12.1. What is a MOS Transistor?
A MOS transistor is a voltage-controlled switch with three terminals:
- Gate (G): Control terminal
- Source (S): Input terminal
- Drain (D): Output terminal
NMOS Transistor
Structure:
Behavior:
- Gate = 1 (High): Switch CLOSED - conducts (D connects to S)
- Gate = 0 (Low): Switch OPEN - does not conduct (High-Z)
Characteristics:
- Good at passing 0: Strong logic 0
- Weak at passing 1: Threshold voltage drop (VDD - Vth)
- Faster: Higher electron mobility
- Pull-down: Used in pull-down networks
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PMOS Transistor
Structure:
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Behavior:
- Gate = 0 (Low): Switch CLOSED - conducts (D connects to S)
- Gate = 1 (High): Switch OPEN - does not conduct (High-Z)
Characteristics:
- Good at passing 1: Strong logic 1
- Weak at passing 0: Threshold voltage drop
- Slower: Lower hole mobility
- Pull-up: Used in pull-up networks
Truth Table:
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