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Transistor, Transistor

A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals.

The transistor is often cited as being one of the greatest achievements in the 20th century, and some consider it one of the most important technological breakthroughs in human history. Some transistors are packaged individually but most are found in integrated circuits.

In 1934 German physicist Dr. Oskar Heil patented another field-effect transistor.

William Shockley saw the potential in this and worked over the next few months greatly expanding the knowledge of semiconductors and could be described as the father of the transistor. The term was coined by John R. Pierce.

A logic gate consists of up to about twenty transistors whereas an advanced microprocessor, as of 2006, can use as many as 1.7 billion transistors (MOSFETs).

Even after MOSFETs became available, the BJT remained the transistor of choice for many analog circuits such as simple amplifiers because of their greater linearity and ease of manufacture.

A transistor can control its output in proportion to the input signal, that is, can act as an amplifier. Or, the transistor can be used to turn current on or off in a circuit like an electrically controlled switch, where the amount of current is determined by other circuit elements.

A bipolar transistor has terminals labeled base , collector and emitter . A small current at the base terminal (that is, flowing from the base to 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.

The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off, apart from a small value due to leakage currents or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.

Bipolar transistors are so named because they conduct by using both majority and minority carriers. The three terminals of the BJT are named emitter , base and collector . The BJT consists of two p-n junctions: the baseemitter junction and the basecollector 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). "The [3] is useful in amplifiers because the currents at the emitter and collector are controllable by the relatively small base current." In an NPN transistor operating in the active region, the emitter-base junction is forward biased (electrons and holes recombine at 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 (electrons and holes are formed at, and move away from the junction) 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. By controlling the number of electrons that can leave the base, the number of electrons entering the collector can be controlled.

Devices designed for this purpose have a transparent window in the package and are called phototransistors.

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 (microwave frequencies; several GHz).

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 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 double 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.

The ball grid array (BGA) is the latest surface mount package (currently only for large transistor arrays ). 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 rating.

The package often dictates the power rating and frequency characteristics. Power transistors have large 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 can/metal plate. At the other extreme, some surface-mount microwave transistors are as small as grains of sand.

Transistor packages are mainly standardized, but the assignment of a transistor's functions to the terminals is not: different transistor types can assign different 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- i.e. BC212L and BC212K).

Transistors with part numbers beginning with 2SA or 2SB are PNP BJTs. Transistors with part numbers beginning with 2SC or 2SD are NPN BJTs. Transistors with part numbers beginning with 2SJ are P-channel FETs (both JFETs and MOSFETs). Transistors with part numbers beginning with 2SK are N-channel FETs (both JFETs and MOSFETs).

Source: Wikipedia > Transistor