MOSFET

A voltage-controlled power switch.

Schematic letter: Q
Unit / key rating: Drain-source voltage (V), current (A), RDS(on) (Ω)

Pinout

G (gate): The control pin; usually driven through a gate resistor.
D (drain): Where the load connects; on a TO-220 the tab is drain too.
S (source): On an N-channel part, usually to ground (or a current-sense resistor).

Real photos

Transistor and MOSFET packages: TO-220, TO-247, D²PAK, SOT and others

What is it for?

A MOSFET controls the current between drain and source with the voltage on its gate. The gate draws almost no current, so a small signal can switch large currents.

It's the main switch in switch-mode supplies, motor drives, inverters and LED drivers. It has a reverse body diode inside.

Things to watch out for

⚠ On diode mode, S to D should read ≈0.5 V (body diode) and OL the other way; near zero both ways means a shorted MOSFET.
⚠ When replacing a blown MOSFET, also measure the gate resistor, current-sense resistor and driver.
⚠ The gate is sensitive to static; don't handle it ungrounded in dry air.

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