NMOS logic

Last updated

N-type metal-oxide-semiconductor logic uses n-type MOSFETs (metal-oxide-semiconductor field-effect transistors) 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 (or subthreshold), triode, saturation (sometimes called active), and velocity saturation.

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 a covered 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 was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at Bell Labs in November 1959. It is the basic building block of modern electronics, and the most widely manufactured device in history, with an estimated total of 13 sextillion (1.3 × 1022) MOSFETs manufactured between 1960 and 2018.

Field-effect transistor transistor that uses an electric field to control the electrical behaviour of the device. FETs are also known as unipolar transistors since they involve single-carrier-type operation

The field-effect transistor (FET) is an electronic device which uses an electric field to control the flow of current. FETs are devices with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

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

Overview

MOS stands for metal-oxide-semiconductor, reflecting the way MOS-transistors were originally constructed, predominantly before the 1970s, with gates of metal, typically aluminium. Since around 1970, however, most MOS circuits have used self-aligned gates made of polycrystalline silicon. These silicon gates are still used in most types of MOSFET based integrated circuits, although metal gates (Al or Cu) started to reappear in the early 2000s for certain types of high speed circuits, such as high performance microprocessors.

In electronics, a self-aligned gate is a transistor manufacturing feature whereby a refractory gate electrode region of a MOSFET is used as a mask for the doping of the source and drain regions. This technique ensures that the gate will slightly overlap the edges of the source and drain.

Polycrystalline silicon high purity, polycrystalline form of silicon

Polycrystalline silicon, also called polysilicon or poly-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.

Integrated circuit electronic circuit manufactured by lithography; set of electronic circuits on one small flat piece (or "chip") of semiconductor material, normally silicon

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny MOS transistors into a small chip results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete electronic components. The IC's mass production capability, reliability, and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

The MOSFETs are n-type enhancement mode transistors, arranged in a so-called "pull-down network" (PDN) between the logic gate output and negative supply voltage (typically the ground). A pull up (i.e. a "load" that can be thought of as a resistor, see below) is placed between the positive supply voltage and each logic gate output. Any logic gate, including the logical inverter, can then be implemented by designing a network of parallel and/or series circuits, such that if the desired output for a certain combination of boolean input values is zero (or false), the PDN will be active, meaning that at least one transistor is allowing a current path between the negative supply and the output. This causes a voltage drop over the load, and thus a low voltage at the output, representing the zero.

In computer science, the Boolean data type is a data type that has one of two possible values, intended to represent the two truth values of logic and Boolean algebra. It is named after George Boole, who first defined an algebraic system of logic in the mid 19th century. The Boolean data type is primarily associated with conditional statements, which allow different actions by changing control flow depending on whether a programmer-specified Boolean condition evaluates to true or false. It is a special case of a more general logical data type —logic doesn't always need to be Boolean.

NMOS NOR WITH RESISTIVE LOAD.PNG

As an example, here is a NOR gate implemented in schematic NMOS. If either input A or input B is high (logic 1, = True), the respective MOS transistor acts as a very low resistance between the output and the negative supply, forcing the output to be low (logic 0, = False). When both A and B are high, both transistors are conductive, creating an even lower resistance path to ground. The only case where the output is high is when both transistors are off, which occurs only when both A and B are low, thus satisfying the truth table of a NOR gate:

Logical NOR

In boolean logic, logical nor or joint denial is a truth-functional operator which produces a result that is the negation of logical or. That is, a sentence of the form is true precisely when neither p nor q is true—i.e. when both of p and q are false. In grammar, nor is a coordinating conjunction.

ABA NOR B
001
010
100
110

A MOSFET can be made to operate as a resistor, so the whole circuit can be made with n-channel MOSFETs only. NMOS circuits are slow to transition from low to high. When transitioning from high to low, the transistors provide low resistance, and the capacitive charge at the output drains away very quickly (similar to discharging a capacitor through a very low resistor). But the resistance between the output and the positive supply rail is much greater, so the low to high transition takes longer (similar to charging a capacitor through a high value resistor). Using a resistor of lower value will speed up the process but also increases static power dissipation. However, a better (and the most common) way to make the gates faster is to use depletion-mode transistors instead of enhancement-mode transistors as loads. This is called depletion-load NMOS logic.

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.

For many years, NMOS circuits were much faster than comparable PMOS and CMOS circuits, which had to use much slower p-channel transistors. It was also easier to manufacture NMOS than CMOS, as the latter has to implement p-channel transistors in special n-wells on the p-substrate. The major drawback with NMOS (and most other logic families) is that a DC current must flow through a logic gate even when the output is in a steady state (low in the case of NMOS). This means static power dissipation, i.e. power drain even when the circuit is not switching. A similar situation arises in modern high speed, high density CMOS circuits (microprocessors, etc.) which also has significant static current draw, although this is due to leakage, not bias. However, older and/or slower static CMOS circuits used for ASICs, SRAM, etc., typically have very low static power consumption.

PMOS logic p-type MOSFETs to implement logic gates

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.

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.

In computer engineering, a logic family may refer to one of two related concepts. A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compatible logic levels and power supply characteristics within a family. Many logic families were produced as individual components, each containing one or a few related basic logical functions, which could be used as "building-blocks" to create systems or as so-called "glue" to interconnect more complex integrated circuits. A "logic family" may also refer to a set of techniques used to implement logic within VLSI integrated circuits such as central processors, memories, or other complex functions. Some such logic families use static techniques to minimize design complexity. Other such logic families, such as domino logic, use clocked dynamic techniques to minimize size, power consumption and delay.

Additionally, just like in DTL, TTL, ECL, etc., the asymmetric input logic levels make NMOS and PMOS circuits more susceptible to noise than CMOS. These disadvantages are why the CMOS logic now has supplanted most of these types in most high-speed digital circuits such as microprocessors (despite the fact that CMOS was originally very slow compared to logic gates built with bipolar transistors).

History

The MOSFET was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at Bell Labs in 1959, and demonstrated in 1960. [1] 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 devices. [2]

In 1965, Chih-Tang Sah, Otto Leistiko and A.S. Grove at Fairchild Semiconductor fabricated several NMOS devices with channel lengths between 8 µm and 65 µm. [3] Dale L. Critchlow and Robert H. Dennard at IBM also fabricated NMOS devices in the 1960s. The first IBM NMOS product was a memory chip with 1  kb data and 50100 ns access time, which entered large-scale manufacturing in the early 1970s. This led to MOS semiconductor memory replacing earlier bipolar and ferrite-core memory technologies in the 1970s. [4]

The earliest microprocessors in the early 1970s were PMOS processors, which initially dominated the early microprocessor industry. [5] In 1973, NEC's μCOM-4 was an early NMOS microprocessor, fabricated by the NEC LSI team, consisting of five researchers led by Sohichi Suzuki. [6] [7] By the late 1970s, NMOS microprocessors had overtaken PMOS processors. [5] CMOS microprocessors were introduced in 1975. [5] [8] [9] However, CMOS processors did not become dominant until the 1980s. [5]

CMOS was initially slower than NMOS logic, thus NMOS was more widely used for computers in the 1970s. [10] The Intel 5101 (1  kb SRAM) CMOS memory chip (1974) had an access time of 800  ns, [11] [12] whereas the fastest NMOS chip at the time, the Intel 2147 (4 kb SRAM) HMOS memory chip (1976), had an access time of 55/70 ns. [10] [12] In 1978, a Hitachi research team led by Toshiaki Masuhara introduced the twin-well Hi-CMOS process, with its HM6147 (4 kb SRAM) memory chip, manufactured with a 3 µm process. [10] [13] The Hitachi HM6147 chip was able to match the performance (55/70 ns access) of the Intel 2147 HMOS chip, while the HM6147 also consumed significantly less power (15  mA) than the 2147 (110 mA). With comparable performance and much less power consumption, the twin-well CMOS process eventually overtook NMOS as the most common semiconductor manufacturing process for computers in the 1980s. [10]

In the 1980s, CMOS microprocessors overtook NMOS microprocessors. [5]

See also

Related Research Articles

Semiconductor device fabrication manufacturing process used to create integrated circuits

Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically the metal-oxide-semiconductor (MOS) devices used in the integrated circuit (IC) chips that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.

Transistor Basic electronics component

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

Static random-access memory Semiconductor memory

Static random-access memory is a type of semiconductor random-access memory (RAM) that uses bistable latching circuitry (flip-flop) to store each bit. SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.

Inverter (logic gate) logic gate implementing negation

In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. The truth table is shown on the right.

Transistor count the number of transistors in a device

The transistor count is the number of transistors on an integrated circuit (IC). It typically refers to the number of MOSFETs on an IC chip, as all modern ICs use MOSFETs. It is the most common measure of IC complexity. The rate at which MOS transistor counts have increased generally follows Moore's law, which observed that the transistor count doubles approximately every two years.

Active-pixel sensor an image sensor consisting of an integrated circuit

An active-pixel sensor (APS) is an image sensor where each pixel sensor unit cell has a photodetector and one or more active MOSFET amplifiers. There are different types of integrated circuit active pixel sensors, including the complementary metal–oxide–semiconductor (CMOS) APS used most commonly in digital camera technologies such as cell phone cameras, web cameras, most modern digital pocket cameras, most digital single-lens reflex cameras (DSLRs), and mirrorless interchangeable-lens cameras (MILCs). Such an image sensor is produced using CMOS technology, which emerged as an alternative to charge-coupled device (CCD) image sensors and eventually outsold them by the mid-2000s.

The 10 µm process is the level of MOSFET semiconductor process technology that was reached around 1971, by leading semiconductor companies such as RCA and Intel.

Four-phase logic is a type of, and design methodology for dynamic logic. It enabled non-specialist engineers to design quite complex ICs, using either PMOS or NMOS processes. It uses a kind of 4-phase clock signal.

A transistor is a semiconductor device with at least three terminals for connection to an electric circuit. The vacuum-tube triode, also called a (thermionic) valve, was the transistor's precursor, introduced in 1907. The principle of a field-effect transistor was proposed by Julius Edgar Lilienfeld in 1925.

In electronics, a native transistor is a variety of the MOS field-effect transistor that is intermediate between enhancement and depletion modes. Most common is the n-channel native transistor.

In electronics, pass transistor logic (PTL) describes several logic families used in the design of integrated circuits. It reduces the count of transistors used to make different logic gates, by eliminating redundant transistors. Transistors are used as switches to pass logic levels between nodes of a circuit, instead of as switches connected directly to supply voltages. This reduces the number of active devices, but has the disadvantage that the difference of the voltage between high and low logic levels decreases at each stage. Each transistor in series is less saturated at its output than at its input. If several devices are chained in series in a logic path, a conventionally constructed gate may be required to restore the signal voltage to the full value. By contrast, conventional CMOS logic switches transistors so the output connects to one of the power supply rails, so logic voltage levels in a sequential chain do not decrease. Simulation of circuits may be required to ensure adequate performance.

In field effect transistors (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the transistor is in an ON state or an OFF state at zero gate–source voltage.

Memory cell (computing) part of computer memory

The memory cell is the fundamental building block of computer memory. The memory cell is an electronic circuit that stores one bit of binary information and it must be set to store a logic 1 and reset to store a logic 0. Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it.

References

  1. "1960 - Metal Oxide Semiconductor (MOS) Transistor Demonstrated". The Silicon Engine. Computer History Museum.
  2. Lojek, Bo (2007). History of Semiconductor Engineering. Springer Science & Business Media. pp. 321–3. ISBN   9783540342588.
  3. Sah, Chih-Tang; Leistiko, Otto; Grove, A. S. (May 1965). "Electron and hole mobilities in inversion layers on thermally oxidized silicon surfaces". IEEE Transactions on Electron Devices . 12 (5): 248–254. doi:10.1109/T-ED.1965.15489.
  4. Critchlow, D. L. (2007). "Recollections on MOSFET Scaling". IEEE Solid-State Circuits Society Newsletter. 12 (1): 19–22. doi:10.1109/N-SSC.2007.4785536.
  5. 1 2 3 4 5 Kuhn, Kelin (2018). "CMOS and Beyond CMOS: Scaling Challenges". High Mobility Materials for CMOS Applications. Woodhead Publishing. p. 1. ISBN   9780081020623.
  6. "1970s: Development and evolution of microprocessors" (PDF). Semiconductor History Museum of Japan. Retrieved 27 June 2019.
  7. "NEC 751 (uCOM-4)". The Antique Chip Collector's Page. Archived from the original on 2011-05-25. Retrieved 2010-06-11.
  8. Cushman, Robert H. (20 September 1975). "2-1/2-generation μP's-$10 parts that perform like low-end mini's" (PDF). EDN.
  9. "CDP 1800 μP Commercially available" (PDF). Microcomputer Digest. 2 (4): 1–3. October 1975.
  10. 1 2 3 4 "1978: Double-well fast CMOS SRAM (Hitachi)" (PDF). Semiconductor History Museum of Japan. Retrieved 5 July 2019.
  11. "Silicon Gate MOS 2102A". Intel . Retrieved 27 June 2019.
  12. 1 2 "A chronological list of Intel products. The products are sorted by date" (PDF). Intel museum. Intel Corporation. July 2005. Archived from the original (PDF) on August 9, 2007. Retrieved July 31, 2007.
  13. Masuhara, Toshiaki; Minato, O.; Sasaki, T.; Sakai, Y.; Kubo, M.; Yasui, T. (1978). "A high-speed, low-power Hi-CMOS 4K static RAM". 1978 IEEE International Solid-State Circuits Conference. Digest of Technical Papers. XXI: 110–111. doi:10.1109/ISSCC.1978.1155749.

Further reading