The FET transistors are mainly classified into two types they are Junction Field Effect Transistor (JFET) and Insulated Gate FET (IG-FET) or Metal Oxide Semiconductor FET (MOSFET). Due to this high impedance values the FET transistors are very sensitive to small input voltages. The FET transistors have high input impedance where as BJT has relatively low. Like BJT the FET transistors are also available in both P-channel and N-channel. These FET transistors are very useful in the chip designing due to their low power consumption behavior. Due to this high efficiency the FET transistors are used in many electronic circuit applications by replacing the corresponding BJT transistors. The FET transistors can be made smaller in size compared to BJT transistor and also they have less power dissipation. The BJT transistors are ‘bipolar’ devices because they operates with both types of charge carriers, such as electrons and holes but the FET transistors are ‘unipolar’ devices because they operate with the charge carriers of either electrons (for N-channel) or holes (for P-channel). In BJT transistor a small input current operates the large load, but in FET a small input voltage operates the large load at the output. In the FET transistors the output current passes between the drain and source terminals and this path is called channel and this channel may be made of either P-type or N-type semiconductor materials. In BJT transistors the output current is controlled by the input current which is applied to the base, but in the FET transistors the output current is controlled by the input voltage applied to the gate terminal. The FET transistors have basically three terminals, such as Drain (D), Source (S) and Gate (G) which are equivalent to the collector, emitter and base terminals in the corresponding BJT transistor. The FET transistors are voltage controlled devices, where as the BJT transistors are current controlled devices. Junction Field Effect Transistor (JFET).
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