The MOSFET switch

Q: What is the threshold voltage of a MOSFET?

The threshold voltage (Vt) is the gate-to-source voltage at which a conducting channel first forms. Below Vt the transistor is off; above it the channel is present and it conducts. Logic levels are chosen so a 1 sits well above Vt and a 0 well below, giving clean switch behavior.

Gate, source, drain, and the insulated handle

6 min read

A MOSFET (metal-oxide-semiconductor field-effect transistor) is a switch with three working terminals (gate, source, drain) whose insulated gate voltage decides whether a conducting channel forms between the source and drain; because the gate is insulated it draws almost no steady current.

Builds onThe transistor Logic as switches

Build it in the lab →

From the transistor you have the one-line idea: a switch you close with a voltage. And from logic as switches you know why a controllable switch is all logic needs. This lesson zooms in on the specific switch modern chips use, the MOSFET, so that when the next lessons say "the gate sees a 1" you know exactly which terminal that is and what physically happens.

MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. The name is a description of its construction: a metal (or polysilicon) control terminal sits on a thin layer of insulating oxide, which sits on a semiconductor body. The word that matters most is field-effect: the switch is turned on by an electric *field* from the gate, not by current flowing into it.

The three working terminals

**Gate (G)** is the control terminal, the handle. Its voltage decides whether the switch conducts. It is separated from everything below it by the insulating oxide, so it never passes current into the channel.
**Source (S) and drain (D)** are the two channel terminals, the two ends of the switch. When the transistor conducts, current flows between them; when it does not, they are isolated.
Body (or bulk) is the underlying semiconductor. In this course it is always tied to a fixed rail (GND for an NMOS, VCC for a PMOS), so you can treat the transistor as a three-terminal switch and ignore the body once it is connected.

The gate is essentially one plate of a tiny capacitor: the oxide is the insulator, the channel region below is the other plate. Charging that capacitor (raising the gate voltage) pulls charge into the region under the oxide and forms a conducting channel between source and drain. Discharging it removes the channel. This is why the gate draws almost no *steady* current: you only push charge in briefly while switching, then it holds, exactly like charging a capacitor.

The threshold: how much voltage closes the switch

The switch does not turn on the instant the gate voltage lifts off zero. A channel forms only once the gate voltage (measured relative to the source) passes a fixed step called the threshold voltage, written Vt. Below Vt there is no channel and the transistor is off; above Vt the channel is present and it conducts. For our purposes the logic values are chosen so that a 1 is comfortably above Vt and a 0 is comfortably below it, so the transistor behaves as a clean open-or-closed switch rather than something in between. Step through a channel bridging the gap as the gate crosses the threshold in the interactive MOSFET simulator.

The kind of MOSFET used in logic is enhancement mode: it is off with zero gate voltage and must be actively *enhanced* (driven past Vt) to turn on. That is the sensible default for a switch, you want it open until you tell it to close.

Two polarities, opposite thresholds

There are two flavors, and they are mirror images. An NMOS builds its channel when the gate is driven *high*, so it turns on for a gate 1. A PMOS builds its channel when the gate is driven *low* (relative to its source, which sits at VCC), so it turns on for a gate 0. The PMOS symbol carries a small bubble on the gate to remind you of that inversion. The next two lessons take each one apart in detail, because which rail each is good at reaching is the key to how gates are wired.

One NMOS wired as a plain switch: EN is the gate, and the source and drain carry A through to F only when the gate is high. Open it in the lab and toggle EN to watch the channel form and vanish.

The gate drawing almost no current does not mean it is free to leave unconnected. A gate wire with nothing driving it floats to an undefined voltage (Z) and can hover right around Vt, leaving the transistor half-on. Every gate input must be driven to a real 0 or 1. A floating gate is one of the most confusing faults to debug because the transistor behaves erratically rather than simply staying off.

Check yourself

An NMOS has its source at 0 V (GND) and Vt = 0.5 V. Roughly what gate voltage turns it on, and does raising the gate voltage cost a steady current?

Frequently asked

What is a MOSFET?

A MOSFET (metal-oxide-semiconductor field-effect transistor) is a voltage-controlled switch with three working terminals: a gate, a source, and a drain. The gate's voltage builds or removes a conducting channel between the source and drain. Because the gate sits on an insulating oxide layer, it draws almost no steady current.

What are the gate, source, and drain of a MOSFET?

The gate is the insulated control terminal whose voltage switches the device on or off. The source and drain are the two channel terminals, the two ends of the switch, between which current flows when the transistor conducts. A fourth terminal, the body, is tied to a fixed rail and can usually be ignored.

What is the threshold voltage of a MOSFET?

The threshold voltage (Vt) is the gate-to-source voltage at which a conducting channel first forms. Below Vt the transistor is off; above it the channel is present and it conducts. Logic levels are chosen so a 1 sits well above Vt and a 0 well below, giving clean switch behavior.

Why does a MOSFET gate draw almost no current?

Because the gate is separated from the channel by a thin insulating oxide layer, so it acts like one plate of a capacitor. Charging or discharging that capacitor to switch the transistor takes a brief pulse of current, but no steady current flows through the insulator.

Next: NMOS in detail, the switch that is excellent at pulling a wire down to 0 and only mediocre at pulling it up, and why that single fact decides where it belongs in every gate.

You've got the theory. Now build it from scratch and watch it work.

Build it in the lab →

Builds towardNMOS: the strong-0 switch