In the planar MOSFET shown below:
1. Apply voltage between drain and source in positive polarity. (Drain-source voltage: VDS)
2. Apply voltage between gate and source in positive polarity. (Gate-source voltage: VGS)
3. As a result, electrons are attracted to P layer under a gate insulator film and P layer becomes N layer. (Such P layer that becomes N layer is called an “inversion layer.”)
4. All regions of MOSFET become N layer (from drain side: “N+”-”N-”-”inversion layer(N)”– “N+”) due to inversion mentioned in 3.
5. As a result, MOSFET works as a resistor, and drain current determined by applied VDS and load flows.