What is a MOSFET?

A MOSFET (metal-oxide-semiconductor field effect transistor) is a voltage-controlled semiconductor switch. Using a MOSFET as a switch is its most common role: a voltage on the insulated gate turns the current between drain and source on or off, while the gate itself draws almost no current. That combination of fast, efficient, voltage-driven switching is what makes it the default choice for electronic switching.

For the full working principle, see how does a MOSFET work.

For an introduction to field effect transistors, see what is a Field Effect Transistor (FET).

What is MOSFET a Gate, Drain and Source schema

How MOSFETs work schema

Characteristics of a MOSFET

As a switch, a MOSFET is defined by how cleanly it moves between fully on and fully off:

  • When on, it presents a very low on-resistance (RDS(on)), so it drops little voltage and wastes little power.
  • When off, it blocks the voltage across its drain terminal with virtually no leakage — effectively an open circuit.
  • It switches fast, because it is a majority-carrier device, which keeps switching losses low even at high frequency.
  • Its gate terminal is voltage-controlled and draws no steady current — unlike a bipolar junction transistor (BJT), which needs a continuous control current — so very little power is needed to hold it on or off.

A MOSFET switch spends almost all of its time either fully off or fully on; the brief transition between the two is where switching loss occurs, which is why fast switching matters.

The detailed voltage limits that bound safe switching — breakdown ratings and derating — are listed in the device datasheets and covered in MOSFET voltage ratings- how to read and apply breakdown voltage specs.

MOSFET types

MOSFET switches include N-channel (best for low-side, efficient switching) and P-channel (simpler high-side), as well as enhancement-mode (normally off) and depletion-mode (normally on).

Power MOSFETs handle higher currents and voltages, while small-signal types suit logic loads. N-channel devices are preferred for low conduction losses in low-side designs.

For example, Infineon's N-channel MOSFET portfolio offers devices optimized for efficient switching across a wide range of applications.

For the full breakdown of MOSFET types and structures, see how does a MOSFET work.

MOSFET operation modes: enhancement and depletion

MOSFET operation

From gate voltage to drain current

Used as a switch, an enhancement-mode MOSFET is normally off. Applying a gate-source voltage a few volts above the threshold voltage (VGS(th)) forms the channel and turns it on, creating a low-resistance path from drain to source; removing that gate voltage turns it off again. Because the gate is insulated, maintaining either state requires almost no power.

One of the most common uses of a transistor is as a switch. In a switching circuit, the MOSFET rapidly toggles between its off and on states to control a load. When that load is inductive — a motor, relay, or solenoid — a flyback diode is mandatory to absorb the voltage spike generated when the device switches off.

Application examples

Switch circuits

Typical MOSFET switching circuits include:

  • LED drivers: the MOSFET controls the current through an LED
  • Relay drivers: microcontrollers use a MOSFET to switch relays safely — and unlike a mechanical relay, the solid-state switch is faster, silent, and has an effectively unlimited number of switching cycles
  • Motor drivers: the MOSFET regulates current in DC motors, often switched with PWM (pulse-width modulation) to control speed

In these circuits, the MOSFET rapidly switches between its off and fully-on states, enabling efficient electronic control of loads.

Power conversion

MOSFETs dominate power conversion — DC-DC converters, motor control, H-bridge drivers, and battery management systems all rely on fast MOSFET switching, including high voltage switching in power supplies and inverters, with an H-bridge using four devices to drive a motor in either direction.

To learn more about how MOSFETs are used across power electronics, see what is a Field Effect Transistor (FET).

Logic (CMOS and NMOS), MCU interfacing, and level shifting

At the system-board level, MOSFETs perform interfacing jobs that ICs alone cannot handle:

  • Level shifting. A 3.3 V microcontroller GPIO cannot directly drive a 5 V or 12 V peripheral. A single N-channel MOSFET level-shifts the logic signal: when the gate is pulled to the higher supply, the source follows the drain and the signal is translated. Bidirectional level shifters handle I²C and similar open-drain buses.
  • Microcontroller load interfacing. Any load drawing more than ~25 mA must be driven through a transistor stage. The MCU GPIO drives the gate; the MOSFET drives the load. This applies to motors, solenoids, high-brightness LEDs, heating elements, and fans.
  • Digital logic. TTL and CMOS logic families are built almost entirely from transistors — modern CMOS from MOSFETs (an NMOS paired with a PMOS device), older families from bipolar transistors. Every NAND gate, flip-flop, multiplexer, and register in a digital system is a configuration of transistors performing switching operations at nanosecond timescales, and billions of them make up the integrated circuits in every processor.
Recommended documents

Frequently Asked Questions about how MOSFETs work as a switch

What are the operation modes of a MOSFET?

By construction, a MOSFET is either enhancement-mode (normally off — it needs a gate voltage to turn on) or depletion-mode (normally on — it needs a gate voltage to turn off). As a switch, it works between two states: cutoff (fully off) and a low-resistance on-state.

The main distinctions are N-channel vs. P-channel, enhancement-mode vs. depletion-mode, and power vs. small-signal devices. For switching, enhancement-mode N-channel power MOSFETs are the most common.

For the full overview, see see how does a MOSFET work.

For low-side switching, choose an enhancement-mode N-channel MOSFET with a low RDS(on) for the lowest conduction loss.

For high-side switching, either a P-channel device (simpler drive) or an N-channel device with a dedicated gate driver. In every case, the device must block your maximum drain-source voltage with margin and carry the load current comfortably.

Match the device to the switching job: it must block your maximum drain-source voltage with margin, carry the load current with a low enough RDS(on), and switch fast enough for your operating frequency, all within its thermal and safe-operating-area limits.

For a full selection walkthrough, see see how does a MOSFET work.

You turn a MOSFET on by applying a gate voltage above its threshold, and off by pulling the gate back below it. For small-signal MOSFETs with low gate capacitance, a microcontroller such as an Arduino can drive the gate directly. Power MOSFETs need a dedicated MOSFET driver (gate-driver IC) to charge the larger gate capacitance fast enough to minimise transition losses.

For more on driving, see how does a MOSFET work.

Infineon provides online product finders and parametric search to filter its MOSFET and gate-driver portfolio by voltage, current, RDS(on), and package, along with SPICE models, simulation tools, and evaluation boards to validate a design before building it.

See available selection tools: