Cross-section of an n-channel MOSFET (on state)
An n-channel MOSFET has n⁺ source and drain regions formed in a p-type substrate, with the surface between them covered by a gate electrode on top of a thin gate insulator. With no voltage on the gate, the region between source and drain is p-type, so no current flows.
As a positive voltage is applied to the gate, holes are pushed away from the surface and a depletion layer forms. Once the voltage exceeds the threshold voltage, electrons gather at the surface and invert it to n-type. This electron layer (the inversion layer) becomes the channel connecting source and drain, and the transistor turns on.
Try it: gate voltage and channel
As you raise the gate voltage VGS, holes under the gate are pushed away and the depletion layer grows; above the threshold voltage, electrons gather at the surface and form an inversion layer (the channel). The threshold voltage is 0.46 V and the flat-band voltage is about −1.0 V (the defaults of the threshold voltage calculator).