Grounded-gate NMOS, commonly known as ggNMOS, is an electrostatic discharge (ESD) protection device used within CMOS integrated circuits (ICs).
Such devices are used to protect the inputs and outputs of an IC, which can be accessed off-chip (wire-bonded to the pins of a package or directly to a printed circuit board) and are therefore subject to ESD when touched.
An ESD event can deliver a large amount of energy to the chip, potentially destroying input/output circuitry; a ggNMOS device or other ESD protective devices provide a safe path for current to flow, instead of through more sensitive circuitry.
When a positive ESD event appears upon the I/O pad (drain), the collector-base junction of the parasitic NPN BJT becomes reverse biased to the point of avalanche breakdown.
The positive VBE forward biases this junction, triggering the parasitic NPN BJT.