What is MOSFET-Metal Oxide Semiconductor Field Effect Transistor?

MOSFET- Metal Oxide Semiconductor Field Effect Transistor

•MOSFET is a three terminal device. The three terminals are source(S), gate (G), drain (D).It is easy to control by the gate terminal.

•Used in low-voltage operation where high switching speed is required

•MOSFET has high blocking voltage capability & improved current carrying capacity.

There are two types of MOSFET

•     n-Channel MOSFET & P-channel MOSFET

Two modes of operation of MOSFET:

•Gate to source voltage (Vgs) > 0 Enhancement mode, Conductivity increases with  increase in Vgs

•Gate to source voltage (Vgs) < 0 Depletion mode, Conductivity decreases with Vgs 

Operation of MOSFET:

             Formation of depletion layer

             Formation of Inversion layer

•When Gate is made +ve w.r.t Source.As Vgs is given, SiO2 layer will act as a capacitor.

• Positive charge is generated on Gate.

• On P type layer negative charge (free electrons) is generated.

•As Vgs increases , negative charge (free electrons) also increases.

At the interface, layer of free electrons is referred as the inversion layer which  is shown in below Fig 

•    Property of inversion layer is same as an n- type semiconductor.
It is a conductive path or “channel” between the drain and the source which permits flow of current between the drain and the source.

Operating principle of a MOSFET

•Initially there is no current injection from the gate terminal,since the metal oxide is a very good insulator.

• In MOSFET the gate to silicon oxide(SiO2) layer and the p-body silicon forms a good quality capacitor.

When a less voltage is applied to this capacitor structure with gate terminal +ve with respect to the source, a depletion region forms at the interface between the SiO2 and the silicon.

•The +ve charge induced on the gate metallization repels the majority hole carriers from the interface region between the gate oxide and the p type body. 

This exposes the negatively charged acceptors and a depletion region is created.

•Further increase in voltage between gate and source(Vgs) causes the depletion layer to grow in thickness.

• At the same instant the electric field at the SiO2-silicon interface gets larger and begins to attract free electrons as shown in above Fig

•The extra holes are neutralized by electrons from the source.

Steady state V-I output  & Transfer

Characteristics of  MOSFET:-

•In the cut-off mode no drain current flows  and the applied drain–source voltage (Vds) is supported by the body-collector p-n junction.

•Therefore, the maximum applied voltage should be below the break down voltage of this junction (Vdss) to avoid destruction of the device.

•When Vgs is increased beyond  Vgs(threshold) drain current starts flowing.For small values of  voltage between gate to source(Vds), id is  proportional to Vds, this mode of operation is called “ohmic mode” of operation. 

• Generally MOSFET is operated  in the cut off or in the ohmic mode in power electronics applications.

•At high amount of electric field, produced by the high Drain – Source voltage, the drift velocity of free electrons in the channel tends to saturate.

•As a result the drain current becomes independent of voltage between drain to source(Vds) and determined  by the gate – source voltage Vgs and this mode is called as active mode of operation of a MOSFET.