Explain the principle of operation of any one type of MOSFET.

Operating Principle of N-channel Enhancement MOSFET

Operating principle of N channel Enhancement MOSFET is demonstrated in fig.(a). When drain is applied with positive voltage with respect to source and no potential is  applied to the gate, two N-regions and one P-substrate form P-N junctions connected back to back with a resistance of P substrate. So a very small drain current i.e., a reverse leakage current flows. If P type substrate is now connected to the source terminal, there is zero voltage across source substrate junction, and drain-substrate junction remains reverse biased.