Complementary metal–oxide–semiconductor (CMOS) (pronounced "see-moss", IPA: /siːmɔːs, ˈsiːmɒs/), is a major class of integrated circuits. Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. A microprocessor incorporates most or all of the functions of a Central processing unit (CPU on a single Integrated A microcontroller (also MCU or µC is a functional Computer system-on-a- chip. Static random access memory (SRAM is a type of Semiconductor memory where the word static indicates that unlike ''dynamic'' RAM (DRAM, it does not Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see 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. An image sensor is a device that converts an optical image to an electric signal A data converter may refer to a Digital-to-analog converter; an Analog-to-digital converter; any other device used in Data A transceiver is a device that has both a Transmitter and a receiver which are combined and share common circuitry or a single housing Frank Wanlass got a patent on CMOS in the 1960's (US Patent 3,356,858). Frank Wanlass invented CMOS Logic circuits in 1963 while working at Fairchild Semiconductor.
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. A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is by adding an impurity of valence -five elements to The metal–oxide–semiconductor field-effect transistor ( MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals
Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Electronic noise is an unwanted signal characteristic of all electronic circuits. In Electrical engineering, power consumption often refers to the Electrical energy over Time supplied to operate an Electrical appliance, although Significant power is only drawn when the transistors in the CMOS device are switching between on and off states. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals 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. Waste heat refers to Heat produced by Machines and industrial processes for which no useful application is found and is regarded as a waste By-product Transistor–transistor logic ( TTL) is a class of Digital circuits built from Bipolar junction transistors (BJT and Resistors It is called nMOS logic uses n-type metal-oxide-semiconductor Field effect transistors (MOSFETs to implement Logic gates and other Digital circuits nMOS transistors have 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. The field-effect transistor (FET is a type of Transistor that relies on an Electric field to control the shape and hence the conductivity of a 'channel' A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that 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. WikipediaNaming The 65 nanometer (65 nm process is an advanced lithographic node used in volume CMOS Semiconductor fabrication. 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]. A metal gate, in the context of a lateral Metal-Oxide-Semiconductor MOS stack is just that--the gate material is made from a metal The term high-κ Dielectric refers to a material with a high Dielectric constant (κ (as compared to Silicon dioxide) used in semiconductor manufacturing Per the International Technology Roadmap for Semiconductors the 45 nm technology node should refer to the average half-pitch of a memory cell manufactured at around the 2007-2008 time
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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.
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. The metal–oxide–semiconductor field-effect transistor ( MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals A logic gate performs a logical operation on one or more logic inputs and produces a single logic output Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see A computer is a Machine that manipulates data according to a list of instructions. Signal processing is the analysis interpretation and manipulation of signals Signals of interest include sound, images, biological signals such as 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. 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. Pull-up Resistors are used in electronic Logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected An electrical network is an interconnection of Electrical elements such as Resistors Inductors Capacitors Transmission lines Voltage In Electrical engineering, node refers to any point on a circuit where two or more circuit elements meet A voltage source is any device or system that produces an Electromotive force between its terminals OR derives a secondary voltage from a primary Almost all Integrated circuits (ICs have at least two pins which connect to the power rails of the circuit they are installed in In Electrical engineering, the term ground or earth has several meanings depending on the specific application areas 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. nMOS logic uses n-type metal-oxide-semiconductor Field effect transistors (MOSFETs to implement Logic gates and other Digital circuits nMOS transistors have Pull-up Resistors are used in electronic Logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected Almost all Integrated circuits (ICs have at least two pins which connect to the power rails of the circuit they are installed in 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. Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. 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. In Science and engineering, a conductor is a material which contains movable Electric charges. 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. A switch is a mechanical device used to connect and disconnect an electric Circuit at will 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. The 90 nanometer (90 nm process refers to the level of CMOS process technology that was reached in the 2002-2003 timeframe by most leading semiconductor companies 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.
Example: NAND gate
As an example, shown on the right is a circuit diagram of a NAND gate in CMOS logic. Definition The NAND operation is a Logical operation on two Logical values typically the values of two Propositions that produces a value A circuit diagram (also known as an electrical diagram Wiring diagram, elementary diagram or electronic Schematic) is a simplified conventional pictorial representation Definition The NAND operation is a Logical operation on two Logical values typically the values of two Propositions that produces a value
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. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical 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. The method of logical effort, a term coined by Ivan Sutherland and Robert Sproull in 1991 is a straightforward technique used to estimate delay in a
Example: NAND gate in physical layout
This example shows a NAND logic device drawn as a physical representation as it would be manufactured. Integrated circuit layout, also known IC layout, IC mask layout, or mask design, is the representation of an Integrated circuit in terms of planar Definition The NAND operation is a Logical operation on two Logical values typically the values of two Propositions that produces a value 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. Definition The NAND operation is a Logical operation on two Logical values typically the values of two Propositions that produces a value 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. Integrated circuit layout, also known IC layout, IC mask layout, or mask design, is the representation of an Integrated circuit in terms of planar Definition The NAND operation is a Logical operation on two Logical values typically the values of two Propositions that produces a value
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. A latchup is the inadvertent creation of a low- impedance path between the power supply rails of an electronic component triggering a Parasitic structure, which then
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. The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide and the substrate 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. Subthreshold leakage or subthreshold conduction or subthreshold drain current is the current that flows between the source and drain of a MOSFET 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. The term high-κ Dielectric refers to a material with a high Dielectric constant (κ (as compared to Silicon dioxide) used in semiconductor manufacturing The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide Measurement The relative static permittivity εr can be measured for static Electric fields as follows first the Capacitance of a test A good overview of leakage and reduction methods are explained in the book Leakage in Nanometer CMOS Technologies ISBN 0-387-25737-3.
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. An operational amplifier, often called an op-amp, is a DC - coupled high- Gain electronic voltage amplifier with differential 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.
See also
- MOSFET
- Magic is open-source software often used as a layout tool for CMOS circuits. The metal–oxide–semiconductor field-effect transistor ( MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals Magic is a Very-large-scale integration layout tool originally written by John Ousterhout and his graduate students at UC Berkeley during the 1980s
References
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.
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