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