Working principle
A MOSFET is a voltage-controlled switch built on a semiconductor die with three terminals: gate, drain and source. In an N-channel enhancement device the gate sits over a thin oxide layer above a P-type body. Applying a positive gate-source voltage attracts electrons to the surface and, once that voltage passes the threshold, inverts the region into a conducting N-channel between drain and source. With no gate voltage the device is off, which is what enhancement mode means.
Operating regions
Below the threshold voltage the MOSFET is in cut-off and conducts only leakage. Just above threshold, with a small drain-source voltage, it behaves like a resistor whose value is set by the gate — the ohmic or linear region, characterised by the on-resistance RDS(on). Raise the drain-source voltage past the saturation point and the channel pinches off: drain current becomes almost independent of drain-source voltage and is controlled by the gate alone, which is the region used for amplification.
Gate drive and switching losses
The gate is an insulated capacitor, so a MOSFET needs essentially no steady-state drive current — but charging and discharging that gate capacitance takes real current, and that determines how fast it can switch. Total losses come in two parts: conduction loss, equal to the drain current squared times RDS(on), and switching loss during the transitions. Driving the gate hard and fully enhancing the channel keeps both small, which is why a switching MOSFET is usually driven well above its threshold.
Where it is used
Power MOSFETs switch the current in nearly every modern power converter: buck and boost regulators, motor drive inverters, class-D amplifiers, battery protection circuits and solid-state relays. Small-signal MOSFETs make up the logic gates inside every CMOS integrated circuit. Their high input impedance, fast switching and low on-resistance at moderate voltages have made them the default power switch below roughly 200 volts.