29-12-2012, 04:43 PM
VLSI Physical Design Lab
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Study of CMOS Inverter
Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors (CMOS sensor), data converters, and highly integrated transceivers for many types of communication.
CMOS is also sometimes referred to as complementary-symmetry metal–oxide–semiconductor (or COS-MOS. 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. CMOS also allows a high density of logic functions on a chip. It was primarily for this reason that CMOS became the most used technology to be implemented in VLSI chips.
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. Aluminum was once used but now the material is polysilicon. Other 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.
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 than logic families with resistive loads. Since this advantage has increased and grown more important, CMOS processes and variants have come to dominate, thus the vast majority of modern integrated circuit manufacturing is on CMOS processes.
CMOS circuits use 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 (e.g., for instructional purposes in an introductory circuits class), typical commercial CMOS products are integrated circuits composed of millions of transistors of both types on a rectangular piece of silicon of between 10 and 400mmThese devices are commonly called "chips", although within the industry they are also referred to as a "die" (singular) or "dice", or "dies" (plural).
Composition
The main principle behind CMOS circuits that allows them to implement logic gates is the use of p-type and n-type metal–oxide–semiconductor field-effect transistors to create paths to the output from either the voltage source or ground. When a path to output is created from the voltage source, the circuit is said to be pulled up. The other circuit state occurs when a path to output is created from ground and the output pulled down to the ground potential.