Grounded-gate
NMOS, commonly known as ggNMOS, is an
electrostatic discharge
Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two electrically charged objects caused by contact, an short circuit, electrical short or dielectric breakdown. A buildup of static electricity can be caused ...
(ESD) protection device used within
CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
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
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
) 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. ESD protection by means of such devices or other techniques is important to product reliability: 35% of all IC failures in the field are associated with ESD damage.
Structure
As the name implies, a ggNMOS device consists of a relatively wide NMOS device in which the gate, source, and body are tied together to ground. The drain of the ggNMOS is connected to the I/O pad under protection. A
parasitic NPN
bipolar junction transistor (BJT) is thus formed with the drain (
n-type) acting as the collector, the base/source combination (n-type) as the emitter, and the
substrate
Substrate may refer to:
Physical layers
*Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached
** Substrate (locomotion), the surface over which an organism lo ...
(
p-type) as the base. As is explained below, a key element to the operation of the ggNMOS is the
parasitic resistance present between the emitter and base terminals of the parasitic npn BJT. This resistance is a result of the finite
conductivity of the p-type doped substrate.
Operation
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. At this point, the positive current flowing from the base to ground induces a voltage potential across the parasitic resistor, causing a positive voltage to appear across the base-emitter junction. The positive V
BE forward biases this junction, triggering the parasitic NPN BJT.
References
{{reflist, refs=
[{{cite conference
, title = ESD design methodology
, last1 = Issaq
, first1 = E.
, last2 = Merri
, first2 = R.
, date = 1993
, pages = 223-237
, location = Lake Buena Vista, Florida
, conference = Electrical Overstress/Electrostatic Discharge Symposium
]
[{{cite conference
, title = A review of EOS/ESD field failures in military equipment
, last1 = Green
, first1 = T.
, date = 1988
, pages = 7-14
, location = Anaheim, California
, conference = Electrical Overstress/Electrostatic Discharge Symposium
]
[{{cite book , last=Wang, first=Albert, date=2002, title=On-Chip ESD Protection for Integrated Circuits: An IC Design Perspective, location=Norwell, MA, USA, publisher=Kluwer Academic Publishing, isbn=0792376471]
https://www.researchgate.net/publication/4133911_Modeling_MOS_snapback_for_circuit-level_ESD_simulation_using_BSIM3_and_VBIC_models
Semiconductor devices
MOSFETs