HMOS
Encyclopedia
Depletion-load nMOS/NMOS (n-channel metal-oxide-semiconductor) is a form of nMOS
logic family
which uses depletion-mode n-type MOSFET
s as load transistors as a method to enable single voltage operation and achieve greater speed than possible with pure enhancement-load devices. This is partly because the depletion-mode MOSFETs can be a better current source
approximation than the simpler enhancement-mode transistor can, especially when no extra voltage is available (one of the reasons early pMOS and nMOS chips demanded several voltages).
The inclusion of depletion-mode n-MOS transistors in the manufacturing process demanded additional manufacturing steps compared to the simpler enhancement-load circuits; this is because depletion-load devices are formed by increasing the amount of dopant
in the load transistors channel region, in order to adjust their threshold voltage
. This is normally performed using ion implantation
.
Some depletion-load nMOS designs are still produced, typically in parallel with newer CMOS counterparts; one example of this is the Z84015 and Z84C15.
s were a hundred times as fast as the (p-channel) MOS transistors then used and were much more reliable, but they also consumed much more power, required more area, and demanded a more complicated manufacturing process. MOS ICs were considered interesting but inadequate for supplanting the fast bipolar circuits in anything but niche market
s, such as low power applications. One of the reasons for the low speed was that MOS transistors had gates made of aluminum which led to considerable parasitic capacitance
s using the manufacturing processes of the time. The introduction of transistors with gates of polycrystalline silicon
(that became the de facto standard
from the mid 1970s to early 2000s) was an important first step in order to reduce this handicap. This new self-aligned silicon-gate
transistor was introduced by Federico Faggin
at Fairchild Semiconductor
in early 1968; it was a refinement (and the first working implementation) of ideas and work by John C. Sarace, Tom Klein and Robert W. Bower
(around 1966–67) for a transistor with less parasitic capacitances that could be manufactured as part of an IC (and not only as a discrete component). This new type of pMOS transistor was 3–5 times as fast (per watt) as the aluminum-gate pMOS transistor, and it needed less area, had much lower leakage and higher reliability. The same year, Faggin also built the first IC using the new transistor type, the Fairchild 3708 (8-bit analog
multiplexer
with decoder
), which demonstrated a substantially improved performance over its metal-gate counterpart. In less than 10 years, the silicon gate MOS transistor replaced bipolar circuits as the main vehicle for complex digital ICs.
s that are the charge (current) carriers in pMOS transistors have lower mobility than the electron
s that are the charge carriers in nMOS transistors (a ratio of approximately 2.5), furthermore pMOS circuits do not interface easily with low voltage positive logic such as DTL-logic
and TTL-logic
(the 7400-series). However, pMOS transistors are relatively easy to make and were therefore developed first — ionic contamination of the gate oxide from etching chemical
s and other sources can very easily prevent (the electron
based) nMOS transistors from switching off, while the effect in (the electron-hole based) pMOS transistors is much less severe. Fabrication of nMOS transistors therefore has to be many times cleaner than bipolar processing in order to produce working devices.
Early work on nMOS was presented in a brief IBM
paper at ISSCC in 1969. Hewlett Packard then started to develop nMOS IC technology to get the promising speed and easy interfacing for its calculator
business. Tom Haswell at HP eventually solved many problems by using purer raw material
s (especially aluminum for interconnects) and by adding a bias voltage to make the gate threshold
large enough; this back-gate bias remained a de facto standard solution to (mainly) sodium
contaminants in the gates until the development of ion implantation
(see below). Already by 1970, HP was making good enough nMOS ICs and had characterized it enough so that Dave Maitland was able to write an article about nMOS in the December, 1970 issue of Electronics magazine. However, nMOS remained uncommon in the rest of the semiconductor industry until 1973.
The production-ready nMOS process enabled HP to develop the industry’s first 4-kbit IC ROM
. Motorola
eventually served as a second source for these products and so became one of the first commercial semiconductor vendors to master the nMOS process, thanks to Hewlett Packard. A while later, the startup company
Intel announced a 1-kbit pMOS DRAM, called 1102, developed as a custom product for Honeywell
(an attempt to replace magnetic core memory in their mainframe computer
s). HP’s calculator engineers, who wanted a similar but more robust product for the 9800 series calculators, contributed IC fabrication experience from their 4-kbit ROM project to help improve Intel DRAM’s reliability, operating-voltage, and temperature range. These efforts contributed to the heavily enhanced Intel 1103 1-kbit pMOS DRAM, which was the world’s first commercially available DRAM
IC. It was formally introduced in October 1970, and became Intel’s first really successful product.
The first depletion-load nMOS circuits were pioneered and made by the DRAM
manufacturer Mostek
, which made depletion-mode transistors available for the design of the original Zilog Z80
in 1975–76. Mostek had the ion implantation
equipment needed to create a doping profile
more precise than possible with diffusion
methods, so that the threshold voltage
of the load transistors could be adjusted reliably. At Intel, depletion load was introduced by Faggin (ex. Fairchild, later founder of Zilog
) in 1974. Depletion-load was first employed for a redesign of one of Intel's most important products at the time, a +5V-only 1Kbit nMOS SRAM called the 2102 (using more than 6000 transistors). The result of this redesign was the significantly faster 2102A, where the highest performing versions of the chip had access times of less than 100ns, taking MOS memories close to the speed of bipolar RAMs for the first time.
Depletion-load nMOS processes were also used by several other manufacturers to produce many incarnation
s of popular 8-bit, 16-bit, and 32-bit CPUs. Similarly to early pMOS and nMOS CPU designs using enhancement mode MOSFETs as loads, depletion-load nMOS designs typically employed various types of dynamic logic
(rather than just static gates) or pass transistor
s used as dynamic clocked latches. These techniques can enhance the area-economy considerably although the effect on the speed is complex. Processors built with depletion-load nMOS circuitry include the 6800 (in later versions), the 6502, Signetics 2650
, 8085, 6809, 8086, Z8000, NS32016, and many others (whether or not the HMOS processors below are included, as special cases).
A large number of support and peripheral ICs were also implemented using (often static) depletion-load based circuitry. However,there were never any standardized logic families
in nMOS, such as the bipolar
7400 series
and the CMOS
4000 series
, although designs with several second source manufacturers often achieved something of a de-facto standard component status. One example of this is the nMOS 8255 PIO
design, originally intended as an 8085 peripheral chip, that has been used in x86 and Z80 embedded system
s and many other contexts for several decades; similar to the 7400 series, modern low power versions are available as CMOS or BiCMOS implementations.
was still quite slow and used mostly for the 4000 series
, Static RAMs, and low-power ASIC
s (such as watch and calculator
ICs). Advanced processes for depletion-load nMOS, such as Intel's family of scaled HMOS (High density, short channel MOS) processes were therefore used for most microprocessors, certain types of memories, and many support and peripheral ICs. The first nMOS process called HMOS (late 1976) was originally used for Intel's then central static RAM products. Although primarily intended for fast SRAM, it was soon employed also for faster and/or less power hungry versions of the 8085, 8086, and other chips. HMOS was also licensed to other manufacturers, such as Motorola, which used it to manufacture the Motorola 68000
series, for instance. According to Intel, HMOS-II (1979) provided twice the density and four times the speed/power product over other typical contemporary depletion-load nMOS processes. HMOS, HMOS II, HMOS III, and HMOS IV were together used for many different kinds of processors; the 8085, 8048, 8051, 8086, 80186, 80286, and many others, but also for several generations of the same basic design, see datasheet
s.
for applications such as the Intel 80386
and certain microcontroller
s. A few years later, in the late 1980s, BiCMOS
was introduced for high-performance microprocessors as well as for high speed analog circuits. Today, most digital circuits, including the ubiquitous 7400 series
, are manufactured using various CMOS processes with a range of different topologies employed. This means that, in order to enhance speed and save die area (transistors and wiring), high speed CMOS designs often employ other elements than just the complementary static
gate
s and the transmission gates of typical slow low-power CMOS circuits (the only CMOS type during the 1960s and 1970s). These methods uses significant amounts of dynamic
circuitry in order to construct the larger building blocks on the chip, such as latches, decoders, multiplexers, and so on, and evolved from the various dynamic methodologies developed for pMOS and nMOS circuits during the 1970s.
s in steady state, where the quiescent current determines the maximum possible load at the output as well as the speed of the gate (i.e. with other factors constant). This contrasts to the power consumption characteristics of static CMOS circuits, which is due only to the transient power draw when the output state is changed and the p- and n-transistors thereby briefly conduct at the same time. However, this is a simplified view, and a more complete picture has to also include the fact that even purely static CMOS circuits have significant leakage in modern tiny geometries, as well as the fact that modern CMOS chips often contain dynamic
and/or domino logic
with a certain amount of pseudo nMOS circuitry.
NMOS logic
N-type metal-oxide-semiconductor logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits...
logic family
Logic family
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...
which uses depletion-mode n-type MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
s as load transistors as a method to enable single voltage operation and achieve greater speed than possible with pure enhancement-load devices. This is partly because the depletion-mode MOSFETs can be a better current source
Current source
A current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. The term constant-current sink is sometimes used for sources fed from a negative voltage supply...
approximation than the simpler enhancement-mode transistor can, especially when no extra voltage is available (one of the reasons early pMOS and nMOS chips demanded several voltages).
The inclusion of depletion-mode n-MOS transistors in the manufacturing process demanded additional manufacturing steps compared to the simpler enhancement-load circuits; this is because depletion-load devices are formed by increasing the amount of dopant
Dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
in the load transistors channel region, in order to adjust their threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...
. This is normally performed using ion implantation
Ion implantation
Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
.
Some depletion-load nMOS designs are still produced, typically in parallel with newer CMOS counterparts; one example of this is the Z84015 and Z84C15.
Silicon gate
In the late 1960s, bipolar junction transistorBipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
s were a hundred times as fast as the (p-channel) MOS transistors then used and were much more reliable, but they also consumed much more power, required more area, and demanded a more complicated manufacturing process. MOS ICs were considered interesting but inadequate for supplanting the fast bipolar circuits in anything but niche market
Niche market
A niche market is the subset of the market on which a specific product is focusing; therefore the market niche defines the specific product features aimed at satisfying specific market needs, as well as the price range, production quality and the demographics that is intended to impact...
s, such as low power applications. One of the reasons for the low speed was that MOS transistors had gates made of aluminum which led to considerable parasitic capacitance
Parasitic capacitance
In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance , is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other...
s using the manufacturing processes of the time. The introduction of transistors with gates of polycrystalline silicon
Polycrystalline silicon
Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells....
(that became the de facto standard
De facto standard
A de facto standard is a custom, convention, product, or system that has achieved a dominant position by public acceptance or market forces...
from the mid 1970s to early 2000s) was an important first step in order to reduce this handicap. This new self-aligned silicon-gate
Self-aligned gate
A self-aligned gate, in electronics, is a transistor manufacturing feature whereby a highly doped gate electrode region of a MOSFET transistor is used as a mask for the doping of the source and drain around it...
transistor was introduced by Federico Faggin
Federico Faggin
Federico Faggin , who received in 2010 the National Medal of Technology and Innovation by Barack Obama, the highest honor bestowed by the United States government on scientists, engineers, and inventors, at the White House in Washington, is an Italian-born and naturalized U.S...
at Fairchild Semiconductor
Fairchild Semiconductor
Fairchild Semiconductor International, Inc. is an American semiconductor company based in San Jose, California. Founded in 1957, it was a pioneer in transistor and integrated circuit manufacturing...
in early 1968; it was a refinement (and the first working implementation) of ideas and work by John C. Sarace, Tom Klein and Robert W. Bower
Robert W. Bower
Dr. Robert W. Bower was born in Santa Monica, CA and is an applied physicist. Immediately after receiving his Ph.D. from The California Institute of Technology in 1973, he worked for over 25 years in many different professions: Engineer, Scientist, Department Head at University of California,...
(around 1966–67) for a transistor with less parasitic capacitances that could be manufactured as part of an IC (and not only as a discrete component). This new type of pMOS transistor was 3–5 times as fast (per watt) as the aluminum-gate pMOS transistor, and it needed less area, had much lower leakage and higher reliability. The same year, Faggin also built the first IC using the new transistor type, the Fairchild 3708 (8-bit analog
Analogue electronics
Analogue electronics are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two different levels. The term "analogue" describes the proportional relationship between a signal and a voltage or current that represents the signal...
multiplexer
Multiplexer
In electronics, a multiplexer is a device that selects one of several analog or digital input signals and forwards the selected input into a single line. A multiplexer of 2n inputs has n select lines, which are used to select which input line to send to the output...
with decoder
Decoder
A decoder is a device which does the reverse operation of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode...
), which demonstrated a substantially improved performance over its metal-gate counterpart. In less than 10 years, the silicon gate MOS transistor replaced bipolar circuits as the main vehicle for complex digital ICs.
nMOS and back-gate bias
There are a couple of drawbacks associated with pMOS: The electron holeElectron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
s that are the charge (current) carriers in pMOS transistors have lower mobility than the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s that are the charge carriers in nMOS transistors (a ratio of approximately 2.5), furthermore pMOS circuits do not interface easily with low voltage positive logic such as DTL-logic
Diode-transistor logic
Diode–transistor logic is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function is performed by a diode network and the amplifying function is performed by a transistor .- Implementations :The DTL circuit shown in...
and TTL-logic
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
(the 7400-series). However, pMOS transistors are relatively easy to make and were therefore developed first — ionic contamination of the gate oxide from etching chemical
Etching (microfabrication)
Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....
s and other sources can very easily prevent (the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
based) nMOS transistors from switching off, while the effect in (the electron-hole based) pMOS transistors is much less severe. Fabrication of nMOS transistors therefore has to be many times cleaner than bipolar processing in order to produce working devices.
Early work on nMOS was presented in a brief IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
paper at ISSCC in 1969. Hewlett Packard then started to develop nMOS IC technology to get the promising speed and easy interfacing for its calculator
Calculator
An electronic calculator is a small, portable, usually inexpensive electronic device used to perform the basic operations of arithmetic. Modern calculators are more portable than most computers, though most PDAs are comparable in size to handheld calculators.The first solid-state electronic...
business. Tom Haswell at HP eventually solved many problems by using purer raw material
Raw material
A raw material or feedstock is the basic material from which a product is manufactured or made, frequently used with an extended meaning. For example, the term is used to denote material that came from nature and is in an unprocessed or minimally processed state. Latex, iron ore, logs, and crude...
s (especially aluminum for interconnects) and by adding a bias voltage to make the gate threshold
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...
large enough; this back-gate bias remained a de facto standard solution to (mainly) sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
contaminants in the gates until the development of ion implantation
Ion implantation
Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
(see below). Already by 1970, HP was making good enough nMOS ICs and had characterized it enough so that Dave Maitland was able to write an article about nMOS in the December, 1970 issue of Electronics magazine. However, nMOS remained uncommon in the rest of the semiconductor industry until 1973.
The production-ready nMOS process enabled HP to develop the industry’s first 4-kbit IC ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
. Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
eventually served as a second source for these products and so became one of the first commercial semiconductor vendors to master the nMOS process, thanks to Hewlett Packard. A while later, the startup company
Startup company
A startup company or startup is a company with a limited operating history. These companies, generally newly created, are in a phase of development and research for markets...
Intel announced a 1-kbit pMOS DRAM, called 1102, developed as a custom product for Honeywell
Honeywell
Honeywell International, Inc. is a major conglomerate company that produces a variety of consumer products, engineering services, and aerospace systems for a wide variety of customers, from private consumers to major corporations and governments....
(an attempt to replace magnetic core memory in their mainframe computer
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
s). HP’s calculator engineers, who wanted a similar but more robust product for the 9800 series calculators, contributed IC fabrication experience from their 4-kbit ROM project to help improve Intel DRAM’s reliability, operating-voltage, and temperature range. These efforts contributed to the heavily enhanced Intel 1103 1-kbit pMOS DRAM, which was the world’s first commercially available DRAM
Dram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...
IC. It was formally introduced in October 1970, and became Intel’s first really successful product.
Depletion-mode transistors
Early MOS logic had one transistor type, which is enhancement mode so that it can act as a logic switch. Since suitable resistors were hard to make, the logic gates used saturated loads; that is, to make the one type of transistor act as a load resistor, the transistor had to be turned always on by tying its gate to the power supply (the more negative rail for PMOS, or the more positive rail for NMOS). Since the current in a device connected that way goes as the square of the voltage across the load, it provides poor pullup speed relative to its power consumption when pulled down. A resistor would be better, and a current source better yet. A depletion-mode device with gate tied to the opposite supply rail would be a better load, acting somewhere between a resistor and a current source.The first depletion-load nMOS circuits were pioneered and made by the DRAM
Dram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...
manufacturer Mostek
Mostek
Mostek was an integrated circuit manufacturer, founded in 1969 by ex-employees of Texas Instruments. Initially their products were manufactured in Worcester, Massachusetts, however by 1974 most of its manufacturing was done in the Carrollton, Texas facility on Crosby Road...
, which made depletion-mode transistors available for the design of the original Zilog Z80
Zilog Z80
The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...
in 1975–76. Mostek had the ion implantation
Ion implantation
Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
equipment needed to create a doping profile
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
more precise than possible with diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
methods, so that the threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...
of the load transistors could be adjusted reliably. At Intel, depletion load was introduced by Faggin (ex. Fairchild, later founder of Zilog
Zilog
Zilog, Inc., previously known as ZiLOG , is a manufacturer of 8-bit and 24-bit microcontrollers, and is most famous for its Intel 8080-compatible Z80 series.-History:...
) in 1974. Depletion-load was first employed for a redesign of one of Intel's most important products at the time, a +5V-only 1Kbit nMOS SRAM called the 2102 (using more than 6000 transistors). The result of this redesign was the significantly faster 2102A, where the highest performing versions of the chip had access times of less than 100ns, taking MOS memories close to the speed of bipolar RAMs for the first time.
Depletion-load nMOS processes were also used by several other manufacturers to produce many incarnation
Incarnation
Incarnation literally means embodied in flesh or taking on flesh. It refers to the conception and birth of a sentient creature who is the material manifestation of an entity, god or force whose original nature is immaterial....
s of popular 8-bit, 16-bit, and 32-bit CPUs. Similarly to early pMOS and nMOS CPU designs using enhancement mode MOSFETs as loads, depletion-load nMOS designs typically employed various types of dynamic logic
Dynamic logic (digital logic)
In integrated circuit design, dynamic logic is a design methodology in combinatorial logic circuits, particularly those implemented in MOS technology. It is distinguished from the so-called static logic by exploiting temporary storage of information in stray and gate capacitances...
(rather than just static gates) or pass transistor
Pass transistor logic
In electronics, pass transistor logic 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...
s used as dynamic clocked latches. These techniques can enhance the area-economy considerably although the effect on the speed is complex. Processors built with depletion-load nMOS circuitry include the 6800 (in later versions), the 6502, Signetics 2650
Signetics 2650
The Signetics 2650, was a very early 8-bit microprocessor. According to Adam Osborne's classic book An Introduction to Microprocessors Vol 2: Some Real Products, it was "the most minicomputer-like" of the microprocessors available at the time....
, 8085, 6809, 8086, Z8000, NS32016, and many others (whether or not the HMOS processors below are included, as special cases).
A large number of support and peripheral ICs were also implemented using (often static) depletion-load based circuitry. However,there were never any standardized logic families
Logic family
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...
in nMOS, such as the bipolar
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
7400 series
7400 series
The 7400 series of transistor-transistor logic integrated circuits are historically important as the first widespread family of TTL integrated circuit logic. It was used to build the mini and mainframe computers of the 1960s and 1970s...
and the CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
4000 series
4000 series
The 4000 series is a family of industry standard integrated circuits which implement a variety of logic functions using Complementary Metal–Oxide–Semiconductor technology, and are still in use today. They were introduced by RCA as CD4000 COS/MOS series in 1968, as a lower power and more versatile...
, although designs with several second source manufacturers often achieved something of a de-facto standard component status. One example of this is the nMOS 8255 PIO
Intel 8255
The Intel 8255 Programmable Peripheral Interface chip is a peripheral chip originally developed for the Intel 8085 microprocessor, and as such is a member of a large array of such chips, known as the MCS-85 Family. This chip was later also used with the Intel 8086 and its descendants.It was later...
design, originally intended as an 8085 peripheral chip, that has been used in x86 and Z80 embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
s and many other contexts for several decades; similar to the 7400 series, modern low power versions are available as CMOS or BiCMOS implementations.
The HMOS processes
In the early to mid 1980s, CMOSCMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
was still quite slow and used mostly for the 4000 series
4000 series
The 4000 series is a family of industry standard integrated circuits which implement a variety of logic functions using Complementary Metal–Oxide–Semiconductor technology, and are still in use today. They were introduced by RCA as CD4000 COS/MOS series in 1968, as a lower power and more versatile...
, Static RAMs, and low-power ASIC
ASIC
ASIC may refer to:* Application-specific integrated circuit, an integrated circuit developed for a particular use, as opposed to a customised general-purpose device.* ASIC programming language, a dialect of BASIC...
s (such as watch and calculator
Calculator
An electronic calculator is a small, portable, usually inexpensive electronic device used to perform the basic operations of arithmetic. Modern calculators are more portable than most computers, though most PDAs are comparable in size to handheld calculators.The first solid-state electronic...
ICs). Advanced processes for depletion-load nMOS, such as Intel's family of scaled HMOS (High density, short channel MOS) processes were therefore used for most microprocessors, certain types of memories, and many support and peripheral ICs. The first nMOS process called HMOS (late 1976) was originally used for Intel's then central static RAM products. Although primarily intended for fast SRAM, it was soon employed also for faster and/or less power hungry versions of the 8085, 8086, and other chips. HMOS was also licensed to other manufacturers, such as Motorola, which used it to manufacture the Motorola 68000
Motorola 68000
The Motorola 68000 is a 16/32-bit CISC microprocessor core designed and marketed by Freescale Semiconductor...
series, for instance. According to Intel, HMOS-II (1979) provided twice the density and four times the speed/power product over other typical contemporary depletion-load nMOS processes. HMOS, HMOS II, HMOS III, and HMOS IV were together used for many different kinds of processors; the 8085, 8048, 8051, 8086, 80186, 80286, and many others, but also for several generations of the same basic design, see datasheet
Datasheet
thumb|A floppy disk controller datasheet.A datasheet, data sheet, or spec sheet is a document summarizing the performance and other technical characteristics of a product, machine, component , material, a subsystem or software in sufficient detail to be used by a design engineer to integrate the...
s.
Further development
In the mid 1980s, faster CMOS variants, using similar HMOS process technology, such as Intel's CHMOS I, II, III, IV, etc started to supplant n-channel HMOSHMOS
Depletion-load nMOS/NMOS is a form of nMOS logic family which uses depletion-mode n-type MOSFETs as load transistors as a method to enable single voltage operation and achieve greater speed than possible with pure enhancement-load devices...
for applications such as the Intel 80386
Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
and certain microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
s. A few years later, in the late 1980s, BiCMOS
BiCMOS
BiCMOS is an evolved semiconductor technology that integrates two formerly separate semiconductor technologies - those of the analog bipolar junction transistor and the digital CMOS transistor - in a single integrated circuit device....
was introduced for high-performance microprocessors as well as for high speed analog circuits. Today, most digital circuits, including the ubiquitous 7400 series
7400 series
The 7400 series of transistor-transistor logic integrated circuits are historically important as the first widespread family of TTL integrated circuit logic. It was used to build the mini and mainframe computers of the 1960s and 1970s...
, are manufactured using various CMOS processes with a range of different topologies employed. This means that, in order to enhance speed and save die area (transistors and wiring), high speed CMOS designs often employ other elements than just the complementary static
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
gate
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...
s and the transmission gates of typical slow low-power CMOS circuits (the only CMOS type during the 1960s and 1970s). These methods uses significant amounts of dynamic
Dynamic logic (digital logic)
In integrated circuit design, dynamic logic is a design methodology in combinatorial logic circuits, particularly those implemented in MOS technology. It is distinguished from the so-called static logic by exploiting temporary storage of information in stray and gate capacitances...
circuitry in order to construct the larger building blocks on the chip, such as latches, decoders, multiplexers, and so on, and evolved from the various dynamic methodologies developed for pMOS and nMOS circuits during the 1970s.
Compared to CMOS
Compared to static CMOS, all variants of nMOS (and pMOS) are relatively power hungry. This is because they rely on load-transistors working as resistorResistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s in steady state, where the quiescent current determines the maximum possible load at the output as well as the speed of the gate (i.e. with other factors constant). This contrasts to the power consumption characteristics of static CMOS circuits, which is due only to the transient power draw when the output state is changed and the p- and n-transistors thereby briefly conduct at the same time. However, this is a simplified view, and a more complete picture has to also include the fact that even purely static CMOS circuits have significant leakage in modern tiny geometries, as well as the fact that modern CMOS chips often contain dynamic
Dynamic logic (digital logic)
In integrated circuit design, dynamic logic is a design methodology in combinatorial logic circuits, particularly those implemented in MOS technology. It is distinguished from the so-called static logic by exploiting temporary storage of information in stray and gate capacitances...
and/or domino logic
Domino logic
Domino logic is a CMOS-based evolution of the dynamic logic techniques which were based on either PMOS or NMOS transistors. It allows a rail-to-rail logic swing. It was developed to speed up circuits....
with a certain amount of pseudo nMOS circuitry.