What a MOSFET is

A transistor is controlled by a current. A MOSFET is controlled by a voltage, and holding it on costs nothing at all — which changes how you wire it, what you can switch with one pin, and what goes wrong when it is half open.

The last page ended with a rule — give the base a twentieth of the load current — and a warning that the sum gets hard once the load is big. A MOSFET does not have that sum. Its control pin is not asking for current at all.

Here are both parts switching the same lamp from the same pin. Watch the control wires rather than the lamps.

Same lamp, same pin, two parts

both off

Into the base

Into the gate

Each lamp gets

Two parts doing one job. Both lamps are off, both pins are low. Press the button and watch the control wires, not the lamps.

Both work. Only one of them is still taking anything from the pin afterwards.

Three legs, and one of them is different

Source and drain are the two ends of the path the current takes — the part of a MOSFET that behaves like a switch contact. Gate is the handle.

The gate is separated from everything else by a layer of glass a few atoms thick. It is not connected to the source, it is not connected to the drain, and no current can cross it. What it can do is sit there holding a voltage, and the electric field from that voltage reaches through the glass and opens a path between the other two legs.

That is the whole part. Everything else in this chapter is a consequence of it.

There is a voltage below which nothing happens

The handle is not proportional from zero. Below a voltage called the threshold — printed on the datasheet as V_GS(th) — there is no path at all, and turning the knob does nothing.

This is a real light dimmer, two resistors and a MOSFET. Drag it slowly.

Turn the knob, watch the lamp

2N7000 · threshold 0.8–3.0 V

The knob0.00 V on the gate

Lamp current

0 mA

Across the MOSFET

6.00 V

Heat in it

0 mW

Nothing. And you have turned the knob 0 % of the way. Below about 2.1 V the gate has not made a channel yet, so there is no path from the lamp to ground at all. That dead travel is the threshold voltage, and every MOSFET has one.

Three things in that drawing are worth keeping:

Most of the travel does nothing. That dead zone is the threshold, and every MOSFET has one.
Just above it, the part is a resistor. The lamp dims because the MOSFET is eating the difference, and eating it means turning it into heat.
Above that it stops caring. More gate voltage changes nothing, because the channel is already as open as it goes.

Off and fully on are the two states you want a switch in. Everything between them is heat in a part that was not sold as a heater. A dimmer is a legitimate use of the middle — a 100 mA lamp will survive it — and the same setting on a 2 A motor is a part you can smell.

What the gate does cost

"Takes no current" is true while it is sitting still and false at the moment it moves. The gate is a small capacitor, so changing its voltage means pushing charge in or pulling charge out, and for that instant it is happy to take an amp. Hold it on for a second and that cost is invisible; switch it on and off at 20 kHz and it is a real current, which is why PWM circuits have driver chips in front of the gate.

Next: what is actually inside one, and why a voltage on the other side of a layer of glass does anything at all.

Three legs, and one of them is different

There is a voltage below which nothing happens

What the gate does cost

Discuss this article