[SOLVED] Deep NWell of a NMOS

Deep NWell for a NMOS transistor

Hello all,

I've read in this forum about the purpose of a deep nwell for a NMOS transistor:
one is to isolate NMOS transistors from each other, which is clearly for me.
The other one is to reduce noises, which I can not understand.

I would be thankful if someone could explain me, please:

1. which sort of noises? What is the reason of these noise?
2. how a DNW (deep nwell) reduce noises?
3. Why are DNWs used for NMOS transistors in high-voltage circuits?


-

P substrate is a common node (distributed resistance mesh)
across the circuit and every switching transition kicks it with
displacement currents (Cdb*dV/dt). These currents may not
entirely return via the ohmic ties. They may at least modulate
nearby channels (body effect) and enter circuit response.

A deep NWell gives an additional layer of junction isolation
with a P pocket inside. You can tie this well separately to
a quiet ground (which may or may not be VSS) to take
away most or all of substrate noise (and let us not forget
minority carriers which might stumble on by).

The DNW-Psub junction can be (not to say, is) higher
breakdown voltage than N+/Psub, and can be let fly
to high voltages while carrying the NMOS inside along
with it. This eliminates adverse body effect and the
limitations of BVgb in applying NMOS (and LDMOS,
which is where you often see this process feature
come in).

0. ... not just to isolate them from each other - this is guaranteed also for NMOSes in p-substrate. But it enables NMOSes in serial connection with Vgs=0 for one serially connected transistor by connecting its p-bulk to the source potential.

1. noises from external sources, as e.g. switched transistor noise
2. by connecting the DNW n+ contacts to the (positive) supply rail (power supply intended to have least noise)
3. NMOSes for HV circuits aren't specifically made in DNWs - higher voltages can often be achieved in the substrate - but of course it's possible to have "HV" transistors in DNWs. "HV" often then means just 12, 10V, even 5V is sometimes called "HV" in 2.5V (or less) core voltage circuits.

Many thanks for the replies. Now, I understand clearly.