A field effect transistor (FET) is a three-terminal semiconductor device in which the current flowing through a conducting channel is controlled by the electric field produced by the voltage applied to its gate. This makes it a voltage-controlled device, in contrast to the bipolar junction transistor whose output current is set by an input current.
The FET's defining property is that it is unipolar: conduction depends on only one kind of majority carrier — electrons in an n-channel device or holes in a p-channel device — whereas a bipolar transistor uses both electrons and holes. Because the controlling gate junction is kept reverse biased, it draws almost no current, so the FET offers a very high input resistance (of the order of megaohms). It also switches fast, adds little noise, and is easy to fabricate in integrated circuits.
These features make the FET a natural choice for small-signal amplifiers, electronic switches, and the dense circuitry inside modern ICs. For the II PUC Electronics board exam, remember the four keywords that describe it: three-terminal, unipolar, voltage-controlled, majority-carrier device. Everything else in this chapter — construction, biasing, characteristics and parameters — builds on this basic idea of a field controlling a channel.