What Is a FET Transistor?

A field-effect transistor (FET) is a three-terminal device that can control the amount of electricity allowed to pass through. FET transistors are able to switch or control electricity very rapidly. Some field-effect transistors are able to handle large amounts of electric power, making it useful in a wide variety of electronic and electrical equipment used in consumer, commercial, and military applications.

The FET transistor usually has source, drain, and gate terminals. Electrical resistance between the source and the drain can be manipulated by changing the electrical potential, or voltage, between the gate and the source. Given this effect, the FET transistor can be used in any electronic circuit, such as power supplies, amplifiers, and receivers.

FET transistor types include the junction FET (JFET) and metal oxide FET (MOSFET). The JFET has a junction between the gate and the source, while the MOSFET transistor has a gate that is insulated from the source. FET transistors may also be categorized as depletion or enhancement devices. For a depletion FET transistor, the main conductive channel between the source and the drain is initially conductive when there is zero voltage between the gate and the source. For an enhancement transistor, the main conductive channel is nearly absent when there is zero voltage between the gate and the source.

In electronic circuitry, switching devices act like an ordinary light switch, turning it on or off. Besides switching, a device may control the current or electrical charge flow rate through a specific path on the circuit. This capability makes it possible to build many useful circuits, such as amplifiers and receivers, used in commercial appliances such as stereos, radios, television sets, and home computers.

Before the FET transistor was discovered, the electronics industry used the bipolar junction transistor (BJT), which itself was proceeded by the vacuum tube (VT), a sealed glass tube with at least three major terminals known as the cathode, plate, and grid located inside a vacuum. The electric charge from cathode to plate flies through a vacuum via a gaseous state, while the grid controls the flow of electricity between the plate and the cathode. By proper design, the vacuum tube can be used as a switch or as an amplifier, as is most common. In this case, it accepts low levels of signals, such as sound or radio, and produces large-level replicas or amplified versions.

When the BJT began to be used on a wide scale, vacuum tubes were reserved for special applications where power economy was not a priority. The BJT was the first solid-state device because the charges traveled through solid material all the way. Further developments introduced the FET transistor, which provided superior low-noise performance, making it best suited for special applications such as radio and optical communications.