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Concept
NMOS logic
Summary
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 transistor body. This inversion layer, called the n-channel, can conduct electrons between 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.
For many years, NMOS circuits were much faster than comparable 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. The major drawback with NMOS (and most other 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.
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 microprocessors despite the fact that CMOS was originally very slow compared to logic gates built with bipolar transistors.
MOS stands for metal-oxide-semiconductor, reflecting the way MOS-transistors were originally constructed, predominantly before the 1970s, with gates of metal, typically aluminium. Since around 1970, however, most MOS circuits have used self-aligned gates made of polycrystalline silicon, a technology first developed by Federico Faggin at Fairchild Semiconductor.
Official source
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