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 VGS=0 i.e. why it is called normally -OFF MOSFET.

This N-channel MOSFET also called N-MOS.

Basic construction
Schematic Symbol

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 VDD is applied  VGS=0, ID.

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 VGS(th)

When VGS<VGS(th),ID=0,Drain current starts only when VGS<VGS(th).

For a given VDS, as VGS  is increased, virtual channel deepens & ID  increases.

The value of ID=K(VGS-VGSth)2

Where K is aconstand & its value is a K=ID(on)/(VGS(on)-VGSth)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


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