Construction and working principle of MOSFET:
A) Depletion type MOSFET:
Construction: The N-channel depletion type MOSFET is formed on P-type silicon substrate with two heavily doped n+ silicon for low resistance connections of terminals Drain (D) and Source (S). The third terminal Gate (G) is isolated from the N-channel by a thin oxide layer. The substrate is normally connected to the source.
Operation: The gate-to-source voltage VGS can be either positive or negative. When VGS is negative, gate becomes negative with respect to source and some of the electrons in the N-channel are repelled leaving behind positive ions. The depletion region is created below the oxide layer, effective channel width is reduced, resulting a high resistance from the drain to source, RDS. At certain negative voltage VGS the channel will be completely depleted, offering very high value of RDS and no current will flow from drain to source i.e. IDS = 0. The value of VGS when this happens is called pinch-off voltage, VP. When VGS = 0, no charge will be induced in N-channel and the resistance RDS will be the normal resistance of the N-channel. When VGS is positive, the gate is positive and hence attracts electrons into the N-channel. Thus channel becomes wider, more carriers are available for current, RDS get reduced and the current IDS increases.
B) Enhancement type MOSFET:
Construction: The N-channel enhancement type MOSFET is formed on P-type silicon substrate with two heavily doped n+ silicon for low resistance connections of terminals Drain (D) and Source (S). It has no physical N-channel. The third terminal Gate (G) is isolated from the substrate by a thin oxide layer. The substrate is normally connected to the source.
Operation: When VGS is positive, gate is positive and hence on opposite side of oxide layer, the electrons are attracted from the P-substrate and accumulated on the surface beneath the oxide layer. If VGS is greater than or equal to a value known as threshold voltage, VT, sufficient electrons are accumulated to form a virtual N-channel and the current flows from the drain to source. Further increase in VGS will result in more accumulation of electrons and hence more current. Thus the MOSFET is a gate voltage controlled device.