Editing NMOS logic (section)

{{More citations needed|date=January 2024}}
{{short description|Form of digital logic family in integrated circuits}}
'''NMOS''' or '''nMOS''' logic (from N-type metal–oxide–semiconductor) uses [[n-type semiconductor|n-type]] (-) [[MOSFET]]s (metal–oxide–semiconductor [[field-effect transistor]]s) to implement [[logic gate]]s and other [[digital circuit]]s.<ref>{{cite book |author-first1=Mohammed|author-last1=Ferdjallah|chapter=5.4 NMOS and PMOS Logic Gates|title=Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL|chapter-url=https://www.oreilly.com/library/view/introduction-to-digital/9780470900550/chap5-sec004.html|publisher=Wiley|year=2011|via=O'Reilly Media, Inc.|language=en|isbn=9780470900550}}</ref><ref>{{cite journal |last1=Kong |first1=Lingan |last2=Chen |first2=Yang |last3=Liu |first3=Yuan |title=Recent progresses of NMOS and CMOS logic functions based on two-dimensional semiconductors |journal=Nano Research |date=June 2021 |volume=14 |issue=6 |pages=1768–1783 |doi=10.1007/s12274-020-2958-7}}</ref> 

NMOS transistors operate by creating an [[inversion layer (semiconductors)|inversion layer]] in a [[p-type semiconductor|p-type]] transistor body. This inversion layer, called the n-channel, can conduct [[electron]]s between [[n-type semiconductor|n-type]] ''source'' and ''drain'' terminals. The n-channel is created by applying voltage to the third terminal, called the ''gate''. Like other MOSFETs, nMOS transistors have four modes of operation: cut-off (or subthreshold), triode, saturation (sometimes called active), and velocity saturation.

NMOS AND-by-default logic can produce unusual glitches or buggy behavior in NMOS components, such as the [[6502]] "illegal opcodes" which are absent in CMOS 6502s. In some cases such as Commodore's [[VIC-II]] chip, the bugs present in the chip's logic were extensively exploited by programmers for graphics effects.

For many years, NMOS circuits were much faster than comparable [[PMOS logic|PMOS]] and [[CMOS]] circuits, which had to use much slower p-channel transistors. It was also easier to manufacture NMOS than CMOS, as the latter has to implement p-channel transistors in special n-wells on the p-substrate, not prone to damage from bus conflicts, and not as vulnerable to electrostatic discharge damage. The major drawback with NMOS (and most other [[logic family|logic families]]) is that a [[direct current]] must flow through a logic gate even when the output is in a [[steady state]] (low in the case of NMOS). This means static [[power dissipation]], i.e. power drain even when the circuit is not switching, leading to high power consumption.

Another disadvantage of NMOS circuits is their thermal output. Due to the need to keep constant voltage running through the circuit to hold the transistors' states, NMOS circuits can generate a considerable amount of heat in operation which can reduce the device's reliability. This was especially problematic with the early large gate process nodes in the 1970s. CMOS circuits for contrast generate almost no heat unless the transistor count approaches 1 million.

CMOS components were relatively uncommon in the 1970s-early 1980s and would typically be indicated with a "C" in the part number. Throughout the 1980s, both NMOS and CMOS parts were widely used with CMOS becoming more widespread as the decade went along. NMOS was preferred for components that performed active processing such as CPUs or graphics processors due to its higher speed and cheaper manufacturing cost as these were expensive compared to a passive component such as a memory chip, and some chips such as the [[Motorola 68030]] were hybrids with both NMOS and CMOS sections. CMOS has been near-universal in integrated circuits since the 1990s.

Additionally, just like in [[diode–transistor logic]], [[transistor–transistor logic]], [[emitter-coupled logic]] etc., the asymmetric input logic levels make NMOS and PMOS circuits more susceptible to noise than CMOS. These disadvantages are why [[CMOS logic]] has supplanted most of these types in most high-speed digital circuits such as [[microprocessor]]s despite the fact that CMOS was originally very slow compared to [[logic gate]]s built with [[bipolar transistor]]s.