MOSFET is the common term for the insulated gate field effect transistor. In enhancement type MOSFET application of electric field causes an increase in the majority carrier density in the conducting region of the transistor.

Construction: The basic construction of an n-channel enhancement type MOSFET is as shown in the figure.

• Two highly doped N+ regions are diffused in a lightly doped substrate of p type silicon substrate. The substrate is sometimes connected to the source otherwise it is brought out as the fourth terminal.

• The drain and source terminals are connected to the N+ type doped regions through the metallic contact.

• The channel is absent in this type.

• The insulating Sio2 layer is still present which isolates gate terminal from the substrate. This device is called the insulated gate FET because of the insulating layer of SiO2.This layer gives an extremely high input resistance. Operation: The operation can be explained with two operating conditions.

 Operation with Vgs = 0

• If Vgs is zero and a positive voltage is applied between its drain and source, then due to absence of n-type channel, a zero drain current will present.

 Operation with Vgs is positive:

• The positive potential at the gate terminal repel the holes present in the p-type substrate. This results in creating an depletion region near the Sio2 insulating layer. But the minority carriers in the p-type substrate will gate attracted towards the gate terminal gather near Sio2 layer.

• As the Vgs becomes more positive the number of electrons present at the Sio2 layer will increase. The increasing concentration of electrons creates an induced channel which connects the n-type doped regions. Hence the conductivity increases and current flows from source to drain through the induced channels.

• Thus the drain current is enhanced by the positive gate voltage.

 Operation with increase Vds

• If Vgs is constant and Vds increases gradually, then the gate terminal becomes more and more negative with respect to drain so less number of electronics are attracted towards gate terminal and induced channel becomes narrow.

• The width of the channel will reduced to the point of pinch off and the saturation condition will be created and breakdown condition encountered.

Transfer and Drain Characteristics :

• The drain and transfer characteristics of N-channel enhancement MOSFET are shown in the figure.