In
integrated circuits, depletion-load NMOS is a form of digital
logic family
In computer engineering, a logic family is 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 compat ...
that uses only a single power supply voltage, unlike earlier
NMOS (n-type
metal-oxide semiconductor) 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
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
s and other logic elements.
Depletion-mode n-type
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
s as load transistors allow 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 NMOS transistors in the
manufacturing process
Manufacturing is the creation or production of goods with the help of equipment, labor, machines, tools, and chemical or biological processing or formulation. It is the essence of secondary sector of the economy. The term may refer to a ...
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
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. Wh ...
in the load transistors channel region, in order to adjust their
threshold voltage
The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important ...
. This is normally performed using
ion implantation
Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the target. Ion implantation is used in semiconductor device fa ...
.
Although the
CMOS process replaced most NMOS designs during the 1980s, some depletion-load NMOS designs are still produced, typically in parallel with newer CMOS counterparts. One example of this is the
Z84015 and Z84C15.
History and background
Following the invention of the
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
by
Mohamed Atalla
Mohamed M. Atalla ( ar, محمد عطاالله; August 4, 1924 – December 30, 2009) was an Egyptian-American engineer, physicist, cryptographer, inventor and entrepreneur. He was a semiconductor pioneer who made important contributions to ...
and
Dawon Kahng
Dawon Kahng ( ko, 강대원; May 4, 1931 – May 13, 1992) was a Korean-American electrical engineer and inventor, known for his work in solid-state electronics. He is best known for inventing the MOSFET (metal–oxide–semiconductor field-effe ...
at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
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.
In 1965,
Chih-Tang Sah
Chih-Tang "Tom" Sah (; born in November 1932 in Beijing, China) is a Chinese-American electronics engineer and condensed matter physicist. He is best known for inventing CMOS (complementary MOS) logic with Frank Wanlass at Fairchild Semiconductor ...
, Otto Leistiko and A.S. Grove at
Fairchild Semiconductor
Fairchild Semiconductor International, Inc. was an American semiconductor company based in San Jose, California. Founded in 1957 as a division of Fairchild Camera and Instrument, it became a pioneer in the manufacturing of transistors and of int ...
fabricated several NMOS devices with channel lengths between
8µm and 65µm. 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
Access time is the time delay or latency between a request to an electronic system, and the access being completed or the requested data returned
* In a computer, it is the time interval between the instant at which an instruction control un ...
, which entered large-scale manufacturing in the early 1970s. This led to MOS
semiconductor memory
Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a sil ...
replacing earlier
bipolar
Bipolar may refer to:
Astronomy
* Bipolar nebula, a distinctive nebular formation
* Bipolar outflow, two continuous flows of gas from the poles of a star
Mathematics
* Bipolar coordinates, a two-dimensional orthogonal coordinate system
* Bipolar ...
and
ferrite-core memory technologies in the 1970s.
Silicon gate
In the late 1960s,
bipolar junction transistor
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...
s were faster than (p-channel) MOS transistors then used and were 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 markets, such as low power applications. One of the reasons for the low speed was that MOS transistors had
gates made of
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It h ...
which led to considerable
parasitic capacitances using the manufacturing processes of the time. The introduction of transistors with gates of
polycrystalline silicon
Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Polysilicon is produ ...
(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
Federico Faggin (, ; born 1 December 1941) is an Italian physicist, engineer, inventor and entrepreneur. He is best known for designing the first commercial microprocessor, the Intel 4004. He led the 4004 (MCS-4) project and the design group d ...
at
Fairchild Semiconductor
Fairchild Semiconductor International, Inc. was an American semiconductor company based in San Jose, California. Founded in 1957 as a division of Fairchild Camera and Instrument, it became a pioneer in the manufacturing of transistors and of int ...
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 lower parasitic capacitances that could be manufactured as part of an IC (and not only as a
discrete component
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are n ...
). 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
Analog or analogue may refer to:
Computing and electronics
* Analog signal, in which information is encoded in a continuous variable
** Analog device, an apparatus that operates on analog signals
*** Analog electronics, circuits which use analo ...
multiplexer
In electronics, a multiplexer (or mux; spelled sometimes as multiplexor), also known as a data selector, is a device that selects between several analog or digital input signals and forwards the selected input to a single output line. The sel ...
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.
NMOS and back-gate bias
There are a couple of drawbacks associated with PMOS: The
electron hole
In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle which is the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or ...
s that are the charge (current) carriers in PMOS transistors have lower mobility than the
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary partic ...
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 chemicals and other sources can very easily prevent (the
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary partic ...
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 integrated circuit (IC) technology 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 materials (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
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
contaminants in the gates until the development of
ion implantation
Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the target. Ion implantation is used in semiconductor device fa ...
(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
Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, 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
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
announced a 1-kbit pMOS DRAM, called ''1102'', developed as a custom product for
Honeywell
Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building technologies, performance ma ...
(an attempt to replace magnetic
core memory
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centr ...
in their
mainframe computer
A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterpris ...
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
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxi ...
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
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.
Enhancement-mode MOSFETs (metal–o ...
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 logic
PMOS or pMOS logic (from p-channel metal–oxide–semiconductor) is a family of digital circuits based on p-channel, enhancement mode metal–oxide–semiconductor field-effect transistors (MOSFETs). In the late 1960s and early 1970s, PMOS lo ...
, or the more positive rail for
NMOS logic
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 ...
). 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 (with the current simply proportional to the voltage) would be better, and a current source (with the current fixed, independent of voltage) better yet. A
depletion-mode device with gate tied to the opposite supply rail is a much better load than an enhancement-mode device, acting somewhere between a resistor and a current source.
The first depletion-load NMOS circuits were pioneered and made by the
DRAM
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxi ...
manufacturer
Mostek, which made depletion-mode transistors available for the design of the original
Zilog Z80
The Z80 is an 8-bit computing, 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first wor ...
in 1975–76. Mostek had the
ion implantation
Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the target. Ion implantation is used in semiconductor device fa ...
equipment needed to create a
doping profile more precise than possible with
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
methods, so that the
threshold voltage
The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important ...
of the load transistors could be adjusted reliably. At Intel, depletion load was introduced in 1974 by Federico Faggin, an ex-Fairchild engineer and later the founder of
Zilog
Zilog, Inc. is an American manufacturer of microprocessors and 8-bit and 16-bit microcontrollers. It is also a supplier of application-specific embedded system-on-chip (SoC) products.
Its most famous product is the Z80 series of 8-bit microp ...
. 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 incarnations of popular 8-bit, 16-bit, and 32-bit CPUs. Similarly to early PMOS and NMOS CPU designs using
enhancement mode
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.
Enhancement-mode MOSFETs (metal–o ...
MOSFETs as loads, depletion-load nMOS designs typically employed various types of
dynamic logic (rather than just static gates) or
pass transistors 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
[ "Motorola is redesigning the M6800 microprocessor family by adding depletion loads to increase speed and reduce the 6800 CPU size to 160 mils."]), the
6502,
Signetics 2650,
8085,
6809
The Motorola 6809 ("''sixty-eight-oh-nine''") is an 8-bit microprocessor with some 16-bit features. It was designed by Motorola's Terry Ritter and Joel Boney and introduced in 1978. Although source compatible with the earlier Motorola 6800, the 6 ...
,
8086
The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allo ...
,
Z8000
The Z8000 ("''zee-'' or ''zed-eight-thousand''") is a 16-bit microprocessor introduced by Zilog in early 1979. The architecture was designed by Bernard Peuto while the logic and physical implementation was done by Masatoshi Shima, assisted by a ...
,
NS32016
The NS32000, sometimes known as the 32k, is a series of microprocessors produced by National Semiconductor. The first member of the family came to market in 1982, briefly known as the 16032 before becoming the 32016. It was the first 32-bit general ...
, 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
Bipolar may refer to:
Astronomy
* Bipolar nebula, a distinctive nebular formation
* Bipolar outflow, two continuous flows of gas from the poles of a star
Mathematics
* Bipolar coordinates, a two-dimensional orthogonal coordinate system
* Bipolar ...
7400 series and the
CMOS 4000 series
The 4000 series is a CMOS logic family of integrated circuits (ICs) first introduced in 1968 by RCA. It had a supply voltage range of 5V to 20V, which is much wider than any contemporary logic family.
Almost all IC manufacturers active during thi ...
, 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 Z80 and x86
embedded system
An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' ...
s and many other contexts for several decades. Modern low power versions are available as CMOS or BiCMOS implementations, similar to the 7400-series.
Intel HMOS
Intel's own depletion-load NMOS process was known as HMOS, for ''High density, short channel MOS''. The first version was introduced in late 1976 and first used for their
static RAM
Static random-access memory (static RAM or SRAM) is a type of random-access memory (RAM) that uses latching circuitry (flip-flop) to store each bit. SRAM is volatile memory; data is lost when power is removed.
The term ''static'' differe ...
products, it was soon being used for faster and/or less power hungry versions of the 8085, 8086, and other chips.
HMOS continued to be improved and went through four distinct generations. 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. This version was widely licensed by 3rd parties, including (among others)
Motorola
Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorola ...
who used it for their
Motorola 68000
The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Sect ...
, and
Commodore Semiconductor Group, who used it for their
MOS Technology 8502
The MOS Technology 8502 is an 8-bit microprocessor designed by MOS Technology and used in the Commodore 128 (C128). It is an improved version of the MOS 6510 used in the Commodore 64 (C64). It was manufactured using the HMOS process, allowing i ...
die-shrunk
MOS 6502.
The original HMOS process, later referred to as HMOS I, had a channel length of 3 microns, which was reduced to 2 for the HMOS II, and 1.5 for HMOS III. By the time HMOS III was introduced in 1982, Intel had begun a switch to their
CHMOS
CHMOS refers to one of a series of Intel CMOS processes developed from their HMOS process. (H stands for high-density). It was first developed in 1981.
CHMOS was used in the Intel 80C51BH, a new version of their standard MCS-51 microcontroll ...
process, a
CMOS process using design elements of the HMOS lines. One final version of the system was released, HMOS-IV. A significant advantage to the HMOS line was that each generation was deliberately designed to allow existing layouts to die-shrink with no major changes. Various techniques were introduced to ensure the systems worked as the layout changed.
HMOS, HMOS II, HMOS III, and HMOS IV were together used for many different kinds of processors; the
8085,
8048,
8051
The Intel MCS-51 (commonly termed 8051) is a single chip microcontroller (MCU) series developed by Intel in 1980 for use in embedded systems. The architect of the Intel MCS-51 instruction set was John H. Wharton. Intel's original versions were po ...
,
8086
The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allo ...
,
80186,
80286
The Intel 80286 (also marketed as the iAPX 286 and often called Intel 286) is a 16-bit microprocessor that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non- multiplexed address and data buses and also th ...
, and many others, but also for several generations of the same basic design, see
datasheet
A datasheet, data sheet, or spec sheet is a document that summarizes the performance and other characteristics of a product, machine, component (e.g., an electronic component), material, subsystem (e.g., a power supply), or software in suffici ...
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 HMOS for applications such as the
Intel 80386
The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors[microcontroller
A microcontroller (MCU for ''microcontroller unit'', often also MC, UC, or μC) is a small computer on a single VLSI integrated circuit (IC) chip. A microcontroller contains one or more CPUs ( processor cores) along with memory and programma ...]
s. A few years later, in the late 1980s,
BiCMOS
Bipolar CMOS (BiCMOS) is a semiconductor technology that integrates two semiconductor technologies, those of the bipolar junction transistor and the CMOS (complementary metal-oxide-semiconductor) logic gate, into a single integrated circuit. I ...
was introduced for high-performance microprocessors as well as for high speed
analog circuit
Analogue electronics ( en-US, analog electronics) are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term "analogue" describes the proportional rela ...
s. 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
A gate or gateway is a point of entry to or from a space enclosed by walls. The word derived from old Norse "gat" meaning road or path; But other terms include '' yett and port''. The concept originally referred to the gap or hole in the wal ...
s and the
transmission gates of typical slow low-power CMOS circuits (the ''only'' CMOS type during the 1960s and 1970s). These methods use 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 NMOS and PMOS circuits during the 1970s.
Compared to CMOS
Compared to static CMOS, all variants of NMOS (and PMOS) are relatively power hungry in steady state. This is because they rely on load transistors working as
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias activ ...
s, 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
Domino logic is a CMOS-based evolution of the dynamic logic techniques based on either PMOS or NMOS transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer ...
with a certain amount of ''pseudo nMOS'' circuitry.
[''Pseudo nMOS means that an enhancement-mode p-channel transistor with grounded gate is used in place of the depletion-mode n-channel transistor. See http://eia.udg.es/~forest/VLSI/lect.10.pdf'']
Evolution from preceding NMOS types
Depletion-load processes differ from their predecessors in the way the ''Vdd'' voltage source, representing ''1'', connects to each gate. In both technologies, each gate contains one NMOS transistor which is permanently turned on and connected to Vdd. When the transistors connecting to ''0'' turn off, this
pull-up transistor determines the output to be ''1'' by default. In standard NMOS, the pull-up is the same kind of transistor as is used for logic switches. As the output voltage approaches a value less than ''Vdd'', it gradually switches itself off. This slows the ''0'' to ''1'' transition, resulting in a slower circuit. Depletion-load processes replace this transistor with a depletion-mode NMOS at a constant gate bias, with the gate tied directly to the source. This alternative type of transistor acts as a current source until the output approaches ''1'', then acts as a resistor. The result is a faster ''0'' to ''1'' transition.
Static power consumption
Depletion-load circuits consume less power than enhancement-load circuits at the same speed. In both cases the connection to ''1'' is always active, even when the connection to ''0'' is also active. This results in high static power consumption. The amount of waste depends on the strength, or physical size, of the pull-up. Both (enhancement-mode) saturated-load and depletion-mode pull-up transistors use greatest power when the output is stable at ''0'', so this loss is considerable. Because the strength of a depletion-mode transistor falls off less on the approach to ''1'', they may reach ''1'' faster despite starting slower, i.e. conducting less current at the beginning of the transition and at steady state.
Notes and references
{{DEFAULTSORT:Depletion-Load Nmos Logic
Logic families
MOSFETs