N-channel and P-channel MOSFETs

Just as BJTs come as NPN and PNP, MOSFETs come as N-channel and P-channel, mirror images of each other. An N-channel MOSFET conducts through a channel of negative carriers (electrons) and is turned on by pulling its gate ABOVE its source. A P-channel MOSFET conducts through positive carriers (holes) and is turned on by pulling its gate BELOW its source. They are complementary partners: one likes its control voltage high, the other likes it low.

In practice the N-channel device is the better switch. Electrons move more easily than holes, so for the same chip area an N-channel part has lower on-resistance, handles more current, and switches faster than an equivalent P-channel part. That is why N-channel parts dominate power switching and sit on the low side (between the load and ground), where their gate is easy to drive against ground. The P-channel is prized for simplicity on the high side (between the supply and the load): you can turn it on just by pulling its gate down toward ground, with no special high-side gate-drive trickery.

The real magic appears when you use both together. A complementary N-and-P pair, with the N pulling the output down and the P pulling it up, wastes almost no power when idle, because one of the two is always off. This complementary pairing is the heart of CMOS logic and of clean push-pull output stages. Choosing N versus P, and which side of the load to put it on, is one of the first design decisions in any switching circuit.