Operation and construction of Enhancement type Mosfet

At its name indicates, this MOSFET operates only in the enhancement mode & has no depletion mode.

It works with large positive gate voltages only.

It differs in construction from the DEMOSFET in that structurally there exists no channel between the drain & source” which shown in figure below.

Hence it does not conduct when V_GS=0 i.e. why it is called normally -OFF MOSFET.

This N-channel MOSFET also called N-MOS.

Basic operation and characteristics-

-We have seen that in MOSFET, there exists no channel between the source & drain. Hence it can never operate with a negative gate voltage because it will induce positive charge in the space between the source & drain which will not allow the passage of electrons between the two.

Hence it operates with positive gate voltage only.

The normal biasing polarities of this E-only MOSFET is shown in figure below (For N-Channel).

even when some positive V_DD is applied  V_GS=0, I_D.

For getting significant amount of drain current, we have to apply sufficiently high positive gate voltage.

The thin layer of p-substrate touching the metal Oxide film which provides channel for electrons is called N-type inversion layer or virtual N-channel.

The minimum gate-source voltage which produces this N-type inversion layer & hence drain current is called threshold voltage V_GS(th)

When V_GS<V_GS(th),I_D=0,Drain current starts only when V_GS<V_GS(th).

For a given V_DS, as V_GS  is increased, virtual channel deepens & I_D  increases.

The value of I_D=K(V_GS-V_GSth)²

Where K is aconstand & its value is a K=I_D(on)/(V_GS(on)-V_GSth)²

N.B.: Similarly a P channel E -only MOSFET(PMOS) is constructed like N-MOS except that all the P & N regions are inter changes.

It operates with negative gate voltage only.

Also Read- Construction of depletion type MOSFET