P-type metal-oxide-semiconductor logic, PMOS or pMOS, is a type of digital circuit constructed using metal-oxide-semiconductor field effect transistors with a p-type semiconductor source and drain printed on a bulk n-type "well". When activated, by lowering the voltage on the gate, the resulting circuit allows the conduction of electron holes between the source and drain, turning the circuit "on". PMOS circuits are less susceptible to electronic noise than other types of MOSFETs, which makes them easy to fabricate. They were widely used in the early days of microprocessor development in the 1970s. They have a number of disadvantages compared to the NMOS and CMOS alternatives, including the need for several different supply voltages, high power dissipation in the conducting state, and relatively large features. Also, the overall switching speed is lower. PMOS was replaced by NMOS as better fabrication techniques were introduced, especially the further elimination of impurities from the siliconstock which lowered noise. NMOS offered major advantages in terms of power use, heat loads, and feature size. NMOS became widely used by the mid-1970s, before it was itself replaced by CMOS logic during the early 1980s.
Description
PMOS uses p-channel metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. PMOS transistors operate by creating an inversion layer in an n-type transistor body. This inversion layer, called the p-channel, can conductholes between p-type "source" and "drain" terminals. The p-channel is created by applying voltage to the third terminal, called the gate. Like other MOSFETs, PMOS transistors have four modes of operation: cut-off, triode, saturation, and velocity saturation. While PMOS logic is easy to design and manufacture, it has several shortcomings as well. The worst problem is that there is a direct current through a PMOS logic gate when the PUN is active, that is, whenever the output is high, which leads to static power dissipation even when the circuit sits idle. Also, PMOS circuits are slow to transition from high to low. When transitioning from low to high, the transistors provide low resistance, and the capacitive charge at the output accumulates very quickly. But the resistance between the output and the negative supply rail is much greater, so the high-to-low 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 PMOS circuits susceptible to noise. Most PMOS integrated circuits require a power supply of 17-24 volt DC. The Intel 4004 PMOS microprocessor, however, uses PMOS logic with polysilicon rather than metal gates allowing a smaller voltage differential. For compatibility with TTL signals, the 4004 uses positive supply voltage VSS=+5V and negative supply voltage VDD = -10V. Though initially easier to manufacture, PMOS logic was later supplanted by NMOS logic using n-channelfield-effect transistors. NMOS is faster than PMOS. Modern integrated circuits are CMOS logic, which uses both p-channel and n-channel transistors.
Gates
The p-type MOSFETs are arranged in a so-called "pull-up network" between the logic gate output and positive supply voltage, while a resistor is placed between the logic gate output and the negative supply voltage. The circuit is designed such that if the desired output is high, then the PUN will be active, creating a current path between the positive supply and the output. PMOS gates have the same arrangement as NMOS gates if all the voltages are reversed. Thus, for active-high logic, De Morgan's lawsshow that a PMOS NOR gate has the same structure as an NMOS NAND gate and vice versa.
History
After the invention of the MOSFET by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959, they demonstrated MOSFET technology in 1960. They fabricated both pMOS and nMOS devices with a 20µm process. However, the nMOS devices were impractical, and only the pMOS type were practical working devices. A more practical nMOS process was developed several years later. The earliest microprocessors in the early 1970s were PMOS processors, which initially dominated the early microprocessor industry. By the late 1970s, NMOS microprocessors had overtaken PMOS processors.
