Depletion-load NMOS logic: Difference between revisions

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{{short description|Form of digital logic family in integrated circuits}}

[[File:Nmos depletion and.svg|right|thumb|A depletion-load and two enhancement-mode NMOS transistor making up [[NAND gate]].]]

In [[integrated circuit]]s, '''depletion-load NMOS''' is a form of digital [[logic family]] that uses only a single power supply voltage, unlike earlier [[NMOS logic|NMOS]] (n-type [[metal-oxide semiconductor]]) logic families that needed more than one differentmultiple power supply voltagevoltages. 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]]s and other logic elements.

[[Depletion and enhancement modes|Depletion-mode]] n-type [[MOSFET]]s as load transistors allow single voltage operation and achieve greater speed than possible with pure enhancement-load devices alone. 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 [[Semiconductor device fabrication|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]].

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{{See also|NMOS logic#History}}

Following the invention of the [[MOSFET]] by [[Mohamed Atalla]] and [[Dawon Kahng]] at [[Bell Labs]] in 1959, they demonstrated MOSFET technology in 1960.<ref name="computerhistory">{{cite journal|url=https://www.computerhistory.org/siliconengine/metal-oxide-semiconductor-mos-transistor-demonstrated/|title=1960 - Metal Oxide Semiconductor (MOS) Transistor Demonstrated|journal=The Silicon Engine|publisher=[[Computer History Museum]]}}</ref> They [[Semiconductor device fabrication|fabricated]] both [[PMOS logic|PMOS]] and NMOS devices with a [[10 µmμm process|20{{nbsp}}µmμm process]]. However, the NMOS devices were impractical, and only the PMOS type were practical working devices.<ref name="Lojek">{{cite book |last1=Lojek |first1=Bo |title=History of Semiconductor Engineering |url=https://archive.org/details/historysemicondu00loje_697 |url-access=limited |date=2007 |publisher=[[Springer Science & Business Media]] |isbn=9783540342588 |pages=[https://archive.org/details/historysemicondu00loje_697/page/n327 321]–3}}</ref>

In 1965, [[Chih-Tang Sah]], Otto Leistiko and [[Andrew Grove|A.S. Grove]] at [[Fairchild Semiconductor]] fabricated several NMOS devices with channel lengths between [[10 µmμm process|8{{nbsp}}µmμm]] and 65{{nbsp}}µmμm.<ref>{{cite journal |last1=Sah |first1=Chih-Tang |author1-link=Chih-Tang Sah |last2=Leistiko |first2=Otto |last3=Grove |first3=A. S. |title=Electron and hole mobilities in inversion layers on thermally oxidized silicon surfaces |journal=[[IEEE Transactions on Electron Devices]] |date=May 1965 |volume=12 |issue=5 |pages=248–254 |doi=10.1109/T-ED.1965.15489 |bibcode=1965ITED...12..248L |url=https://pdfslide.net/documents/electron-and-hole-mobilities-in-inversion-layers-on-thermally-oxidized-silicon-57e531d33262d.html|url-access=subscription }}</ref> 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{{nbsp}}[[kibibit|kb]] data and 50{{ndash}}100 [[nanosecond|ns]] [[access time]], which entered large-scale manufacturing in the early 1970s. This led to MOS [[semiconductor memory]] replacing earlier [[bipolar junction transistor|bipolar]] and [[ferrite-core memory]] technologies in the 1970s.<ref>{{cite journal |last1=Critchlow |first1=D. L. |title=Recollections on MOSFET Scaling |journal=IEEE Solid-State Circuits Society Newsletter |date=2007 |volume=12 |issue=1 |pages=19–22 |doi=10.1109/N-SSC.2007.4785536 |doi-access=free }}</ref>

===Silicon gate===

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There are a couple of drawbacks associated with PMOS: The [[electron hole]]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 [[Diode–transistor logic|DTL-logic]] and [[Transistor–transistor logic|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 (microfabrication)|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 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.<ref>These calculators (like the [[Datapoint 2200]] and others) were in many ways small [[desktop computer]]s, but preceded the [[Apple II series|Apple II]] and the [[IBM Personal Computer|IBM PC]] by many years.</ref> 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 [[threshold voltage|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.<ref>''Shown by its mere mention in a large roundup article written by GE engineer Herman Schmid that appeared in the December, 1972 issue of IEEE Transactions on Manufacturing Technology. Although it cites Maitland’s 1970 article in Electronics, Schmid’s article does not discuss NMOS fabrication in detail but it does cover PMOS and even CMOS fabrication extensively.''</ref>

The production-ready NMOS process enabled HP to develop the industry’s first 4-kbit IC [[Read-only memory|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 [[HP 9800 series|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 [[Dynamic random-access memory|DRAM]] IC. It was formally introduced in October 1970, and became Intel’s first really successful product.<ref>{{cite web|url=http://www.hp9825.com/html/prologues.html |title=Prologues |publisher=Hp9825.com |date= |accessdate=2022-03-15}}</ref>

===Depletion-mode transistors===

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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]], or the more positive rail for [[NMOS logic]]). 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 [[Dynamic random-access memory|DRAM]] manufacturer [[Mostek]], which made depletion-mode transistors available for the design of the original [[Zilog Z80]] in 1975–76.<ref>''Zilog relied on [[Mostek]] and [[Synertek]] to produce the Z80 and other chips before their own production facilities were ready.''</ref> Mostek had the [[ion implantation]] equipment needed to create a [[doping (semiconductor)|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 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 [[Static random-access memory|SRAM]] called the ''2102'' (using more than 6000 transistors<ref>''Each bit demands six transistors in a typical [[static random-access memory|static RAM]].''</ref>). 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.<ref>''See for instance: http://www.intel4004.com/sgate.htm or http://archive.computerhistory.org/resources/text/Oral_History/Faggin_Federico/Faggin_Federico_1_2_3.oral_history.2004.102658025.pdf'' {{Webarchive|url=https://web.archive.org/web/20170110232713/http://archive.computerhistory.org/resources/text/Oral_History/Faggin_Federico/Faggin_Federico_1_2_3.oral_history.2004.102658025.pdf |date=2017-01-10 }}</ref>

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 [[Channel (transistor)|enhancement mode]] MOSFETs as loads, depletion-load nMOS designs typically employed various types of [[dynamic logic (digital logicelectronics)|dynamic logic]] (rather than just static gates) or [[Pass transistor logic|pass transistor]]s used as dynamic [[latchFlip-flop (electronics)|clocked latch]]es. 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 [[Motorola 6800|6800]] (in later versions<ref name = "M6800 redesign">{{Cite journal| title = Motorola Redesigns 6800 | journal = Microcomputer Digest | volume = 3 | issue = 2 | page =4 | publisher = Microcomputer Associates | ___location = Santa Clara, CA | date = August 1976 | url = http://www.bitsavers.org/pdf/microcomputerAssociates/Microcomputer_Digest_v03n02_Aug76.pdf}} "Motorola is redesigning the M6800 microprocessor family by adding depletion loads to increase speed and reduce the 6800 CPU size to 160 mils."</ref>), the [[MOS Technology 6502|6502]], [[Signetics 2650]], [[Intel 8085|8085]], [[Motorola 6809|6809]], [[Intel 8086|8086]], [[Zilog Z8000|Z8000]], [[NS32000|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,

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===Intel HMOS===

{{redirect|HMOS|operating system|HarmonyOS}}

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,<ref>{{cite journal |first1=A.M. |last1=Volk |first2=P.A. |last2=Stoll |first3=P. |last3=Metrovich |title=Recollections of Early Chip Development at Intel |journal=Intel Technology Journal |volume=5 |issue=Q1 |pages= |date=2001 |url=https://www.intel.com/content/dam/www/public/us/en/documents/research/2001-vol05-iss-1-intel-technology-journal.pdf}}</ref> it was soon being used for faster and/or less power hungry versions of the 8085, 8086, and other chips.

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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]] 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.<ref>{{cite conference |conference=ISSCC 82 |date=1982 |title=HMOS III Technology}}</ref><ref>{{cite journal |first1=G.E. |last1=Atwood |first2=H. |last2=Dun |first3=J. |last3=Langston |first4=E. |last4=Hazani |first5=E.Y. |last5=So |first6=S. |last6=Sachdev |first7=K. |last7=Fuchs |title=HMOS III technology |journal=IEEE Journal of Solid-State Circuits |volume=17 |issue=5 |pages=810–5 |date=October 1982 |doi=10.1109/JSSC.1982.1051823 |bibcode=1982IJSSC..17..810A |s2cid=1215664 |url=}}</ref>

HMOS, HMOS II, HMOS III, and HMOS IV were together used for many different kinds of processors; the [[Intel 8085|8085]], [[Intel MCS-48|8048]], [[Intel 8051|8051]], [[Intel 8086|8086]], [[Intel 18680186|80186]], [[Intel 28680286|80286]], and many others, but also for several generations of the same basic design, see [[datasheet]]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 [[i386|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 [[analogue electronics|analog circuitcircuits]]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 [[:wikt:complementary|complementary]] ''[[CMOS|static]]'' [[logic gate|gate]]s and the [[transmission gate]]s of typical slow low-power CMOS circuits (the ''only'' CMOS type during the 1960s and 1970s). These methods use significant amounts of [[dynamic logic (digital logicelectronics)|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==