MOSFET

MOSFET

The metal oxide semiconductor field effect transistor is a semiconductor device widely used for switching purposes and for the amplification of electronic, signals. It has three terminals source(s), gate(G) and drain (D).

The MOSFET can function in two ways

(i) Depletion mode

(ii) Enhancement mode

Construction and operation of depletion type MOSFET

n-channel depletion MOSFET

• Fig shows N-channel depletion MOSFET

• It operates both depletion mode and enhancement mode

Construction

• A type material is formed silicon base it is referred as substrate

• The substrate internally connected to the source terminal

• Two heavy depend region n+ are diffused in p-type substrate

• One n+ region act as source(s) and another n+ region act as drain(D)

• A thin insulating layer SiO2 is grown over the surface.

• The primary difference between the construction of depletion type and enhancement type MOSFET is that there is absence of channel in two enhancement type. Where as there is a diffused channel in the depletion type

Operation

• When V_gs= 0, V_Ds voltage is applied across two drain to source

• As a result is an attraction for the positive potential at the drain by two free electrons of n-channel and current is established through the channel.

• When V_gs = +ve voltage

• The positive gate voltage will draw additional electrons from the p-type substrate due reverse leakage current. As two V_Gs continues to increase in the positive direction. The drain current will increase at a rapid rate

• [V_gs =V_e↑, I_D↑] when V_gs = -V_e voltage. The negative potential at the gate will tend to pressure electrons toward to the p-substrate and attract holes. From the p-type substrate. The more negative voltage, the increase the rate of recombination. The resulting drain current reduced.

[V_gs = -V_e↑, I_D ↓]

Characteristics of n-channel depletion type MOSFET

Construction and working of n-channel enhancement type MOSFET

Construction

• It operates only in enhancement mode

• No physical channel between two n-region as shown in Fig.

• A p-type material is formed as substrate. It is internally connected to source terminal

• Two n+ highly doped regions are diffused in p-substrate

• One n+ region act as drain (D) and one n+ region act as source(S)

• Insulating layer Sio₂ is grown over the surface of the structure.

• No-channel between the two n+ regions.

Operation

• when V_gs = 0, voltage is applied between drain and source, the absence of n- channel will result in a zero drain current (V_gs = 0, V_Ds= +ve I_D=0)

If both V_gs and V_ds is +ve voltage is +ve voltage

• When +ve voltage to gate repelled the holes to p-type substrate

• As V_gs increases it will induced the n-type channel between the two n+ region

• V_gs is increased beyond the threshold level, the density of free carriers induced channel will increases, I_D current increases.

• If r_gs is constants and v_Ds increases the pinching off process occurs, and narrow channel at the drain end.

I_D = k(V_Gs – V_T)²