Q. Explain the construction and operation of Enhancement MOSFET?

The construction of an N-channel enhancement MOSFET is shown in figure below. Two highly doped N⁺ regions are diffused in a lightly doped substrate of P-type silicon substrate.

One N⁺ region is called the source S and other one is called the drain D. They are separated by 1 mil(10^-3 inch).A thin insulating layer of SiO2 is grown over the surface of the structure and holes are cut into the oxide layer ,allowing contact with source and drain. Then a thin layer of metal aluminum is formed over the layer of SiO2.This metal layer covers the entire channel region and it forms the gate G.

The metal area of the gate , in conjunction with the insulating oxide layer of SiO2 and the semiconductor channel forms a parallel plate capacitor. This device is called the insulated gate FET because of the insulating layer of SiO2.This layer gives extremely high input impedance for the MOSFET.

If the substrate is grounded and a positive voltage is applied at the gate, the positive charge on G induces an equal negative charge on the substrate side between the source and drain regions. Thus, an electric field is produced between the source and drain regions. The direction of the electric field is perpendicular to the plates of the capacitor through the oxide. The negative charge of electrons which are minority carriers in the P-type substrate forms an inversion layer. As the positive voltage on the gate increases, the induced negative charge in the semiconductor increases. Hence the conductivity increases and current flows from source to drain through the induced channel. Thus the drain current is enhanced by the positive gate voltage.