CMOS

From Overclocking Wiki

An assortment of integrated circuits.

Virtualy omnipresent in today's technology, complimentary metal-oxide-semiconductor (abbreviated CMOS) is a design technique and fabrication process used to make integrated circuits.

CMOS parts, compared to bipolar device families such as TTL, are much more immune to power supply noise. They also draw much less power in static operation, though their power draw becomes much higher as frequencies rise.

While "CMOS" originally referred to the spcific fabrication process of using a metal-oxide to form a semiconductor using field-effect transistors with a metal gate electrode over an oxide-based insulator layered on top of semiconductor material, modern parts are made from polysilicon or silicon-on-insulator processes.

CMOS more generically means the design process of using two complimentary transistors to drive an output. One transistor is off when the other transistor is on, and such an arrangement provides a very natrural switching mechanism.

The original line of CMOS SSI circuits was distributed by RCA and known as the 4000 series as all the parts had numbers in the 4000's. These chips are still avaialable today and often used in hobbyist projects.

Specific applications

Because carefully designed CMOS parts can be implemented to consume very little power, they are often used in applications which are run by battery. In PCs, one of the most common such applications is the CMOS configuration memory and battery backed-up real time clock. The configuration memory retains the options and settings for the BIOS even while power is off as the CMOS memory where the data is stored is powered by a watch battery. The battery also operators an oscilator which keeps a time of day clock avialable for the system.

Electrostatic discharge

CMOS circuits are very sensitive to electrostatic discharge (ESD). Early circuits were painfully fragile, causing them to be very unreliable unless tedious and expensive construction techniques were employed. Modern circuits employ clamps and bleed resistors which make the devices less sensitive thought still not immune.

Anyone handling devices made with CMOS parts, including memory DIMMs, processors, and expansion cards, must take ESD handling precautions to avoid damaging the devices.

Because bipolar devices do not feature FETs, they are not susceptible to ESD damage.

Design example

A NAND gate implemented with CMOS design.

The adjacent figure implements a NAND gate using CMOS design principles.

When both the A and B inputs are high, Q1 and Q2 are both conducting and will pull the output to ground. Q3 and Q4 are both open and not conducting. If either input is low, then one of the Q1 or Q2 transistors will be open and one of Q3 and Q4 will be saturated. Q3 or Q4 will pull the output high.

Q1 and Q2 are n-channel transistors, while Q3 and Q4 are p-channel devices. Using devices of both types, often in compliment to eachother as in this circuit, is why CMOS design is called "complimentary".

Note that CMOS designs are not limited to digital circuits. The FETs in the parts can be operated in biased and unsaturated modes and make efficient preamplifiers, operation amplifiers, and other linear circuits.