A semiconductor device having three terminals: source, gate, and drain. Current flow in a narrow conduction channel between drain and source is controlled by the voltage applied between gate and source, which can deplete the conduction channel of charge carriers. If the source and drain regions are composed of n-type semiconductor the conduction channel is n-type; these devices are known as n-channel devices. Devices with p-type source, drain, and channel are called p-channel devices. In contrast to bipolar transistors, FETs are unipolar devices; the current flow is electrons (in n-channel devices) or holes (in p-channel devices).
In the junction FET the channel is a composite part of the structure. In the MOSFET the gate is insulated from the source and drain regions and the channel forms when the gate voltage is applied. Unlike the bipolar transistor both types of FET require virtually no input current to the gate except a pulse to charge or discharge the gate capacitance. Junction FETs have relatively slow switching speeds compared with MOSFETs and bipolar transistors, and are therefore not used in logic circuits. FETs appear in vast numbers in microprocessor chips and similar devices.