Materials Science and Engineering/Doctoral review questions/Daily Discussion Topics/01202008
CMOS and nMOS
editnMOS
editnMOS logic uses n-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits. nMOS transistors have three modes of operation: cut-off, triode, and saturation (sometimes called active).
The n-type MOSFETs are arranged in a so-called "pull-down network" (PDN) between the logic gate output and negative supply voltage, while a resistor is placed between the logic gate output and the positive supply voltage. The circuit is designed such that if the desired output is low, then the PDN will be active, creating a current path between the negative supply and the output.
As an example, here is a NOR gate in nMOS logic. If either input A or input B is high (logic 1, = True), the respective MOS transistor acts as a very low resistance between the output and the negative supply, forcing the output to be low (logic 0, = False). When both A and B are high, both transistors are conductive, creating an even lower resistance path to ground. The only case where the output is high is when both transistors are off, which occurs only when both A and B are low, thus satisfying the truth table of a NOR gate:
| A | B | A NOR B |
|---|---|---|
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 0 |
While nMOS logic is easy to design and manufacture (a MOSFET can be made to operate as a resistor, so the whole circuit can be made with nMOSFETs), it has several shortcomings as well. The worst problem is that a DC current flows through an nMOS logic gate when the PDN is active, that is whenever the output is low. This leads to static power dissipation even when the circuit sits idle.
Also, nMOS circuits are slow to transition from low to high. When transitioning from high to low, the transistors provide low resistance, and the capacitative charge at the output drains away very quickly. But the resistance between the output and the positive supply rail is much greater, so the low to high transition takes longer. Using a resistor of lower value will speed up the process but also increases static power dissipation.
Additionally, the asymmetric input logic levels make nMOS circuits susceptible to noise.
These disadvantages are why nMOS logic was supplanted by CMOS logic both in low-power and in high-speed digital circuits, such as microprocessors, during the 1980s.
CMOS
edit
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.
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). 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, 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.