FET Basics

Field Effect Transistor: (2m)

›  Field effect transistor is a unipolar-transistor, which acts as a voltage-controlled current device and is a device in which current at two electrodes is controlled by the action of an electric field at another electrode.

›  Field effect transistor is a device in which the current is controlled and transported by carriers of one polarity (majority) only and an electric field near the one terminal controls the current between other two.

Advantages: (2m)

1. Unipolar device, operation depends on only one type of charge carriers (h or e)
2. Voltage controlled Device (gate voltage controls drain current)
3. Very high input impedance (»10⁹-10¹² W)
4. Source and drain are interchangeable in most Low-frequency applications
5. Low Voltage Low Current Operation is possible (Low-power consumption)
6. Less Noisy as Compared to BJT        
7. No minority carrier storage (Turn off is faster)
8. Self limiting device
9. Very small in size, occupies very small space in ICs
10. Low voltage low current operation is possible in MOSFETS
11. Zero temperature drift of output is possible

Comparison of FET and BJT: (2/4 m)

›  The conventional bipolar transistor has two type of current carriers of both polarities (majority and minority) and FET has only one type of current carriers, p or n (holes or electrons)

›  The BJT is current controlled and FET is voltage controlled current between two other terminals

›  JFET junction is reverse-biased, the gate current is practically zero, and a very high impedance at input whereas the base current of the BJT is always some value greater than zero, for example, in microA