MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)

The metal-oxide-semiconductor field-effect transistor (MOSFET) is the most widely manufactured transistor in electronics. It is a field-effect device with four functional regions: source, drain, gate and body (substrate). The defining feature is the gate, which is separated from the semiconductor channel by a thin layer of insulating oxide. Because of this insulation the gate draws essentially no steady current and controls the channel purely by the electric field it produces, giving the MOSFET a very high input impedance.

MOSFETs come in n-channel and p-channel types, and in enhancement and depletion modes. The common enhancement-mode device is off at zero gate voltage; applying a gate-to-source voltage above a threshold attracts carriers to form a conducting channel between source and drain. By switching between a high-resistance off state and a low-resistance on state, MOSFETs act as electronic switches. Pairing complementary n-channel and p-channel transistors produces CMOS logic, which draws very little static power and forms the basis of almost all modern processors and memory.

Power MOSFETs are designed to handle large currents and voltages, typically using vertical structures to spread current and dissipate heat. They are characterised by a low on-state resistance and fast switching, which makes them efficient in switched-mode power supplies, DC-DC converters, motor controllers and class-D amplifiers, especially at higher switching frequencies. For very high voltages and currents the IGBT is often preferred, while at lower voltages power MOSFETs usually give the best efficiency. Because the thin gate oxide is easily damaged, MOSFETs require care against electrostatic discharge.

Frequently asked questions

What does enhancement mode mean?

An enhancement-mode MOSFET is non-conducting at zero gate voltage and turns on only when the gate-source voltage exceeds a threshold, enhancing a channel between source and drain.

Why is CMOS so power-efficient?

CMOS pairs complementary transistors so that, in a steady logic state, one device is always off. This blocks any continuous current path, so static power consumption is very low.

What limits a power MOSFET's high-voltage use?

On-state resistance rises sharply with voltage rating, increasing conduction losses. For very high voltages the IGBT, with its lower forward drop, is often more efficient.