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Encyclopedia > NMOS

NMOS logic uses n-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits. NMOS transistors have three modes of operation: cut-off, triode, and saturation (sometimes called active). The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape of the nonconductive depletion layer within a semiconductor material, thus controlling the conductivity of a channel in that material. ... A logic gate is an arrangement of electronically-controlled switches used to calculate operations in Boolean algebra. ... Digital circuits are electric circuits based on a number of discrete voltage levels. ...

The n-type MOSFETs are arranged in a so-called "pull-down network" (PDN) between the output and low-voltage (ground) rail, while a resistor is placed between the output and the high-voltage rail. The circuit is designed such that if the desired output is low , then the PDN will be active, creating a current path between the low-voltage rail and the output. In the physical sciences, potential difference is the difference in potential between two points in a conservative vector field. ...

As an example, here is a NOR gate in NMOS logic. If either input A or input B is high (logic 1, = True), the respective MOS transistor acts as a very low resistance between the output and the low-voltage supply rail, forcing the output to be low (logic 0, = False). When both A and B are high, both transistors are conductive, creating an even lower resistance path to ground. The only case where the output is high is when both transistors are off, which only occurs when both A and B are low, thus satisfying the truth table of a NOR gate: Re-upload of image originally mis-titled NMOS_NAND_WITH_RESISTIVE_LOAD.PNG by Allen Timothy Chang [User:Allentchang] (Move did not work) File links The following pages link to this file: NMOS Categories: GFDL images ... This article is about logical nor Nór. ...

A B A NOR B
0 0 1
0 1 0
1 0 0
1 1 0

While NMOS logic is easy to design and manufacture (a MOSFET can be made to operate as a resistor, so the whole circuit can literally be made with one type of component), it has several shortcomings as well. The worst problem is that a DC current flows through an NMOS gate when the PDN is active, that is whenever the output is low. This leads to static power dissipation even when the circuit sits idle.

Also, NMOS circuits are slow to transition from low to high. When transitioning from high to low, the transistors provide low resistance, and the capacitative charge at the output drains away very quickly. But the resistance between the output and the high supply rail is much greater, so the low to high transition takes longer. Using a resistor of lower value to speed up the process is a mixed blessing as this also increases power dissipation. Additionally, the asymmetric behaviour makes NMOS circuits susceptible to noise.

These disadvantages are why NMOS logic has been supplanted by CMOS both in low-power and in high-speed digital circuits, such as microprocessors, during the 1980s. CMOS (pronounced see-moss) is the name used for a major class of integrated circuits. ... Microprocessors, including an Intel 80486DX2 and an Intel 80386 A microprocessor (abbreviated as µP or uP) is an electronic computer central processing unit (CPU) made from miniaturized transistors and other circuit elements on a single semiconductor integrated circuit (IC) (aka microchip or just chip). ...

Results from FactBites:

 New Mexico Ornithological Society (397 words) The NMOS Field Notes Database is now available for searching! The 45th Annual Meeting of the New Mexico Ornithological Society will be held Saturday, May 5, 2007, at Camp Washington Ranch and Retreat near Carlsbad in Eddy County. Sorry, we are experiencing technical difficulties with the database at the moment.
 NMOS Database Overview (194 words) The New Mexico Ornithological Society (NMOS) has partnered with the Natural Heritage New Mexico (NHNM) to develop a searchable database of the Society's publications containing field note observations of the Society's membership. The NMOS Field Notes have been compiled and entered into the database by NMOS volunteers and NHNM employees. For example, if you would like to know all of the species identified by a specific observer in one, two, or more counties, select these variables and the results of your query will be presented in a table which can be viewed and subsequently saved to your computer.
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