CMOS
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Complementary metal–oxide–semiconductor (CMOS) (pronounced "see-moss", IPA: /siːmɔːs, ˈsiːmɒs/), is a major class of integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for a wide variety of analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. Frank Wanlass got a patent on CMOS in the 1960's (US Patent 3,356,858).
CMOS is also sometimes referred to as complementary-symmetry metal–oxide–semiconductor. The words "complementary-symmetry" refer to the fact that the typical digital design style with CMOS uses complementary and symmetrical pairs of p-type and n-type metal oxide semiconductor field effect transistors (MOSFETs) for logic functions.
Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Significant power is only drawn when the transistors in the CMOS device are switching between on and off states. Consequently, CMOS devices do not produce as much waste heat as other forms of logic, for example transistor-transistor logic (TTL) or NMOS logic, which uses all n-channel devices without p-channel devices. CMOS also allows a high density of logic functions on a chip.
The phrase "metal–oxide–semiconductor" is a reference to the physical structure of certain 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 (usually aluminum in the very old days), current gate electrodes (including those up to the 65 nanometer technology node) are almost always made from a different material, polysilicon, but the terms MOS and CMOS nevertheless continue to be used for the modern descendants of the original process. Metal gates have made a comeback with the advent of high-k dielectric materials in the CMOS process, as announced by IBM and Intel for the 45 nanometer node and beyond [1].
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[edit] Technical details
"CMOS" 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 circuitry dissipates less power and is denser than other implementations having the same functionality. As this advantage has grown and become more important, CMOS processes and variants have come to dominate, so that the vast majority of modern integrated circuit manufacturing is on CMOS processes.[citation needed]
[edit] 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 equipment, 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 centimeters.[citation needed] These devices are commonly called "chips", although within the industry they are also referred to as "die" (singular) or "dice", "dies", or "die" (plural).
In CMOS logic, a collection of n-type MOSFETs are arranged in a pull-down network between the output node and the lower-voltage power supply rail, named Vss, which often has ground potential. Instead of the load resistor of NMOS logic gates, CMOS logic gates have a collection of p-type MOSFETs in a pull-up network between the output and the higher-voltage rail, named Vdd. Pull-up and pull-down refer to the idea that the output node, which exhibits some internal capacitance, is charged or discharged by the connected pull-up and pull-down networks. By asserting or de-asserting the inputs to the CMOS circuit, individual transistors along the pull-up and pull-down networks become conductive, and a path is connected from the output node to one of the voltage rails. A digital CMOS circuit cannot be in a pull-up and pull-down state at the same time, except when switching between the two states. Each input is connected to both the pull-up and pull-down networks in a complementary fashion, so that when an n-type transistor on the pull-down path is off, the p-type on the pull-up path is on, and vice-versa.
CMOS logic dissipates less power than NMOS logic circuits 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.
n-type MOSFETs are complementary to p-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 n-type and p-type transistor have their gates connected to the same input, the n-type MOSFET will be on when the p-type MOSFET is off, and vice-versa.
[edit] Example: NAND gate
As an example, shown on the right is a circuit diagram of a NAND gate in CMOS logic.
If both of the A and B inputs are high, then both the n-type transistors (bottom half of the diagram) will conduct, neither of the p-type transistors (top half) 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.
[edit] Example: NAND gate in physical layout
This example shows a NAND logic device drawn as a physical representation as it would be manufactured. The physical layout perspective is a "bird's eye view" of a stack of layers. The circuit is constructed on a P-type substrate. The polysilicon, diffusion, and n-well are referred to as "base layers" and are actually inserted into trenches of the P-type substrate. The contacts penetrate an insulating layer between the base layers and the first layer of metal (metal1) making a connection.
The inputs to the NAND (illustrated in green coloring) are in polysilicon. The CMOS transistors (devices) are formed by the intersection of the polysilicon and diffusion: N diffusion for the N device; P diffusion for the P device (illustrated in salmon and yellow coloring respectively). The output ("out") is connected together in metal (illustrated in cyan coloring). Connections between metal and polysilicon or diffusion are made through contacts (illustrated as black squares. The physical layout example matches the NAND logic circuit given in the previous example.
The N device is manufactured on a P-type substrate. The P devices is manufactured in an N-type well (n-well). A P-type substrate "tap" is connected to VSS and an N-type n-well tap is connected to VDD to prevent latchup.
[edit] Power: switching and leakage
CMOS circuits dissipate 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 used, 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 transitions. 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). But as supply voltages have come down to conserve power the Vdd to Vss short circuit is avoided.
However, to speed up the designs, manufacturers have switched to gate materials which lead to lower voltage thresholds and 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 dissipate considerable power even when not switching. Leakage power reduction using new material and system design is critical to sustaining scaling of CMOS. The industry is contemplating the introduction of High-k Dielectrics to combat the increasing gate leakage current by replacing the silicon dioxide that are the conventional gate dielectrics with materials having a higher dielectric constant. A good overview of leakage and reduction methods are explained in the book Leakage in Nanometer CMOS Technologies ISBN 0-387-25737-3.
[edit] Analog CMOS
Besides digital applications, CMOS technology is also used for analog applications. For example, there are CMOS operational amplifier ICs available in the market. CMOS technology is also widely used for RF applications all the way to microwave frequencies. Indeed, CMOS technology is used for mixed-signal (analog+digital) applications.
[edit] See also
[edit] References
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[edit] External links
- CMOS gate description and interactive illustrations
- LASI is a "general purpose" IC layout CAD tool. It is a free download and can be used as a layout tool for CMOS circuits.
