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 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