Q3VLSI Design

The basic NMOS inverter consists of a driver (switching) NMOS transistor with its source grounded and drain connected to the output, in series with a load element (either a resistor, a depletion-mode NMOS transistor with gate tied to source, or an enhancement-mode NMOS with gate tied to drain) connected between the output and VDD. The CMOS inverter instead replaces this passive/always-on load with a PMOS transistor whose gate is driven by the same input as the NMOS, so exactly one of the two transistors conducts strongly for any static input level (PMOS on and NMOS off for input low, giving output high; NMOS on and PMOS off for input high, giving output low), eliminating the static current path present in the NMOS-only inverter and giving CMOS its hallmark near-zero static power dissipation.