopant (such as boron or phosphorus) for each ten million atoms of the base semiconductor (such as silicon or germanium).

Operation of a transistor

A typical transistor looks like a sandwich with one type of semiconductor as the slices of bread and the second type of semiconductor as the filling. For example, a thin slice of a p-type semiconductor might be placed between two thicker slices of an n-type semiconductor. The middle slice of the transistor is known as the base, while the two outer slices are called the collector and the emitter.

Suppose that this transistor is placed into an electric circuit, and current is allowed to flow through it. The current flows into the transistor through the collector, across the base, and out through the emitter.

The flow of this current can be controlled by attaching a second source of electric current to the base itself. The amount of current that flows through the transistor will be determined by this second source of electric current. If a relatively small current is allowed to flow into the base, the transistor does not permit a very large flow of current through it. If a relatively large current is allowed to flow into the base, the transistor allows a much larger flow of current through it.

For example, suppose that a particular transistor typically permits a flow of 0.01 milliamperes when the electrical flow into the base is at a minimum. Then suppose that the flow into the base is increased by a small

amount. That small increase will allow a much larger flow of electric currentsay up to 2.5 milliamperesthrough it. The transistor has been made to act, therefore, like an amplifier.

Many other kinds of transistors have been developed to perform other electronic functions. One of the greatest breakthroughs in transistor research was the invention in 1959 of the integrated circuit (IC). An integrated circuit is an electronic device that contains large numbers (usually thousands or millions) of microscopic-sized transistors etched on a single piece (chip) of material.

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transistor

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. The vast majority of transistors now are built as parts of integrated circuits. Transistors are used in virtually all electronic devices, including radio and television receivers, computers, and space vehicles and guided missiles.

See microelectronics.

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transistor

transistor A semiconductor device having, in general, three terminals that are attached to electrode regions within the device. Current flowing between two of these electrodes is made to vary in response to voltage or current variations imposed on the third electrode. The device is capable of current or voltage amplification depending on the particular circuit implementation employed. It can also be used as a switch by driving it between its maximum and minimum of current flow.

The transistor was invented in 1948 by Shockley, Brattain, and Bardeen at Bell Telephone Labs. As performance and manufacturing techniques improved, the transistor enabled a huge growth in computer technology.

See also bipolar transistor, field-effect transistor, MOSFET.

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transistor

transistor Electronic device made of semiconductor material that can amplify electrical signals. The material, such as silicon or germanium, is ‘doped’ with minute amounts of phosphorus, arsenic or antimony to produce n-type material, in which negative charges (electrons) carry current; or with aluminium, gallium or indium to give a p-type material. Joining together a piece of each produces a diode. Sandwiching one type between two of the other produces a transistor. Transistors were first developed (1948) by US physicists John Bardeen, Walter Brattain, and William Shockley, making possible many advances in technology, especially in computers, portable radios and televisions, satellites, and control systems.

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The Transistor

The Transistor

The transistor allows the miniaturization of electronic equipment and is regarded as the nerve cell of the Information Age. Its first use was in the telephone switching machine in Englewood, New Jersey, in 1952. By the end of the twentieth century, the transistor could be found everywhere, in supercomputers, televisions, radios, toys, greeting cards, and garage door openers.

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transistor

tran·sis·tor / tranˈzistər/ • n. a semiconductor device with three connections, capable of amplification in addition to rectification. ∎  (also transistor radio) a portable radio using circuits containing transistors rather than vacuum tubes.

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transistor

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