Electrical Engineering ⇒ Topic : Enhancement MOSFET

Enhancement MOSFET

The construction of an insulated gate FET or metal oxide semiconductor FET (MOSFET) is shown in Figure (a). It consists of the following:

• High resistive P-type substrate,
• Two blocks of heavily doped N-type material are diffused into the surface, and
• A surface is coated with silicon dioxide

             Figure (a) Enhancement MOSFET (NMOS).

To make contact with the N-type blocks, holes are made through the silicon dioxide and metal is deposited through the holes to form drain and source terminals. On the surface between drain and source, a metal is deposited and this plate can function as gate.

When drain is made positive with respect to source and no potential is applied to gate, a small current flows. In this case, two N-blocks and a P-type substrate form back to back P-N junction material. If positive potential is given to the gate with respect to the source, negative charge carriers will be attracted towards the gate.

The charge carriers accumulate close to the surface of the substrate. If the gate potential is increased, more and more electrons accumulate under the gate. These electrons are unable to cross the silicon dioxide. An N-type channel is being stretched from drain to source and drain current flows. The magnitude of drain current depends upon the channel resistance. The channel resistance depends upon the number of charge carriers attracted to the gate. In other words, the gate potential controls the drain current. Since the gate potential enhances the conductivity of the channel, this device is known as enhancement mode MOSFET.

The drain and transfer characteristics of an enhancement mode MOSFET is shown in Figure (b). The P-channel MOSFET is constructed by diffusing P-type drain and source blocks on an N-type substrate shown in Figure (c).

                                                    Figure (b) Drain and transfer characteristics for enhancement NMOS.

                                             figure (c) Enhancement PMOS

The circuit symbol of enhancement MOSFET is shown in Figure (d).

Figure (4) Symbol for N- and P-channel enhancement MOS.