PMOS logic

PMOS or pMOS logic, from p-channel metal–oxide–semiconductor, is a family of digital circuits based on p-channel, enhancement mode metal–oxide–semiconductor field-effect transistors (MOSFETs). In the late 1960s and early 1970s, PMOS logic was the dominant semiconductor technology for large-scale integrated circuits before being superseded by NMOS and CMOS devices.

History and application

Mohamed Atalla and Dawon Kahng manufactured the first working MOSFET at Bell Labs in 1959. They fabricated both PMOS and NMOS devices but only the PMOS devices were working. It would be more than a decade before contaminants in the manufacturing process (particularly sodium) could be managed well enough to manufacture practical NMOS devices.

Compared to the bipolar junction transistor, the only other device available at the time for use in an integrated circuit, the MOSFET offers a number of advantages:

Given semiconductor device fabrication processes of similar precision, a MOSFET requires only 10% of the area of a bipolar junction transistor. The main reason is that the MOSFET is self-insulating and does not require p–n junction isolation from neighboring components on the chip.

A MOSFET requires fewer process steps and is therefore simpler and cheaper to manufacture (one diffusion doping step compared to four for a bipolar process).

Since there is no static gate current for a MOSFET, the power consumption of an integrated circuit based on MOSFETs can be lower.

Disadvantages relative to bipolar integrated circuits were:

The switching speed was considerably lower, due to large gate capacitances.