Transistor

Field-effect transistors (FETs) are solid-state active devices based on a different principle than BJTs but producing much the same result. FETs are three-terminal devices, just as are BJTs. The input terminal of an FET is termed its gate and constitutes one of the electrodes of a reverse-biased diode. FETs achieve current control by channeling current through a narrow n-type or p-type pathway whose conductivity is adjusted by the input signal. The output current controlled by an FET passes between the two remaining terminals called a source and a drain. The current through an FET must find its way through a narrow channel formed by the input-diode junction. Since this input diode is reverse biased, this channel tends to have few charge carriers. The input signal to the FET can deplete or enhance the number of available charge carriers in this channel, regulating the current in the drain circuit. Because the input diode is reverse biased, the FET demands almost no current from the signal source, therefore almost no power must be supplied. The power gain commonly achieved in an FET amplifier is very high.

A particular type of FET called a MOSFET (metal oxide semiconductor field-effect Transistor) can have an input resistance as high as 10¹⁸ ohms. Because of their very high input resistance, FETs are instantly destroyed if they receive even a small static-electric charge from careless handling. Sliding across a plastic chair may impart enough charge to a technician's body to destroy a field-effect transistor's input diode at the first touch. FETs must be handled only by persons who ground themselves before touching these devices to first dissipate static charges.

FETs are particularly useful as amplifiers of very weak signals such as those produced by high-quality microphones. FETs have more desirable overload characteristics than BJTs, so that FETs are able to handle many signals simultaneously, some strong and some weak, without suffering troublesome distortion. Before FETs were used in automobile receivers, these radios were easily overloaded by strong signals; the introduction of FETs made a tremendous improvement in automobile-radio receiver performance.