• 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 conductng regions of the transisior.

Consruction and Working of Deplition 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.

   • A P-ytpe semiconductor material is used as substrate. The source and drain are connected to n-type region through metallic contact. Gate terminal is insulated from the n-channel by a thin SiO2 layer.

   • 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.

   • 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.

   • 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.

Characteristics of n-channel D-MOSFET

1) The transfer characteristics of n-channel depletion MOSFET shown by Figure indicate that the it has a current flowing through it even when VGS is 0V.This indicates that these D-MOSFET conduct even when the gate terminal is left unbiased., which is further emphasized by the VGS0 curve of Figure b.

2)Under this condition, the current through the MOSFET is seen to increase with an increase in the value of VDS, untill VDS becomes equal to pinch-off voltage VP. After this, IDS will get saturated to a particular level IDSS which increases with an increase in VGS. 3) Further, the locus of the pinch-off voltage also shows that VP increases with an increase in VGS. However it is to be noted that, if one needs to operate these devices in cut-off state, then it is required to make VGS negative and once it becomes equal to -VT, the conduction through the device stops (IDS = 0) as it gets deprived of its n-type channel.

The diagrams of N-channel depletion type MOSFET along with its characteristics is as shown in the figure: