Field-effect transistor

The field-effect transistor (FET) is a type of transistor that relies on an electric field to control the shape and hence the conductivity of a 'channel' in a semiconductor material. The concept of the field effect transistor predates the bipolar junction transistor (BJT), though it was not physically implemented until after BJTs, due to the limitations of semiconductor materials and relative ease of manufacturing BJTs compared to FETs at the time.

Terminals

All FETs except J-FETs have four terminals, which are known as the gate, drain, source and body/base/bulk/substrate. Compare these to the terms used for BJTs: base, collector and emitter. BJTs and J-FETs have no body terminal.
The names of the terminals refer to their functions. The gate terminal may be thought of as controlling the opening and closing of a physical gate. This gate permits electrons to flow through or blocks their passage by creating or eliminating a channel between the source and drain. Electrons flow from the source terminal towards the drain terminal if influenced by an applied voltage. The body simply refers to the bulk of the semiconductor in which the gate, source and drain lie. Usually the body terminal is connected to the highest or lowest voltage within the circuit, depending on type. The body terminal and the source terminal are sometimes connected together since the source is also sometimes connected to the highest or lowest voltage within the circuit, however there are several uses of FETs which do not have such a configuration, such as transmission gates and cascode circuits.