Complementary Metal-Oxide-Semiconductor (CMOS) Circuits

The term 'Complementary Metal-Oxide-Semiconductor', or simply 'CMOS', refers to the device technology for designing and fabricating integrated circuits that employ logic using both n- and p-channel MOSFET's. CMOS is the other major technology utilized in manufacturing digital IC's aside from TTL, and is now widely used in microprocessors, memories, and digital ASIC's.

See Also:

CMOS Parameters

MOSFET's

Logic Gates

TTL

The input to a CMOS circuit is always to the gate of the input MOS transistor, which exhibits a very high resistance. This high gate resistance is due to the fact that the gate of a MOS transistor is isolated from its channel by an oxide layer, which is a dielectric. As such, 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. This power dissipation during a switching action is known as 'dynamic power'. In a typical CMOS IC, output switching may take about a hundred picoseconds, and may occur every 10 nanoseconds (or 100 millions times per second). Switching an output from one logic level to another requires the charging and discharging of various load capacitances, which dissipates power that is proportional to these capacitances and the frequency of switching.

Figure 1 shows an example of a CMOS circuit - an inverter that employs a p-channel and an n-channel MOS transistor. A logic '1' Vin voltage at the input would make T1 (p-channel) turn off and T2 (n-channel) turn on, pulling Vout to near Vss, or logic '0'. A logic '0' Vin voltage, on the other hand, will make T1 turn on and T2 turn off, pulling Vout to near Vdd, or logic '1'. Note that the p- and n-channel MOS transistors in the circuit are complementary, so they are always in opposite states, i.e., for any given Vin level, one of them is 'on' while the other is 'off'.

Figure 1. A CMOS Inverter

CMOS circuits were invented by Frank Wanlass of Fairchild Semiconductor in 1963, although the first CMOS I.C.'s were not produced until 1968, this time at RCA. The original CMOS devices consumed less power than TTL but ran slower too, so early applications centered on circuits where battery consumption was more important than speed of operation. Steadily CMOS technology has improved, subsequently becoming the technology of choice for digital circuits. Aside from low power consumption, CMOS circuits are also easy and cheap to fabricate, allowing denser circuit integration than their bipolar counterparts.

CMOS circuits are quite vulnerable to ESD damage, mainly by gate oxide punchthrough from high ESD voltages. Because of this issue, modern CMOS IC's are now equipped with on-chip ESD protection circuits, which reduce (but not totally eliminate) risks of ESD damage. Proper handling and processing of CMOS IC's to prevent ESD damage are also a 'must'.

In the 70's and 80's, CMOS IC's are run using digital voltages that are compatible with TTL so both can be inter-operated with each other. By the 1990's, however, the need for much lower power consumption for mobile devices has resulted in the deployment of more and more CMOS devices that run on much lower power supply voltages. The lower operating voltages also allowed the use of thinner, higher-performance gate dielectrics in CMOS IC's.

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