A MOSFET is a transistor controlled by voltage: a voltage between its gate and source decides whether current can flow between its drain and source. When fully on, it behaves like a very small resistance; when off, like an open switch. Because the gate draws almost no current, a tiny signal from a controller can switch amps of current.

The three terminals

  • Gate — the control input, insulated from the rest by a thin oxide layer.
  • Drain and source — the path that is switched.

N-channel and P-channel versions differ in polarity: an n channel mosfet turns on when the gate is driven above its source, a p channel mosfet when the gate is pulled below its source. Inside every power MOSFET there is also a body diode between drain and source, which matters for how it behaves when off.

Where MOSFETs appear on a laptop board

  • Buck converter switches — pairs of MOSFETs (high side and low side) switching at high frequency in every main regulator, including the CPU core rail.
  • Input and battery path switches — MOSFETs that connect or isolate the adapter and the battery, controlled by the charger IC.
  • Load switches — MOSFETs that turn rails on and off for different power states, for example creating +3VS from +3VALW.
  • Signal switches and level shifters — small MOSFETs that invert or translate control signals.

In schematics they carry designators starting with Q, or PQ in the power section on many designs. Dual MOSFETs in one package are common.

How MOSFETs fail

  • Short between drain and source — the most common power failure. A shorted switch can pull a rail to ground or connect two rails together.
  • Gate damage — a gate that leaks to the source or drain, so the MOSFET no longer switches cleanly.
  • Open — after overheating; less common.

A shorted high-side MOSFET in a buck converter is especially dangerous: it can connect the system rail straight to a low-voltage rail and damage everything on it.

Testing a MOSFET

On the board, with power off, measure in diode mode between each pair of pins and compare with an identical MOSFET elsewhere on the board or on a known-good board. A near-zero reading between drain and source in both directions indicates a short. Out of circuit, a simple test with a diode-mode meter can charge the gate and show the channel turning on and off. The full method is in MOSFETs on Laptop Motherboards: How to Test Them.

Key parameters

  • Gate threshold voltage — the gate voltage at which it starts to conduct.
  • RDS(on) — the resistance when fully on; lower means less heat.
  • Maximum drain-source voltage and current — must not be exceeded.

When replacing one, match the package, polarity, voltage and current ratings and, for power stages, RDS(on) and gate characteristics.