Definition

field-effect transistor (FET)

Part of the Wireless and mobile glossary:

See also bipolar transistor and transistor.

A field-effect transistor (FET) is a type of transistor commonly used for weak-signal amplification (for example, for amplifying wireless signals).� The device can amplify analog or digital signals.� It can also switch DC or function as an oscillator.

In the FET, current flows along a semiconductor path called the channel. �At one end of the channel, there is an electrode called the source. � At the other end of the channel, there is an electrode called the drain. � The physical diameter of the channel is fixed, but its effective electrical diameter can be varied by the application of a voltage to a control electrode called the gate.� The conductivity of the FET depends, at any given instant in time, on the electrical diameter of the channel.� A small change in gate voltage can cause a large variation in the current from the source to the drain.� This is how the FET amplifies signals.

Field-effect transistors exist in two major classifications.� These are known as the junction FET (JFET) and the metal-oxide- semiconductor FET (MOSFET).

The junction FET has a channel consisting of N-type semiconductor (N-channel) or P-type semiconductor (P-channel) material; the gate is made of the opposite semiconductor type.� In P-type material, electric charges are carried mainly in the form of electron deficiencies called holes. � In N-type material, the charge carriers are primarily electrons.� In a JFET, the junction is the boundary between the channel and the gate.� Normally, this P-N junction is reverse-biased (a DC voltage is applied to it) so that no current flows between the channel and the gate.� However, under some conditions there is a small current through the junction during part of the input signal cycle.

In the MOSFET, the channel can be either N-type or P-type semiconductor. � The gate electrode is a piece of metal whose surface is oxidized.� The oxide layer electrically insulates the gate from the channel.� For this reason, the MOSFET was originally called the insulated-gate FET (IGFET), but this term is now rarely used.� Because the oxide layer acts as a dielectric, there is essentially never any current between the gate and the channel during any part of the signal cycle.� This gives the MOSFET an extremely large input impedance. � Because the oxide layer is extremely thin, the MOSFET is susceptible to destruction by electrostatic charges.� Special precautions are necessary when handling or transporting MOS devices.

The FET has some advantages and some disadvantages relative to the bipolar transistor.� Field-effect transistors are preferred for weak-signal work, for example in wireless communications and broadcast receivers. � They are also preferred in circuits and systems requiring high impedance.� The FET is not, in general, used for high-power amplification, such as is required in large wireless communications and broadcast transmitters.

Field-effect transistors are fabricated onto silicon integrated circuit (IC) chips.� A single IC can contain many thousands of FETs, along with other components such as resistors, capacitors, and diodes.

This was last updated in April 2005
Posted by: Margaret Rouse

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