Transistor

Description

Transistor, in electronics, common name for a group of electronic devices used as amplifiers or oscillators in communications, control, and computer systems . Until the advent of the transistor in 1948, developments in the field of electronics were dependent on the use of thermionic vacuum tubes, magnetic amplifiers, specialized rotating machinery, and special capacitors as amplifiers. Capable of performing many functions of the vacuum tube in electronic circuits, the transistor is a solid-state device consisting of a tiny piece of semiconducting material, usually germanium or silicon, to which three or more electrical connections are made. The basic components of the transistor are comparable to those of a triode vacuum tube and include the emitter, which corresponds to the heated cathode of the triode tube as the source of electrons. The transistor was developed at Bell Telephone Laboratories by the American physicists Walter Houser Brattain, John Bardeen, and William Bradford Shockley. For this achievement, the three shared the 1956 Nobel Prize in physics. Shockley is noted as the initiator and director of the research program in semiconducting materials that led to the discovery of this group of devices; his associates, Brattain and Bardeen, are credited with the invention of an important type of transistor.

What can a transistor do ?

Transistors are used as amplifiers, oscillators, or switches in communication, control, and computer systems. Commercial applications include very small hearing aids and compact portable radio and television receivers. Transistors have completely replaced vacuum tubes in electronic computers, which require a great many amplifiers. Transistors are also used in miniaturized diagnostic instruments, such as those used to transmit electrocardiograph, respiratory, and other data from the bodies of astronauts on space flights . Nearly all transmitting equipment used in space-exploration probes employs transistorized circuitry. Transistors also aid in diagnosing diseases. Miniature radio transmitters using transistors can also be implanted in the bodies of animals for ecological studies of feeding habits, patterns of travel, and other factors. A recent commercial application is the transistorized ignition system in automobiles.

Transistor types.Bipolar Transistor

fig.2 - Biploar transistor

The bipolar junction transistor consists of three layers of highly purified silicon (or germanium) to which small amounts of boron (p-type) or phosphorus (n-type) have been added. The boundary between each layer forms a junction, which only allows current to flow from p to n.

Connections to each layer are made by evaporating aluminum on the surface; the silicon dioxide coating protects the nonmetalized areas. A small current through the base-emitter junction causes a current 10 to 1000 times larger to flow between the collector and emitter. The many uses of the junction transistor, from sensitive electronic detectors to powerful hi-fi amplifiers, all depend on this current amplification.

fig.3 - Symbol

fig.4 Internal view of the bipolar transistor

Transistor Operation

In the transistor, a combination of two junctions may be used to achieve amplification. The first type, called the n-p-n junction transistor, consists of a very thin layer of p-type material between two sections of n-type material. The n-type material is the emitter element of the transistor, constituting the electron source. To permit the forward flow of current across the n-p junction, the emitter has a small negative voltage with respect to the p-type layer, or base component, that controls the electron flow. The n-type material in the output circuit serves as the collector element, which has a large positive voltage with respect to the base to prevent reverse current flow. Electrons moving from the emitter enter the base, are attracted to the positively charged collector, and flow through the output circuit. The input impedance, or resistance to current flow, between the emitter and the base is low, whereas the output impedance between collector and base is high. Therefore, small changes in the voltage of the base cause large changes in the voltage drop across the collector resistance, making this type of transistor an effective amplifier.

fig.5 - Amplifier

The voltage from a source is applied to the base of the transistor (labeled P). Small changes in this applied voltage across R1 (input) result in large changes in the voltage across the resistor labeled R2 (output). One possible application of this circuit would be to amplify sounds. In this case the input would be a microphone and the resistor R2 would be a speaker. "Hi-fi" amplifiers have many more transistors, both to increase the power output and to reduce the distortion that occurs in simple circuits like this.

Similar in operation to the n-p-n type is the p-n-p junction transistor, which also has two junctions and is equivalent to a triode vacuum tube. Other types with three junctions, such as the n-p-n-p junction transistor, provide greater amplification than the two-junction transistor.

Future developments

Quiteron Quiteron, electronic device being studied as a potential replacement for the transistor in integrated circuits and computers. Developed at IBM in the early 1980s, the quiteron is made of superconducting materials separated by insulators. Its action depends on a quantum physics phenomenon called the tunnel effect, due to which changes in resistance (produced by changes in applied voltage) can take place in a billionth of a second or less. These resistance changes can serve the same switching function that a transistor does, but at much higher speeds.

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