CMOS

CMOS (complementary metal-oxide-semiconductor) 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 and signal processing equipment. Manufacture is generally by a semiconductor device fabrication process.

As in NMOS logic, a collection of n-type MOSFETs is arranged in a pull-down network (PDN) between the output and the low-voltage power supply rail. However, unlike NMOS, CMOS also has a collection of p-type MOSFETs (complementary to the n-type) in a pull-up network (PUN) between the output and high-voltage rail, in place of a resistor. As an example, here is a NOR gate in CMOS logic. Note how (in a steady state) only either the PDN or the PUN can be active at any one time. This implies there is no current flow and hence virtually no power dissipation while the circuit is at rest, a major virtue that sets CMOS circuits apart from their NMOS and TTL predecessors.

          A
         _|_             A      B
      __|   |__          |      |
     |         |        _o_    _o_
low__|    B    |___ ___|   |__|   |__high
     |   _|_   |   |
     |__|   |__|   |
                  out
pull-down network       pull-up network

Another advantage of CMOS over NMOS is that both low-to-high and high-to-low output transitions are very fast since the transistors have low resistance when active. This strong, symmetric response also makes CMOS more resistant to noise.

As switching speeds increase, though, the power dissipation of CMOS begins to be felt. This is because the currents necessary to charge and discharge the various load capacitances cause voltage drops in the transistors. Also, during transitions, for a short time both the PDN and the PUN are partially conductive, which creates direct current flow between the high- and low-voltage rails. The majority of power consumed by CMOS circuits is in fact dissipated during transitions.

History

CMOS circuits were invented in 1963 by Frank Wanlass at Fairchild Semiconductor. Originally a low-power but slow alternative to TTL, CMOS had become the predominant technology in digital integrated circuits some twenty-five years later. This is essentially because area occupation, operating speed, energy efficiency and manufacturing costs have and do continue to benefit from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In addition, the simplicity and comparatively low power dissipation of CMOS circuits have allowed for integration densities not possible on the basis of bipolar junction transistors.