OE-10.1 Introduction to MOS Technology

What this is and why it exists

Everything in this subject rests on one device behaving as a voltage-controlled switch. Time spent here pays back through every later unit, so it is worth being slow.

The number that decides when the switch turns on is the threshold voltage. And the thing worth knowing about it is that it is not a constant.

That is the body effect. The voltage between source and body shifts the threshold, which matters as soon as transistors are stacked. In real logic they are stacked constantly.

The vocabulary

• Gate — the terminal whose voltage controls the switch.
• Source and drain — the two terminals current flows between.
• Body — the substrate the device sits in, which is also a terminal.
• Channel — the conducting path that appears under the gate.
• Threshold voltage — the gate voltage at which the channel forms.
• Enhancement mode — off with no gate voltage; a voltage turns it on.
• Depletion mode — on with no gate voltage; a voltage turns it off.
• Body effect — the shift in threshold caused by source-to-body voltage.

The mental model

A voltage on the gate attracts charge carriers to the surface underneath it. Below a certain gate voltage there are too few to matter and the device is off. Above it a conducting channel forms and current can flow between source and drain. That gate voltage is the threshold, and the device is a switch controlled by it.

Enhancement and depletion modes differ only in where that threshold sits relative to zero. An enhancement device is off with nothing applied and needs a voltage to turn on. That is what logic wants, because a gate with no input should not be conducting. A depletion device conducts with nothing applied and needs a voltage to turn off, which suits it to being a load rather than a switch.

Now the body effect, and it is worth understanding rather than memorising because it explains a real design problem. The body is a terminal too. Raise the source above the body and the threshold rises — the device becomes harder to turn on.

Why does that matter? Because in real logic, transistors are stacked in series. The one at the bottom has its source at ground and behaves as designed. The one above it has its source at whatever the bottom one is holding, which is above ground. Its threshold is therefore higher, and it conducts less well than the same transistor would alone. Stack more and it gets worse.

That is why a gate with many stacked inputs is slower than one with few. It is also why the effect appears in every later unit rather than only this one.

What you should now be able to explain or do

Describe MOS action as a voltage-controlled switch, and say what the gate voltage does physically. Distinguish enhancement from depletion mode and say which suits logic and why. State what the threshold voltage decides. Explain the body effect in terms of the source-to-body voltage. Say why stacked transistors conduct less well, and connect that to gate speed.

Check yourself