IMPORTANCE OF FIELD EFFECT TRANSISTOR

The field-effect transistor (FET) is a transistor that uses an electric field to control the shape and hence the electrical conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are also known as unipolar transistors as they involve single-carrier-type operation. The FET has several forms, but all have high input impedance. While the conductivity of a non-FET transistor is regulated by the input current (the emitter to base current) and so has a low input impedance, a FET's conductivity is regulated by a voltage applied to a terminal (the gate) which is insulated from the device. The applied gate voltage imposes an electric field into the device, this in turn attracts or repels charge carriers to or from the region between a source terminal and a drain terminal. The density of charge characters in turn influences the conductivity between the source and drain

The Field Effect Transistor is a device which enables us to use one electrical signal to control another. The name ‘transistor’ is a shortened version of the original term, transfer resistor, which indicates how the device works. Most transistors have three connections. The voltage on (or current into/out of) one wire has the effect of controlling the ease with which current can move between the other two terminals. The effect is to make a ‘resistance’ whose value can be altered by the input signal. We can use this behaviour to ‘transfer’ patterns of signal fluctuation from a small input signal to a larger output signal. 

A wide variety of devices are called transistors. Here will just look at one example, called an N-channel Junction-FET (J-FET). This sort of transistor is made by forming a channel of N-type material in a substrate of P-type material. Three wires are then connected to the device. One at each end of the channel. One connected to the substrate. In a sense, the device is a bit like a PN-junction diode, except that we've connected two wires to the N-type side.
Electrons can move along the channel, so when we apply a voltage between the two end-wires a current will flow along the channel. We can maintain this by continually putting electrons in one end (the source) and removing them at the other (the drain). The effective resistance between the two ends will depend upon the size & shape of the channel and the properties of the N-type material. Note, however, that electrons moving in the channel will — just as with the diode — be repelled by the fixed charges in the P-type substrate. As a result, the current doesn't fill the whole channel. It avoids the depletion zones near the walls.