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 n-type MOSFETs are arranged in a so-called "pull-down network" (PDN) between the logic gate output and negative supply voltage, while a resistor is placed between the logic gate output and the positive supply voltage.

The circuit is designed such that if the desired output is low, then the PDN will be active, creating a current path between the negative supply and the output. 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 negative supply, 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 occurs only when both A and B are low, thus satisfying the truth table of a NOR gate: While NMOS logic is easy to design and manufacture (a MOSFET can be made to operate as a resistor, so the whole circuit can be made with nMOSFETs), it has several shortcomings as well. The worst problem is that a DC current flows through an NMOS logic 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.

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