CMOS Transistor Basics

VLSI Concept · Fundamentals

An inverter is just two switches that are never on at the same time.

A CMOS gate pairs a PMOS transistor (conducts when input is low) with an NMOS transistor (conducts when input is high), wired so exactly one of them connects the output to power or ground at any time — never both. Stacking and combining these transistor networks is literally how AND/OR/NAND/NOR gates are built at the silicon level.

PMOSConducts when gate input = 0 (pulls output to VDD / logic '1').

NMOSConducts when gate input = 1 (pulls output to GND / logic '0').

Why it mattersThis complementary structure is why CMOS barely draws current in steady state — the root of the static vs dynamic power split.

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