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CMOS logic uses a combination of p-type and n-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits found in computers, telecommunications and signal processing equipment. Although CMOS logic can be implemented with discrete devices (for instance, in an introductory circuits class), typical commercial CMOS products are integrated circuits composed of millions (or hundreds of millions) of transistors of both types on a rectangular piece of silicon of between 0.1 and 4 square cm. These bits of silicon are commonly called chips, although within the industry they are also referred to as die, perhaps because they are the result of dicing (that is, cutting up) the circular silicon wafer which is the basic unit of semiconductor device fabrication.

In CMOS logic gates, as in NMOS logic gates, a collection of n-type MOSFETs is arranged in a pull-down network between the output and the lower- voltage power supply rail (often named V_ss). Instead of the load resistor of NMOS gates, CMOS gates have a collection of p-type MOSFETs in a pull-up network between the output and higher-voltage rail (often named V_ss. P-type MOSFETs are complementary to n-type because they turn on when their gate voltage goes low, and because they can pull the output all the way to V_dd. Thus, the p-type MOSFETs are on when the corresponding n-type MOSFETs are off, and vice-versa.

As an example, here is a NAND gate in CMOS logic. If inputs A and B remain at the high or low voltage, only one of the pull-up or pull-down networks can be conductive. In any of these states, the gate has no conductive path between the two voltage rails, and dissipates only leakage power (MOSFETs that are nominally off actually leak a small amount of current). The very low static power dissipation of CMOS is a major virtue that sets CMOS circuits apart from their NMOS and TTL predecessors.

Another advantage of CMOS over NMOS is that both low-to-high and high-to-low output transitions are fast since the pull-up transistors have low resistance when switched on, unlike the load resistors in NMOS logic. In addition, the output signal swings the full voltage between the low and high rails. This strong, more nearly symmetric response also makes CMOS more resistant to noise.

CMOS circuits burn power by charging and discharging the various load capacitances (mostly gate and wire capacitance, but also source and drain capacitances) whenever they are switched. The charge moved is the capacitance multiplied by the voltage change. Multiply by the switching frequency to get the current burned, multiply by voltage again to get the characteristic switching power dissipated by a CMOS device: .

Crowbar power became noticeable in the 1990s as wires on chip became narrower and the long wires became more resistive. CMOS gates at the end of those resistive wires see slow input transistions. During the middle of these transitions, both the NMOS and PMOS networks are partially conductive, and current flows directly from V_dd to V_ss. Careful design which avoids weakly driven long skinny wires has ameliorated this effect, and crowbar power is nearly always substantially smaller than switching power.

Both NMOS and PMOS transistors have a threshold gate-to-source voltage, below which the current through the device drops exponentially. Historically, CMOS designs operated at supply voltages much larger than their threshold voltages (V_dd might have been 5 V, and V_th for both NMOS and PMOS might have been 700 mV). As supply voltages have come down to conserve power, voltage thresholds have had to come down as well. The exponential current curve has not changed, however, and as a result a modern NMOS transistor with a V_th of 200 mV has a significant subthreshold leakage current. Designs (e.g. desktop processors) which try to optimize their fabrication processes for minimum power dissipation during operation have been lowering V_th so that leakage power begins to approximate switching power. As a result, these devices burn considerable power even when not switching.

1 History

CMOS originally meant complementary metal-oxide-semiconductor, which described the stack of materials that made up the transistor. The gate electrode was initially fabricated of metal. Later CMOS processes switched to polycrystalline silicon ("polysilicon"), which can better tolerate the high temperatures used to anneal the silicon after ion implantation. As of 2004 there is some research into using metal gates once again, but all commonly used processes have polysilicon gates.

CMOS circuits were invented in 1963 by Frank Wanlass at Fairchild Semiconductor. The first CMOS integrated circuits were made by RCA in 1968. Originally a low-power but slow alternative to TTL, CMOS found early adopters in the watch industry and in other fields where battery life was more important than speed. Some twenty-five years later, CMOS has become the predominant technology in digital integrated circuits. This is essentially because area occupation, operating speed, energy efficiency and manufacturing costs have benefited and continue to benefit from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In addition, the simplicity and comparatively low power dissipation of CMOS circuits have allowed for integration densities not possible on the basis of bipolar junction transistorA Bipolar Junction Transistor (BJT), is a type of transistor, an amplifying or switching device constructed of doped semiconductor. A bipolar transistor is a sandwich of differently doped sections, either NPN or PNP. The center section is called the bases.

Early CMOS circuits were very susceptible to damage from electrostatic dischargeAn electrostatic discharge ESD is a sudden flow of electric current through a material that is normally an insulator. A large potential difference across the insulator generates a strong electric field, converting the material's atoms into ions that condu (ESD). Subsequent generations were thus equipped with sophisticated protection circuitry that helps absorb electric charges with no damage to the fragile gate oxides and PN-junctions. Still, antistatic handling precautions continue to be enforced to prevent excessive energies from building up. Please remember this when adding a memory module to your computer, for instance.

Conversely, early generations such as the 4000 seriesThe 4000 series is the general classification used to refer to the industry standard integrated circuits which implement a variety of logic functions using CMOS technology. They were created in the 1970s as a lower power and more versatile alternative to that used aluminum as gate material were extremely tolerant to supply voltage variations and operated anywhere from 3 to 18 volts DCDirect current DC or continuous current ) is the continuous flow of electricity through a conductor such as a wire from high to low potential. In direct current, the electric charges flow always in the same direction, which distinguishes it from alternati. For many years, CMOS logic was designed to operate from the then industry-standard of 5V imposed by TTL. By 1990, lower power dissipation was usually more important than easy interfacing to TTL, and CMOS voltage supplies began to drop along with the geometric dimensions of the transistors. Lower voltage supplies not only saved power, but allowed thinner, higher performance gate insulators to be used. Some modern CMOS circuits operate from voltages below one volt.

See Logical effortThe method of logical effort a term coined by Ivan Sutherland and Robert Sproull in 1991, is a straight-forward technique used to estimate delay in a CMOS circuit. Circuit topology and gate sizing are key components utilized in this method. Logical effort for a method of calculating delay in a CMOS circuit.

In computingOriginally, the word computing was synonymous with counting and calculating, and a computer was a person who computes. Since the advent of the electronic computer, it has come to also mean the operation and usage of these machines, the electrical processe the term CMOS is often used to mean the non-volatile memory that stores motherboard BIOS information. This was called the CMOS RAM or just the CMOS because it usually used a low-power CMOS memory chip, powered by a small battery. The term remains common but has become a misnomer as nearly all modern computer components except the "CMOS memory" are constructed in CMOS technology. For the "CMOS", in modern computers, flash memory is usually employed.

Most modern PCs have I/O addresses assigned to read or write to the CMOS memory which typically stores BIOS configuration and RTC.

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