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


Sachidananda Dhar

The main aim of this website to provide dynamic study material to student who can't afford any coaching class and also help to all people of electrical community. We need your support to grow.

Leave a Reply

Your email address will not be published.