• MOSFET is the common term for insulated gate field effect transistor.

There are two basic forms of MOSFET:

1) Enhancement MOSFET 2) Depletion MOSFET.

• Principal of Operation: By applying a transverse electric field across an insulator,deposited on the semiconducting material, the thickness and hence the resistance of the conducting channel of the semiconducting material can be controlled.

• In depletion MOSFET, the controlling electric field reduces the number of mejority carriers available for conduction where as in enhancement MOSFET,application of electric field causes an increase in the majority carrier density in the conducting regions of the transistor.

• Construction and Working of Depletion MOSFET:

1. The basic construction of n-channel depletion type MOSFET is as shown in the figure. Where an N-channel is diffused between the source and drain to the basic structure of MOSFET

   Operation with VGS: When VGS = 0 then the gate,substrate and source terminals are connected to ground.A positive voltage VDD is connected between drain and source.Due to the positive voltage applied to the drain terminal, free electrons from the channel are attracted to the drain and the drain current starts following. This ID = IDSS.

2. Operation with negative VGS : Due to negative voltage applied, the gate will tend to repel free electrons towards the p-type substrate and attract the hole from the substrate. These electrons and holes will recombine inside the channel and this will reduce the number of free electrons available for conduction. Therefore drain current will decrease with increase in negative voltage.

3. Effect of positive gate to source voltage: when the positive voltage is applied,it will draw additional electrons from the p-type substrate due to reverse leakage current. Hence the drain current will increase so as the conductivity of channel.Thus the level of free electron will enhance due to application of positive gate to source voltage. Hence the corresponding region of operation is known as enhancement region.

The drain and transfer characteristics of depletion MOSFET is as shown in figure: