CMOS

From ArticleWorld

CMOS stands for complimentary metal-oxide-semiconductor. It forms an important category of integrated circuit chips. CMOS chips essentially consist of two metal-oxide semiconductor field effect transistors (MOSFETS), one which N-type or negative and another which is P-type or positive; hence the word ‘complimentary’. CMOS circuitry is very common in random-access memory (RAM) and applications involving switching. This technology has been popularized because of the fact that these devices have considerably high speeds and require amazingly low amounts of power. These properties have made it especially important in applications requiring low consumption of power leading to greater battery life.

Technical features of CMOS

Complimentary metal-oxide semiconductor involves logic that is denser than other implementations of identical functionality. Only one of the two transistors can be turned on at one time, making CMOS ideal for switching purposes in electronics.

Logic gates are implemented by the positive and negative type MOSFETs which are present inside the CMOS chip. CMOS logic is known for lesser energy dissipation than other systems performing the same functions. This means that CMOS devices do not heat up excessively.

The size of a CMOS chip may vary from just 0.1 to 4 cm². Each chip may consist of several hundred million transistors. These chips are fabricated in many variants.

The P-type MOSFETs are complementary to the N-type ones since they are brought on when their gate voltage dips well below the voltage of the source. Also, when the N-type is off, the P-type turns on, and vice versa. In CMOS logic there exists a collection of negative-type MOSFETs arranged in a pull-down network between the lower-voltage supply rail and the output. This voltage is referred to as Vss. In a pull-up network between the higher-voltage rail and the output, CMOS logic gates have a collection of positive type MOSFETs.

Power is dissipated in CMOS circuits by the charging and discharging of the various capacitances involved. These may be the gate, wire, drain or the source capacitances. The total power dissipated can be calculated by using the formula: P=CV²f, where C is the capacitance, V is the voltage change and f is the frequency.

History of CMOS

CMOS circuits were developed in 1963 by Fairchild Semiconductor. Frank Wanlass of the company spearheaded research in this direction. It was initially seen as a low-power device that could be used in place of TTL components. It found use in the watch manufacturing industry, where battery life has always assumed more importance than speed of operation. As years progressed, CMOS became the most predominant technology in digital integrated circuits. CMOS technology has greatly contributed to the reduction in sizes of semiconductor-based equipment. Operating speeds too have been greatly enhanced by the use of CMOS.

Many of the early CMOS circuits were prone to damage due to static electricity. Technological improvements in newer generations of CMOS devices have seen the emergence of adequate protection circuits to absorb static charges. Thus delicate layers of oxide and P-N junctions could be guarded against damage.

Some of the hallmarks of newer CMOS devices are simplicity, low power dissipation and the ability for better integration densities. CMOS logic functionality was in the beginning available only on the 4000 series of integrated circuits. The future saw the 7400 series too being fabricated in CMOS and NMOS forms apart from other variants.