CMOS logic uses two complementary MOSFET types: NMOS and PMOS.
They respond to input voltage in opposite useful directions.
NMOS
An NMOS transistor is good at pulling a node down toward ground.
In the simplified switch model:
- input high turns NMOS on
- input low turns NMOS off
- when on, it can provide a path from output toward ground
This makes NMOS useful for pull-down networks.
PMOS
A PMOS transistor is good at pulling a node up toward the supply voltage.
In the simplified switch model:
- input low turns PMOS on
- input high turns PMOS off
- when on, it can provide a path from output toward the positive supply
This makes PMOS useful for pull-up networks.
Complementary Behavior
The key is that one input can control two opposite paths:
| Input | NMOS | PMOS |
|---|---|---|
| Low | Off | On |
| High | On | Off |
This complementarity is the basis of CMOS.
CMOS stands for complementary metal-oxide-semiconductor. It uses NMOS and PMOS together so a logic output is usually pulled strongly toward one valid voltage range or the other.
Boundary
The switch model hides many analog details, but it gives the right digital direction:
NMOS is the usual pull-down switch. PMOS is the usual pull-up switch. CMOS uses them together so inputs select a high or low output path.