CMOS (Complementary metal–oxide–semiconductor) is a technique for constructing digital logic circuits from two complementary MOS transistors - pMOS and nMOS. CMOS is the dominant technology used for VLSI and ULSI circuit chips used for anywhere from SRAM to microcontrollers and microprocessors.

Overview

Main article: CMOS Complementary Topology

CMOS primarily makes use of what would otherwise be two separate circuit technologies - pMOS and nMOS. To better understand this, consider an nMOS transistor. Because it can pull no higher than V_DD - V_t we get a degraded 1 output. Likewise with pMOS, we can pull no lower than V_t - a degraded 0 output. By combining both types, we can borrow the desired characteristics from both transistors such as a strong 0 and a strong 1.

Inverter Example

Main article: inverter

The simplest gate that can be implemented is the NOT gate which simply inverts the input. We can implement an inverter using a single nMOS and pMOS transistors. The pMOS transistor is connected to V_DD while the nMOS transistor is connected to GND. When A is 0, the nMOS transistor turns OFF and the pMOS transistor turns ON. This results in Q being pulled up to 1 since the pMOS transistor will conduct V_DD. Conversely, when A is 1, the nMOS transistor turns ON and the pMOS transistor turns OFF, thereby pulling Q down to GND.

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