From transistor to logic gate

A MOSFET can be reduced to its simplest function: a voltage-controlled switch. Current either flows between source and drain or it doesn't, depending on the voltage applied to the gate. This makes it fundamentally different from a mechanical switch — nothing moves, there's no wear, and switching takes only a fraction of a nanosecond.

In an enhancement-mode NMOS transistor, no conducting channel exists between source and drain without gate voltage. Only once the gate voltage exceeds the threshold voltage does the channel form, and the transistor conducts. Think of the gate as a switch's control input: 0 V at the gate → switch open; enough positive voltage at the gate → switch closed.

Alongside the NMOS transistor there's its counterpart, the PMOS transistor. It behaves as a mirror image: while the NMOS conducts at a high gate level, the PMOS blocks at exactly that point — and instead conducts at a low level. This complementarity isn't a footnote; it's the core principle behind all of CMOS technology ("Complementary MOS").

The NMOS is typically used as the connection to ground (0 V), the PMOS as the connection to the supply voltage. This interplay lets both transistor types together switch an output cleanly between High and Low, without ever creating a direct short between supply and ground.