3. Field-Effect Transistor

The field-effect transistor or FET is a three-terminal semiconductor device that controls an electric current by an electric field. The FET actually pre-dates the BJT as the first patent was granted for such a device in 1928. Its impact on industry however was felt only about a decade after the development of the transistor in 1948. The FET is a unipolar device having only one p-n junction, and it differs from the BJT in several important respects, the main one being the FET’s inherently high input impedance. There are two types of FETS: the junction gate FET (JFET or JUGFET) and the metal oxide semiconductor FET or MOSFET. The MOSFET, sometimes called the insulated gate FET or IGFET, itself comes in two versions: the depletion MOSFET and the enhancement MOSFET. Because of a difference in construction, the MOSFET has a higher input impedance than the JFET. The FET like the BJT can provide amplification of a signal and operate as a switch. It is important in many applications and forms the subject of this chapter. At the end of the chapter, the student will be able to: