Power transistor

A power MOSFET is a transistor built for brawn rather than brains. Instead of billions of tiny separate switches, its die acts as one big switch, made of thousands of small cells working in parallel. It can block hundreds of volts, carry several amps, and turn on and off tens or hundreds of thousands of times a second.

It's made from doped silicon. Unlike the transistors on a logic chip, current flows vertically, from the top surface down through the die to its back. Layers and wells of n-type and p-type silicon are built into the wafer, with a gate on top that opens a channel when charged. The back of the die is the drain, so it's soldered directly to the metal tab of a lead frame, which carries both current and heat away. Bond wires, often thicker than in ordinary chips, connect the top to the legs, and mold compound forms the body. Packages like the TO-220 have a metal tab that bolts to a heat sink.

On the power supply board, power MOSFETs chop current on and off so fast that a small transformer can do the work of a big one. That's why modern power supplies are light and efficient.