ANALOGUE ELECTRONICS I

The field-effect transistor (FET) is a transistor that uses an electric field to control the electrical behaviour of the device. It is a three-terminal device used for a variety of applications that match, to a large extent, those of the BJT transistor. Although there are important differences between the two types of devices, there are also many similarities. The primary difference between the two types of transistors is the fact that: a BJT is a current-controlled device; that is, the base current controls the amount of collector current. A FET is a voltage-controlled device, where the voltage between two of the terminals (gate and source) controls the current through the device.

FETs are unipolar devices because, unlike BJTs that use both electron and hole current, they operate only with one type of charge carrier. The two main types of FETs are the junction field-effect transistor (JFET) and the metal oxide semiconductor field-effect transistor (MOSFET). The term field-effect relates to the depletion region formed in the channel of a FET as a result of a voltage applied on one of its terminals (gate).