Question

Draw a labelled diagram of a field effect transistor and explain how the properties change as a forward bias and a reverse bias are applied.

Answer 1

Labelled diagram Of Field effect transistors

Field effect transistors are made of two pn junctions. It has three terminalsa npn. It has p substrate sandwiched between two n layers. over p-substrate in between two layer gate oxide is deposited and is known as gate terminal. One layer acts as source and other n -layer as drain terminal.

Reverse Biased:

When drain is applied with positive voltage with respect to source and no potential is applied to the gate two N-regions and one P-substrate from two P-N junctions connected back to back with a resistance of the P-substrate. So a very small drain current that is, reverse leakage current flows. If the P-type substrate is now connected to the source terminal, there is zero voltage across the source substrate junction, and the^–drain-substrate junction remains reverse biased.

And thus it remains in OFF state.

Forward biased

When the gate to source voltage is applied and the substrate, negative (i.e. minority) charge carriers within the substrate are attracted to the positive gate and accumulate close to the-surface of the substrate. As the gate voltage is increased, more and more electrons accumulate under the gate. Since these electrons can not flow across the insulated layer of silicon dioxide to the gate, so they accumulate at the surface of the substrate just below the gate. These accumulated minority charge carriers N -type channel stretching from drain to source. When this occurs, a channel is induced by forming what is termed an inversion layer (N-type). Now a drain current start flowing. The strength of the drain current depends upon the channel resistance which, in turn, depends upon the number of charge carriers attracted to the positive gate. Thus drain current is controlled by the gate potential.

Thus it starts conducting and drain current increases as voltage between gate to source increases. It becomes ON.