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Encyclopedia > Complementary Metal Oxide Semiconductor

CMOS (pronounced "see-moss") is the name used for a major class of integrated circuits. CMOS chips include microprocessor, microcontroller, static RAM, and other digital logic circuit designs. The central characteristic of the technology is that it only uses significant power when its transistors are switching between on and off states, and so it is effective for dealing with the problems of chip power consumption and heat dissipation. It also generally allows a fairly high density of logic functions on a chip.


CMOS stands for complementary metal-oxide-semiconductor. The word 'complementary' refers to the fact that the design uses pairs of transistors for logic functions. The two transistors of a pair complement each other in that only one of the two is nominally on at any one time. When one of them is on, the output is essentially connected to the supply voltage, and when the other is on, the output is essentially connected to ground.


The phrase 'metal-oxide-semiconductor' is a reference to the nature of the fabrication process originally used to build CMOS chips. That process created field effect transistors having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. Instead of metal, today the gate electrodes are almost always made from a different material, polysilicon, but the name CMOS nevertheless continues to be used for the modern descendents of the original process. (See also MOSFET.)

Contents

Development history

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 transistors.


Early CMOS circuits were very susceptible to damage from electrostatic discharge (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 series that used aluminum as gate material were extremely tolerant to supply voltage variations and operated anywhere from 3 to 18 volts DC. 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.


In the early fabrication processes, the gate electrode was made of aluminum. 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.


Technical details

CMOS (complementary metal oxide semiconductor) refers to both a particular style of digital circuitry design, and the family of processes used to implement that circuitry on integrated circuits (chips). CMOS logic on a CMOS process dissipates less energy and is more dense than other implementations of the same functionality. As this advantage has grown and become more important, CMOS processes and variants have come to dominate, so that as of 2004 the vast majority of integrated circuit manufacturing by dollar volume is on CMOS processes.


Structure

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.

Enlarge
CMOS Implementation of the NAND Operation

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 Vss). 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 the higher-voltage rail (often named Vdd). The p-type transistor network is complementary to the n-type transistor network, so that when the n-type is off, the p-type is on, and vice-versa.


CMOS logic dissipates less power than NMOS logic because CMOS dissipates power only when switching (dynamic power). On a typical ASIC in a modern 90 nanometer process, switching the output might take 120 picoseconds, and happen once every ten nanoseconds. NMOS logic dissipates power whenever the output is low (static power), because there is a current path from Vdd to Vss through the load resistor and the n-type network.


P-type MOSFETs are complementary to n-type because they turn on when their gate voltage goes sufficiently below their source voltage, and because they can pull the drain all the way to Vdd. Thus, if both a p-type and n-type transistor have their gates connected to the same input, the p-type MOSFET will be on when the n-type MOSFET is off, and vice-versa.


As an example, here is a NAND gate in CMOS logic. If both of the A and B inputs are high, then both the n-type transistors will conduct, neither of the p-type transistors will conduct, and a conductive path will be established between the output and Vss, bringing the output low. If either of the A or B inputs is low, one of the n-type transistors will not conduct, one of the p-type transistors will, and a conductive path will be established between the output and Vdd, bringing the output high.


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.


See Logical effort for a method of calculating delay in a CMOS circuit.


Power

CMOS circuits burn power by charging and discharging the various load capacitances (mostly gate and wire capacitance, but also drain and some source 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, and multiply by voltage again to get the characteristic switching power dissipated by a CMOS device: P = CV2f.


A different form of power consumption 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 Vdd to Vss. The power thus used is called crowbar power. 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 (Vdd might have been 5 V, and Vth 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 Vth 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 Vth so that leakage power begins to approximate switching power. As a result, these devices burn considerable power even when not switching.


The "CMOS" on PC motherboards

In computing the term CMOS is often used to mean the non-volatile memory component that stores BIOS information on the motherboard of a personal computer. 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 now usually employed.


External links

  • CMOS gate description and interactive illustrations (http://tech-www.informatik.uni-hamburg.de/applets/cmos/cmosdemo.html)

  Results from FactBites:
 
Semiconductor OneSource: Semiconductor Glossary -- Search For : cmos (503 words)
Complementary Metal Oxide Semiconductor structure; consists of N-channel and P-channel MOS transistors; due to very low power consumption and dissipation as well minimization of the current in "off" state CMOS is a very effective device configuration for implementation of digital functions; CMOS is a key device in state-of-the-art silicon microelectronics.
a pair of two complementary transistors (a p-channel and an n-channel) with the source of the n-channel transistor connected to the drain of the p-channel one and the gates connected to each other.
thin (thickness depends on wafer diameter, but is less than 1 mm), circular slice of single-crystal semiconductor material used in manufacturing of semiconductor devices and integrated circuits; depending on material wafer diameter may range from about 25 mm to 300 mm; cut from the ingot of single crystal semiconductor.
CMOS - Wikipedia, the free encyclopedia (1456 words)
That process created field effect transistors having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material.
Instead of metal, today the gate electrodes are almost always made from a different material, polysilicon, but the name CMOS nevertheless continues to be used for the modern descendants of the original process.
CMOS (complementary metal oxide semiconductor) refers to both a particular style of digital circuitry design, and the family of processes used to implement that circuitry on integrated circuits (chips).
  More results at FactBites »

 
 

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