Difference between N-well and Deep N-well

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The regular N-well only has to stand off the low voltage
supply / low voltage transistor Vds(max). In the interests
of density the well will be as shallow as practical (or
manufacturable) to get the depletion width.

But a deeper implant is needed for high voltage, to get
vertical depletion room and the curvature needed to not
unacceptably "enhance" breakdown. This then makes it
also side-diffuse during the implant drive and limits the
packing density.

You may see both together in a BCDMOS process. You
might see only one in an SOI, trench defined.
 

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