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The field-effect transistor (FET), sometimes called a unipolar transistor, uses either electrons (N-channel FET) or holes (P-channel FET) for conduction. The three main terminals of the FET are named source, gate and drain. On some FETs a fourth connection to the body (substrate) is provided, but normally the body is connected internally to the source.

A voltage applied between the gate and source controls the current flowing between the source and drain. In FETs the source/ drain current flows through a conducting channel néar the gate. This channel connects the source region to the drain region. The channel conductivity is varied by the electric field generated by the voltage applied between the gate/source terminals. In this way the current flowing between the source and drain is controlled. Like bipolar transistors, FETs can be made to conduct with light (photons) as well as voltage. Devices designed for this purpose are called phototransistors.

FETs are divided into two families: junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more commonly known as metal-oxide-semiconductor FET (MOSFET), from their original construction as a layer of metal (the gate), a layer of oxide (the insulation), and a layer of semiconductor. Unlike IGFETs, the JFET gate forms a PN diode with the channel which lies between the source and drain. Functionally, this makes the N-channel JFET the solid state equivalent of the vacuum tube triode which, similarly, forms a diode between its grid and cathode. Also, both devices operate in the depletion mode, they both have a high input impedance, and they both conduct current under the control of an input voltage.

MESFETs are JFETs, in which the reverse biased PN junction is replaced by a semiconductor-metal Schottky-junction. These, and the HEMFETs (high electron mobility FETs), 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).

FETs are further divided into depletion-mode and enhancement-mode types. Mode refers to the polarity of the gate voltage with respect to the source at the threshold of conduction. For N-channel depletion-mode FETs the gate is negative with respect to the source while for N-channel enhancement-mode FETs the gate is positive, at the threshold of conduction. For both modes, if the gate voltage is made more positive the source/drain current will incréase. For P-channel devices the polarities are reversed. Néarly all JFETs are depletion-mode types and most IGFETs are enhancement-mode types.