depletion-load NMOS
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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 compati ...
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 circ ...
s and other logic elements. Depletion-mode n-type MOSFETs 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 A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it. A current source is the dual of a voltage source. The term ''current sink'' is sometimes used for sources fed ...
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 ran ...
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. When ...
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 s ...
. 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 by Mohamed Atalla 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 development, research and scientific developm ...
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, Otto Leistiko and A.S. Grove at Fairchild Semiconductor fabricated several NMOS devices with channel lengths between 8µm and 65µm. Dale L. Critchlow and
Robert H. Dennard Robert Heath Dennard (born September 5, 1932) is an American electrical engineer and inventor. Biography Dennard was born in Terrell, Texas, U.S. He received his B.S. and M.S. degrees in Electrical Engineering from Southern Methodist University, ...
at IBM also fabricated NMOS devices in the 1960s. The first IBM NMOS product was a
memory chip 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 ...
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 uni ...
, 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.


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
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made of
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which led to considerable parasitic capacitances 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 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 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). 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
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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 Decoder may refer to: Technology * Audio decoder converts digital audio to analog form * Binary decoder, digital circuits such as 1-of-N and seven-segment decoders * Decompress (compression decoder), converts compressed data (e.g., audio/video/i ...
), 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 ) 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 particles because they have no ...
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 ) 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 particles because they have no ...
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 International Solid-State Circuits Conference is a global forum for presentation of advances in solid-state electrical network, circuits and System-on-a-chip, Systems-on-a-Chip. The conference is held every year in February at the San Francisco ...
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
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.
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, Motorol ...
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. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...
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
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in their mainframe computers). 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.


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 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 t ...
). 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 manufacturer
Mostek Mostek was a semiconductor integrated circuit manufacturer, founded in 1969 by L. J. Sevin, Louay E. Sharif, Richard L. Petritz and other ex-employees of Texas Instruments. At its peak in the late 1970s, Mostek held an 85% market share of the d ...
, which made depletion-mode transistors available for the design of the original
Zilog Z80 The Z80 is an 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 working samples were ...
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 chemica ...
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 s ...
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. 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 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 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small te ...
, Signetics 2650, 8085, 6809,
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 (allowi ...
, 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 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 compati ...
in NMOS, such as the bipolar
7400 series The 7400 series of integrated circuits (ICs) are a popular logic family of transistor–transistor logic (TTL) logic chips. In 1964, Texas Instruments introduced the SN5400 series of logic chips, in a ceramic semiconductor package. A lo ...
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 th ...
, 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 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, Motorol ...
who used it for their Motorola 68000, 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 it ...
die-shrunk
MOS 6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small te ...
. 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 The MCS-48 microcontroller series, Intel's first microcontroller, was originally released in 1976. Its first members were 8048, 8035 and 8748. The 8048 is probably the most prominent member of the family. Initially, this family was produced us ...
, 8051,
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 (allowi ...
,
80186 The Intel 80186, also known as the iAPX 186, or just 186, is a microprocessor and microcontroller introduced in 1982. It was based on the Intel 8086 and, like it, had a 16-bit external data bus multiplexed with a 20-bit address bus. The 80188 ...
,
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 the ...
, and many others, but also for several generations of the same basic design, see
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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 transistorsmicrocontrollers. 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. In ...
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 relat ...
s. Today, most digital circuits, including the ubiquitous
7400 series The 7400 series of integrated circuits (ICs) are a popular logic family of transistor–transistor logic (TTL) logic chips. In 1964, Texas Instruments introduced the SN5400 series of logic chips, in a ceramic semiconductor package. A lo ...
, 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 Static may refer to: Places *Static Nunatak, a nunatak in Antarctica United States * Static, Kentucky and Tennessee *Static Peak, a mountain in Wyoming **Static Peak Divide, a mountain pass near the peak Science and technology Physics *Static el ...
'' gates and the
transmission gate A transmission gate (TG) is an analog gate similar to a relay that can conduct in both directions or block by a control signal with almost any voltage potential. It is a CMOS-based switch, in which PMOS passes a strong 1 but poor 0, and NMOS passes ...
s of typical slow low-power CMOS circuits (the ''only'' CMOS type during the 1960s and 1970s). These methods use significant amounts of
dynamic Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical dyna ...
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 resistors, 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 Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical dyna ...
and/or domino logic 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