PMOS logic
Dominant digital logic family before NMOS and CMOS.
PMOS (p-channel metal–oxide–semiconductor) logic is a family of digital circuits based on p-channel, enhancement mode 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and early 1970s, PMOS logic was the dominant semiconductor technology for large-scale integrated circuits before being superseded by NMOS and CMOS devices. The first working MOSFET, fabricated at Bell Labs in 1959, was a PMOS device, and PMOS circuits were the first to achieve commercial large-scale integration, including the first microprocessors.
- first working MOSFET
- 1959, Bell Labs
- first commercial PMOS circuit
- 1964, 20-bit shift register by General Microelectronics
- key improvement
- polysilicon self-aligned gate technology (1968)
- first PMOS microprocessor
- Intel 4004 (1971)
- typical power supply
- 17-24 V DC (high-voltage); -16 V (low-voltage silicon gate)
- replaced by
- NMOS (by late 1970s) and later CMOS (by 1980s)
Lore & Background
Mohamed Atalla and Dawon Kahng manufactured the first working MOSFET at Bell Labs in 1959, fabricating both PMOS and NMOS devices, but only the PMOS devices were working. Contaminants in manufacturing, particularly sodium, prevented practical NMOS devices for over a decade. Compared to bipolar junction transistors, MOSFETs required only 10% of the area, fewer process steps, and lower power consumption, though early PMOS had lower switching speed and high threshold voltages requiring -24 V to -28 V supplies.
General Microelectronics introduced the first commercial PMOS circuit in 1964, a 20-bit shift register with 120 MOSFETs. Their 1965 attempt to develop 23 custom circuits for the Victor 3900 calculator proved too ambitious given reliability issues, leading to the company's demise. Other companies continued with PMOS circuits such as large shift registers (General Instrument) and the analogue multiplexer 3705 (Fairchild Semiconductor). A major improvement came in 1968 with polysilicon self-aligned gate technology, developed by Tom Klein and Federico Faggin at Fairchild, which allowed tighter tolerances, smaller MOSFETs, reduced gate capacitances, and lower threshold voltages, enabling -16 V supplies.
Fairchild did not pursue PMOS development as intensively as some managers wanted. Two of them, Gordon Moore and Robert Noyce, founded Intel in 1968, joined later by Federico Faggin and Les Vadasz. Intel introduced its first PMOS static RAM (256-bit Intel 1101) in 1969, the 1024-bit dynamic RAM Intel 1103 in 1970 (which replaced magnetic core memory), and the first PMOS microprocessor, the Intel 4004, in 1971. Many early microprocessors were PMOS, including the Intel 4040 and 8008, National Semiconductor's IMP-16, PACE, and SC/MP, Texas Instruments' TMS1000, and Rockwell's PPS-4 and PPS-8.
Reader's Guide
PMOS logic was the foundational technology for large-scale integrated circuits in the late 1960s and early 1970s, enabling the first commercial microprocessors and memory chips. Its dominance stemmed from the relative ease of manufacturing p-channel MOSFETs compared to n-channel devices, which were hindered by contamination issues. The introduction of polysilicon self-aligned gates in 1968 significantly improved performance and reduced power supply voltages, allowing PMOS to power early calculators, clocks, and computers. However, PMOS suffered from static power dissipation, slow high-to-low transitions, and asymmetric input logic levels. By 1972, NMOS technology had matured, offering three times the switching speed due to higher electron mobility in n-type channels, and quickly replaced PMOS for most applications. CMOS, though invented earlier, required more complex manufacturing and did not become the dominant technology until the 1980s. PMOS's legacy lies in its role as the first practical MOS logic family, directly enabling the microprocessor revolution and the transition from bipolar to MOS integrated circuits.
Did You Know?
- The first working MOSFET, fabricated in 1959, was a PMOS device; NMOS devices were not yet practical due to contamination.
- General Microelectronics' 1965 attempt to build 23 custom PMOS circuits for the Victor 3900 calculator failed due to reliability issues, leading to the company's demise.
- The Intel 1103, a 1024-bit PMOS dynamic RAM introduced in 1970, began replacing magnetic core memory in computers.
- The Intel 4004 PMOS microprocessor used polysilicon gates, allowing a smaller voltage differential with VSS=+5V and VDD=-10V for TTL compatibility.
Frequently Asked Questions
What is PMOS logic?
PMOS logic is a family of digital circuits built around p-channel, enhancement-mode MOSFET transistors. It served as the dominant semiconductor approach for large-scale integrated circuits from the late 1960s through the early 1970s.
Who created the first working PMOS transistor?
The first functional MOSFET, which was a PMOS device, was fabricated at Bell Labs in 1959. That single breakthrough laid the groundwork for every subsequent MOSFET-based circuit family.
What was the first microprocessor built using PMOS logic?
The Intel 4004, released in 1971, holds the title of the first PMOS microprocessor. It proved that PMOS technology could support complex, integrated computing on a single chip.
Why did PMOS logic get replaced by other technologies?
PMOS circuits were gradually supplanted by NMOS in the late 1970s and then by CMOS in the 1980s, both of which delivered better speed and lower power draw. The high supply voltages PMOS typically required (17–24 V DC) also made it increasingly impractical as design demands grew.
What was the first commercial PMOS integrated circuit?
General Microelectronics shipped a 20-bit shift register in 1964, marking the first commercially available PMOS circuit. That early product helped establish PMOS as a viable path toward large-scale integration.
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