A MOSFET controls current through a semiconductor channel using an electric field produced by an insulated gate.

In an n-channel enhancement MOSFET, the source and drain are n-type regions separated by a p-type body. The gate is insulated from the semiconductor by a thin dielectric, so ideally almost no DC current flows into the gate.

A positive gate-to-source voltage attracts electrons toward the surface. Above a threshold voltage $V_{TH}$, an inversion layer forms and creates a conducting channel between source and drain.

A p-channel enhancement MOSFET uses the complementary doping and carrier type. Its controlling voltage and current polarities are reversed: making the gate sufficiently negative relative to the source creates a hole channel.

The gate voltage therefore controls channel conductivity electrostatically rather than by supplying the main channel current.

This high input resistance makes MOSFETs fundamental to digital logic, analog amplification and power switching.