NMOS transistor as a switch
Operation, truth table, and strong 0 versus weak 1
An NMOS transistor is a switch controlled by its gate. With a high enough gate-to-source voltage it conducts; with the gate at the supply, it passes a strong 0 but a threshold-dropped weak 1.
Try the guided transistor switch →NMOS switch at a glance
| Gate control | Channel | Output with a ground connection |
|---|---|---|
| 0 | Open (off) | Z: disconnected |
| 1 | Closed (on) | 0: connected to ground |
This is a digital switch model: it shows
0, 1, Z, and contention, not analog voltages. The strong-0/weak-1 explanation below describes a real NMOS with its gate driven at the supply voltage; the lab does not calculate the threshold voltage drop.From the MOSFET you know an NMOS forms its channel when the gate is driven high. This lesson answers the more useful question: once it is conducting, how well does it pass a
0 versus a 1? The answer is lopsided, and that lopsidedness is the single reason NMOS is used the way it is in every CMOS gate.When it conducts
An NMOS turns on when its gate is a
1 and is off when its gate is a 0. Remember it as N for eNable-on-1. While on, its source and drain are connected and a signal can pass; while off, they are isolated. So far it is the plain switch from the last lesson.Strong 0, weak 1
The subtlety is what happens to the value it passes. An NMOS conducts only while its gate voltage stays at least a threshold (
Vt) above its source. Watch what that means in the two cases:- Passing a
0(pulling the output down toGND). The output falls toward0 V. As the source terminal drops, the gate (1, nearVCC) stays far above it, so the gate-to-source voltage stays large and the transistor keeps conducting hard all the way down. The output reaches a clean, full-strength0. This is a strong 0. - Passing a
1(trying to pull the output up toVCC). As the output rises, the source terminal rises with it. But the gate is fixed at1(aboutVCC), so the gate-to-source voltage keeps shrinking. The moment the output climbs to withinVtof the gate, the gate-to-source voltage drops toVtand the channel cuts off. The output stops rising at roughlyVCC - Vt, short of a full1. This is a degraded weak 1.
An NMOS delivers a strong 0 and a weak 1. It is superb at dragging a wire down to ground and mediocre at pushing it up to the supply. That is why, in every static gate you will build, NMOS is used only in the pull-down network (the path to
GND), never to pull an output up. Use each transistor on its strong side.An analogy: an NMOS pulling a wire high is like trying to push a rope up over a pulley. You can get it moving, but you lose grip near the top and it never quite reaches. Pulling the same rope down is easy and you can pull it all the way to the floor. The NMOS pulls down beautifully and pushes up poorly. The PMOS is the exact opposite.
| gate | input passed | output |
|---|---|---|
| 0 | 0 | Z |
| 0 | 1 | Z |
| 1 | 0 | strong 0 |
| 1 | 1 | weak 1 |
0: off, nothing passes (output floats to Z). Gate 1: it passes a full-strength 0 but only a threshold-dropped weak 1.A common mistake is to reach for a lone NMOS to route a
1 somewhere (a naive pass-transistor mux, say) and then wonder why downstream gates behave oddly. That weak 1 sits below full VCC, so a gate reading it can leak current and switch slowly, or misread it entirely. If you must pass a full 1 through a switch, you need a transmission gate (an NMOS and PMOS in parallel), which restores the strong 1.Check yourself
An NMOS has its gate tied to
VCC (a permanent 1) and you feed a 1 into one channel terminal. What does the other terminal read, and why not a full 1?Answer
It reads a weak 1, roughly
VCC - Vt, not a full VCC. As the output rises it lifts the source with it, shrinking the gate-to-source voltage; once the output gets within Vt of the gate, the channel cuts off and it stops rising. The NMOS simply cannot deliver the top Vt of the range, which is why complementary static CMOS uses PMOS for its pull-up network.Frequently asked
When does an NMOS transistor conduct?
An NMOS conducts when its gate is a
1 (driven high, past the threshold above its source) and is off when its gate is a 0. Remember it as N for eNable-on-1.Why does an NMOS pass a strong 0 but a weak 1?
When it pulls an output down, the source stays low so the gate-to-source voltage stays large and it conducts fully, giving a strong
0. When it tries to pass a 1, the rising output lifts the source until the gate-to-source voltage falls to the threshold and the channel cuts off, so the output stops about a threshold voltage short of the supply: a weak 1.Can a single NMOS be used as an analog or pass switch?
Yes, an NMOS on its own works as a simple pass switch: drive the gate high and the channel conducts in either direction. The limitation is the weak 1: signals near the supply come through degraded by a threshold voltage. That is why real analog switches and signal multiplexers pair the NMOS with a PMOS as a transmission gate, which passes both levels at full strength.
Why is NMOS used in the pull-down network?
Because it delivers a full-strength
0 when connecting an output to ground, but only a degraded 1 when pulling up. Using it on its strong side, to pull the output down to GND, gives a clean logic 0, so every static CMOS gate builds its pull-down network from NMOS.Next: the PMOS, the mirror-image switch that closes on a
0 and delivers a strong 1, so it takes the job of pulling outputs up.You've got the theory. Now try it in the lab and watch it work.
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