MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

Notes

The gate has been polysilicon rather than metal since the 1970s, and metal only returned with high-k metal gate at 45 nm.

Overview

A MOSFET (metal-oxide-semiconductor field-effect transistor) is a transistor in which a voltage on a gate, insulated from the semiconductor by a thin layer, controls the current flowing between two terminals called the source and the drain. MOSFETs are the building blocks of almost all digital chips, and they are also sold as separate power switches for power supplies, motor drives and chargers. The name comes from the original metal, oxide and silicon stack; modern devices often use other gate materials and three-dimensional shapes but keep the name.

How it works

A MOSFET has a gate, a source, a drain and a body. In an n-channel device the source and drain are n-type regions set into a p-type body. When the gate voltage rises above the threshold voltage, it draws electrons to the surface under the gate and forms a thin conducting channel between source and drain. Below threshold the channel is almost empty and only a small leakage current flows. Because the gate is insulated, it draws almost no steady current, although switching quickly means charging and discharging the gate capacitance.

Above threshold, a small drain voltage makes the device behave like a resistor whose value the gate sets (the linear region). At larger drain voltages the current levels off and depends mainly on the gate voltage (saturation), which is how analogue amplifiers use it. A p-channel device is the mirror image, conducting when the gate is pulled low, and pairing the two types gives CMOS logic. Most are enhancement mode, off at zero gate voltage; depletion-mode devices conduct until the gate turns them off.

The insulator was originally thermally grown silicon dioxide. As it thinned towards about a nanometre, gate leakage grew, and leading processes switched to high-k dielectrics with metal gates around the 45 nm node. To keep the gate in control at short lengths, the channel then became a fin (FinFET) and later stacked sheets (gate-all-around). Discrete power MOSFETs, designed for high voltage and current, are compared by on-resistance, gate charge and safe operating area.

Examples

• The insulated-gate transistor made by John Atalla and Dawon Kahng at Bell Labs in 1959, using a thermally grown oxide.
• RCA's CD4000 family of CMOS logic chips, introduced in 1968, built from paired n-channel and p-channel MOSFETs.
• Texas Instruments NexFET power MOSFETs, offered in N-channel and P-channel versions for power supply designs.
• Infineon CoolSiC silicon carbide MOSFETs, used where higher efficiency and switching frequency are needed.

Why it matters

MOSFET behaviour sets the speed, power and leakage of every digital and most analogue chips, so device engineers, compact modelling teams and circuit designers all work from its characteristics. In power electronics, the choice of discrete MOSFET decides much of a converter's efficiency, heat and cost.

In the chip lifecycle, the MOSFET is defined during process development, captured in models and the design kit for circuit design, characterised on first silicon, and qualified with the product, for example against AEC-Q101 for automotive discretes.

It matters most to: Device Engineer, Device Modeling Engineer, Device Physicist, High-Voltage and Power Discrete Design Engineer, Power Device Researcher

Common confusions

Not the same as a bipolar junction transistor

A bipolar transistor is controlled by a current into its base. A MOSFET is controlled by a voltage on an insulated gate, which is why it draws almost no steady input current.

Not the same as CMOS

CMOS is a circuit style, and the process that supports it, combining n-channel and p-channel MOSFETs so that little current flows except while switching. The MOSFET is the single device. MOSFET vs CMOS

Not the same as IGBT

An IGBT uses a MOSFET-style insulated gate to control a bipolar structure, and is a separate device family. A power MOSFET conducts through its channel alone.

Sources

1. 1960: Metal Oxide Semiconductor (MOS) Transistor Demonstrated (opens in a new tab), Computer History Museum, 2026-10-22
2. 1963: Complementary MOS Circuit Configuration is Invented (opens in a new tab), Computer History Museum, 2026-10-22
3. MOSFETs (opens in a new tab), Texas Instruments, 2026-10-22
4. MOSFETs (opens in a new tab), Infineon Technologies, 2026-10-22
5. MOSFET (Wikipedia) (opens in a new tab), Wikimedia Foundation, 2026-10-22

Related terms

• CMOS
• Threshold voltage
• FinFET
• Gate oxide

Where it appears on the Hub

Used on 13 pages of the Hub. In each group, the pages that mention it most come first.