The four MOSFETs

N-channel or P-channel, enhancement or depletion — four parts, four symbols, and you can read which one you are holding straight off the schematic. One of the four is in almost everything you will build.

A datasheet will tell you which of four kinds a part is, in a phrase that means nothing until somebody unpacks it. Pick each one and drag the gate voltage.

Four MOSFETs, one question

N-channel enhancement · off

Gate, relative to the source0.0 V

N-channel enhancement: gate about 2 V above the source, or more. Nearly every switch you will ever build. Off until the gate goes positive, which is what makes it safe to wire to a pin that starts up as an input. The gaps in the symbol are the giveaway — there is no channel until the gate makes one, which is why it starts up off.

Two questions, and the symbol answers both.

Are the bars broken or solid? Three bars with gaps means enhancement: there is no channel until the gate makes one, so the part is off with nothing connected. One solid bar means depletion: the channel is built in, the part conducts at rest, and the gate's job is to squeeze it shut.

Which way does the arrow point? In means N-channel — turn it on by taking the gate above the source. Out means P-channel — turn it on by taking the gate below the source.

N-channel enhancement is the one to learn. It is off until you ask, which is the only safe behaviour for something wired to a pin that starts up as an input, and for the same money it conducts better than the P-channel equivalent. Depletion parts exist and you will not meet one in a kit.

The gate is measured against the source, not against ground

This is the sentence that decides where the MOSFET goes in the circuit, and reading it is not the same as believing it. Three wirings, one load, one 3.3 V pin.

Above the load, or below it

N-channel · low side

Load gets

0.00 V

Current

0 mA

Of what it wanted

0 %

Nothing is on yet. Three wirings, one load, one 3.3 V pin. Two of them work.

The middle one is the trap. An N-channel part above the load turns on, its source rises with the load, V_GS shrinks as it does — and it settles at the point where the gate is only just above the threshold. The load runs at a fraction of its current, the MOSFET gets hot, and nothing about the wiring looks wrong.

So:

Switching something on the ground side — the usual case — use an N-channel part underneath the load. Source to ground, drain to the load, gate to the pin.
Switching the positive rail — when the load's ground has to stay connected — use a P-channel part above it, and remember you now need something that can pull its gate all the way back up to the rail to turn it off. That is what the small transistor is doing in every high-side switch circuit you will see.

The reverse-polarity protection on a battery input is the same P-channel part doing the same trick, wired so that the body diode points the way you want current to go.

The gate is measured against the source, not against ground

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