PMOS logic

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PMOS clock IC, 1974 CT7004.jpg
PMOS clock IC, 1974

P-type metal-oxide-semiconductor logic uses p-channel metal-oxide-semiconductor field effect transistors (MOSFETs) 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 conduct holes between p-type "source" and "drain" terminals.

MOSFET Transistor used for amplifying or switching electronic signals.

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of field-effect transistor that is fabricated by the controlled oxidation of a semiconductor, typically silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The MOSFET is the basic building block of modern electronics. Since its invention by Mohamed Atalla and Dawon Kahng at Bell Labs in November 1959, the MOSFET has become the most widely manufactured device in history, with an estimated total of 13 sextillion (1.3 × 1022) MOS transistors manufactured between 1960 and 2018.

In electronics, a logic gate is an idealized or physical device implementing a Boolean function; that is, it performs a logical operation on one or more binary inputs and produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and unlimited fan-out, or it may refer to a non-ideal physical device.

Contents

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 (or subthreshold), triode, saturation (sometimes called active), and velocity saturation.

While PMOS 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 PMOS FETs), it has several shortcomings as well. The worst problem is that there is a direct current (DC) 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.

Direct current Unidirectional flow of electric charge

Direct current (DC) is the unidirectional flow of an electric charge. A battery is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for this type of current was galvanic current.

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 (similar to charging a capacitor through a very low resistance). But the resistance between the output and the negative supply rail is much greater, so the high-to-low transition takes longer (similar to discharge of a capacitor through a high resistance). 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. [1]

Most P-MOS integrated circuits require a power supply of 17-24 volt DC. [2] 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. [3]

Intel 4004 4-bit central processing unit

The Intel 4004 is a 4-bit central processing unit (CPU) released by Intel Corporation in 1971. It was the first commercially available microprocessor, and the first in a long line of Intel CPUs. The chip design, implemented with the MOS silicon gate technology, started in April 1970, and was created by Federico Faggin who led the project from beginning to completion in 1971. Marcian Hoff formulated and led the architectural proposal in 1969, and Masatoshi Shima contributed to the architecture and later to the logic design. The first commercial sale of the fully operational 4004 occurred in March 1971 to Busicom Corp. of Japan for its 141-PF electronic calculator, for which it was originally designed and built as a custom chip.

Metal gate

A metal gate, in the context of a lateral metal-oxide-semiconductor (MOS) stack, is just that—the gate material is made from a metal.

Though initially easier to manufacture, [4] PMOS logic was later supplanted by NMOS logic using n-channel field-effect transistors. NMOS is faster than PMOS. Modern integrated circuits are CMOS logic, which uses both p-channel and n-channel transistors.

N-type metal-oxide-semiconductor logic uses n-type MOSFETs to implement logic gates and other digital circuits. These nMOS transistors operate by creating an inversion layer in a p-type transistor body. This inversion layer, called the n-channel, can conduct electrons between n-type "source" and "drain" terminals. The n-channel is created by applying voltage to the third terminal, called the gate. Like other MOSFETs, nMOS transistors have four modes of operation: cut-off, triode, saturation, and velocity saturation.

CMOS Technology for constructing integrated circuits

Complementary metal–oxide–semiconductor (CMOS), also known as complementary-symmetry metal–oxide–semiconductor (COS-MOS), is a type of MOSFET fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFETs for logic functions. CMOS technology is used for constructing integrated circuits (ICs), including microprocessors, microcontrollers, memory chips, and other digital logic circuits. CMOS technology is also used for analog circuits such as image sensors, data converters, RF circuits, and highly integrated transceivers for many types of communication.

Gates

The p-type MOSFETs are arranged in a so-called "pull-up network" (PUN) 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. [4] Thus, for active-high logic, De Morgan's laws show that a PMOS NOR gate has the same structure as an NMOS NAND gate and vice versa.

De Morgans laws pair of transformation rules that are both valid rules of inference

In propositional logic and boolean algebra, De Morgan's laws are a pair of transformation rules that are both valid rules of inference. They are named after Augustus De Morgan, a 19th-century British mathematician. The rules allow the expression of conjunctions and disjunctions purely in terms of each other via negation.

PMOS inverter with load resistor. PMOS-inverter.svg
PMOS inverter with load resistor.
PMOS NAND gate with load resistor. PMOS-NAND-gate.svg
PMOS NAND gate with load resistor.
PMOS NOR gate with load resistor. PMOS-NOR-gate.svg
PMOS NOR gate with load resistor.

History

Following the invention of the MOSFET by Egyptian engineer Mohamed Atalla and Korean engineer Dawon Kahng at Bell Labs in 1959, they demonstrated MOSFET technology in 1960. [5] They fabricated both pMOS and nMOS devices with a 20 µm process. However, the nMOS devices were impractical, and only the pMOS type were practical working devices. [6] 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. [7]

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Latch-up

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Depletion-load NMOS logic form of nMOS logic family

In integrated circuits, depletion-load NMOS is a form of digital logic family that uses only a single power supply voltage, unlike earlier nMOS logic families that needed more than one different power supply voltage. Although manufacturing these integrated circuits required additional processing steps, improved switching speed and the elimination of the extra power supply made this logic family the preferred choice for many microprocessors and other logic elements.

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References

  1. "Microwave Engineering: Concepts and Fundamentals". 2014. p. 629. Retrieved 2016-04-10. Also, the asymmetric input logic levels make PMOS circuits susceptible to noise.
  2. Fairchild (January 1983). "CMOS, the Ideal Logic Family" (PDF). p. 6. Retrieved 2015-07-03. Most of the more popular P-MOS parts are specified with 17V to 24V power supplies while the maximum power supply voltage for CMOS is 15V.
  3. "Intel 4004 datasheet" (PDF) (published 2010-07-06). 1987. p. 7. Retrieved 2011-07-06.
  4. 1 2 Microelectronic Device Data Handbook (PDF) (NPC 275-1 ed.). NASA / ARINC Research Corporation. August 1966. p. 2-51.
  5. "1960 - Metal Oxide Semiconductor (MOS) Transistor Demonstrated". The Silicon Engine. Computer History Museum.
  6. Lojek, Bo (2007). History of Semiconductor Engineering. Springer Science & Business Media. pp. 321–3. ISBN   9783540342588.
  7. Kuhn, Kelin (2018). "CMOS and Beyond CMOS: Scaling Challenges". High Mobility Materials for CMOS Applications. Woodhead Publishing. p. 1. ISBN   9780081020623.

Further reading