A MOSFET (metal-oxide-semiconductor field-effect transistor) is a transistor in which a voltage on an insulated gate electrode controls the current flowing between two terminals, the source and the drain, through a thin semiconductor channel. In semiconductors it is the most widely manufactured device ever made, forming the switches of every processor and memory chip as well as the power stages that feed them.
Because the gate is separated from the channel by an insulating layer, a MOSFET draws almost no steady gate current. It is controlled by charge rather than by continuous current, which is the property that makes CMOS logic and efficient switching power supplies possible.
How a MOSFET controls current
A MOSFET has four terminals: gate, source, drain and body. The gate sits on a thin dielectric, historically silicon dioxide and in modern chips a high-k material such as hafnium oxide. When the gate voltage exceeds the threshold voltage (Vth), the electric field attracts carriers to the surface and forms a conductive inversion channel between source and drain.
In an n-channel MOSFET, the channel carries electrons and turns on with a positive gate voltage. A p-channel MOSFET carries holes and turns on with a negative gate voltage relative to the source. Electrons are more mobile, so n-channel devices conduct more current for the same size, which is why power stages favor them.
| Region | Condition | Behavior | Typical use |
|---|---|---|---|
| Cutoff | Gate below threshold | Practically no drain current | Switch off |
| Linear (triode) | Gate above threshold, small drain voltage | Acts like a voltage-controlled resistor | Switch fully on |
| Saturation | Gate above threshold, larger drain voltage | Current set mainly by gate voltage | Amplifiers and current sources |
| Subthreshold | Gate slightly below threshold | Small exponential leakage current | Ultra-low-power circuits |
Key MOSFET parameters
The parameters that matter depend on whether the device is a tiny logic transistor or a discrete power MOSFET. For power devices, datasheets focus on resistance, voltage rating and gate charge; for logic transistors, the focus is on drive current, leakage and switching energy.
| Parameter | Typical range | What it affects |
|---|---|---|
| Drain-source voltage rating (VDS) | About 20 V to 1,000 V or more | Maximum blocking voltage |
| On-resistance (RDS(on)) | Below 1 mΩ to several ohms | Conduction losses and heating |
| Gate threshold (Vth) | About 1 to 4 V | Drive voltage needed |
| Total gate charge (Qg) | A few nC to hundreds of nC | Switching speed and driver load |
| Junction temperature limit | Typically 150 to 175 °C | Safe operating margin |
A central trade-off in power design is on-resistance versus gate charge. A larger die lowers RDS(on) and conduction losses but increases capacitance and switching losses. The best device for a converter therefore depends on its switching frequency and load current.
MOSFET structures in chips and power electronics
Logic MOSFETs have evolved from flat planar transistors to three-dimensional structures that wrap the gate around the channel. Stronger gate control reduces short-channel effects and leakage, which is essential as features shrink through successive photolithography nodes.
| Structure | Gate arrangement | Main use |
|---|---|---|
| Planar MOSFET | Gate on top of a flat channel | Older logic nodes, analog circuits |
| FinFET | Gate on three sides of a vertical fin | Logic from roughly the 22 nm class onward |
| Gate-all-around (nanosheet) | Gate surrounds stacked channels | Leading-edge logic nodes |
| Trench power MOSFET | Vertical current flow through a trench gate | Low-voltage power stages |
| Silicon carbide MOSFET | Wide-bandgap vertical device | High-voltage converters and inverters |
On a motherboard or graphics card, the voltage regulator module uses power MOSFETs, often integrated with drivers as smart power stages, to convert 12 V to about 1 V for the processor at hundreds of amperes. Their efficiency and temperature directly influence how much power the chip can draw.
MOSFET compared with related devices
The MOSFET is one member of the broader transistor family. Its insulated gate distinguishes it from the bipolar junction transistor, which is controlled by a base current, and from the IGBT, which combines a MOSFET gate with bipolar conduction for high-voltage, high-current switching.
| Device | Control method | Strength | Weakness |
|---|---|---|---|
| MOSFET | Gate voltage | Fast switching, easy to drive | Resistance rises at high voltage |
| Bipolar transistor | Base current | Linear, low noise amplification | Continuous drive current |
| IGBT | Gate voltage | High voltage and current | Slower switching, tail current |
| GaN HEMT | Gate voltage | Very fast, compact chargers | Sensitive gate, higher cost |
Common failure modes and design mistakes
- Driving the gate too weakly keeps the device in its linear region during transitions, causing heavy switching losses and overheating.
- Exceeding the gate voltage rating, often about ±20 V, can rupture the thin oxide permanently.
- Electrostatic discharge damages unprotected gates during handling.
- Ignoring thermal design lets RDS(on) rise with temperature, which increases losses further.
- Inductive voltage spikes at turn-off can exceed VDS and trigger avalanche breakdown without snubbers or clamping.
In simple terms
A MOSFET is a tiny electronic switch with no moving parts. A small voltage on its control pin opens or closes a path for electric current. It is a bit like a water tap that opens when you press gently on the handle.
Billions of these switches inside a processor turn on and off to compute. Larger versions on a motherboard control the power that feeds the processor. In both cases, the switch needs almost no energy to stay in place.
A practical example
A hardware engineer designs the processor power supply for a compact workstation board. The processor can demand up to 200 A at about 1.1 V, so the design uses a multiphase buck converter with eight phases, each built around a smart power stage containing two n-channel MOSFETs and their driver.
The engineer compares candidate stages by on-resistance and gate charge at a switching frequency of 600 kHz. A device with slightly higher RDS(on) but much lower gate charge wins, because switching losses dominate at that frequency. In thermal tests, the stages stay below 90 °C under full load, and converter efficiency remains above 90 percent across most of the load range.