a "load" that can be thought of as a resistor, see below) is placed between the positive supply voltage and each logic gate output. The MOSFETs are n-type enhancement mode transistors, arranged in a so-called "pull-down network" (PDN) between the logic gate output and negative supply voltage (typically the ground). These silicon gates are still used in most types of MOSFET based integrated circuits, although metal gates ( Al or Cu) started to reappear in the early 2000s for certain types of high speed circuits, such as high performance microprocessors. 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. MOS stands for metal-oxide-semiconductor, reflecting the way MOS-transistors were originally constructed, predominantly before the 1970s, with gates of metal, typically aluminium. ![]() 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. power drain even when the circuit is not switching, leading to high power consumption.Īdditionally, 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. This means static power dissipation, i.e. 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). 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. ![]() Like other MOSFETs, nMOS transistors have four modes of operation: cut-off (or subthreshold), triode, saturation (sometimes called active), and velocity saturation.įor many years, NMOS circuits were much faster than comparable PMOS and CMOS circuits, which had to use much slower p-channel transistors. The n-channel is created by applying voltage to the third terminal, called the gate. This inversion layer, called the n-channel, can conduct electrons between n-type "source" and "drain" terminals. These nMOS transistors operate by creating an inversion layer in a p-type transistor body. N-type metal–oxide–semiconductor logic uses n-type (-) MOSFETs (metal–oxide–semiconductor field-effect transistors) to implement logic gates and other digital circuits. ![]() Form of digital logic family in integrated circuits
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