The term 'Complementary Metal-Oxide-Semiconductor' (CMOS), refers to the device technology for fabricating integrated circuits using both n- and p-channel MOSFET's. Today, CMOS is the major technology in manufacturing digital IC's and is widely used in microprocessors, memories, and other digital IC's. The input to a CMOS circuit is always to the gate of the input MOS transistor. The gate offers a very high resistance because it is isolated from the channel by an oxide layer. The current flowing through a CMOS input is virtually zero, and the device is operated mainly by the voltage applied to the gate, which controls the conductivity of the device channel. The low input currents required by a CMOS circuit results in lower power consumption, which is the major advantage of CMOS over TTL. In fact, power consumption in a CMOS circuit occurs only when it is switching between logic levels. Moreover, CMOS circuits are easy and cheap to fabricate resulting in high packing density than their bipolar counterparts. CMOS circuits are quite vulnerable to ESD damage, mainly by gate oxide punchthrough from high ESD (Electro static Discharge) voltages. Therefore, proper handling CMOS IC's is required to prevent ESD damage and generally, these devices are equipped with protection circuits.

Fig. 2(a) and Fig. 2(b) show the circuit symbols of an n-channel and a p-channel transistor respectively. An nMOS transistor is 'ON' if the gate voltage is at logic '1', or more precisely if the potential between the gate and the source terminals (V_GS) is greater than the threshold voltage V_T. An 'ON' transistor implies the existence of a continuous channel between the source and the drain terminals. On the other hand, an 'OFF' nMOS transistor indicates the absence of a connecting channel between the source and the drain. Similarly, a pMOS transistor is 'ON' if the potential V_GS is lower (or more negative) than the threshold voltage V_T..