Why is an Nwell guard ring needed with deep nwell?

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Artist27

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While making an analog layout, We always use nwell ring with deep nwell? why is it so? Can I use just the deep nwell layer?
 

Deep NWell probably doesn't reach the surface
and N+ maybe doesn't reach down far enough
to contact it; ordinary (surface-adjacent) NWell
bridges the gap.
 
this cross section is how DNW commonly look, DNW need NW for body connection.

 
And the NW could be just in on point to bias the DNW, but it should be a ring in order to isolate the internal PW from the substrate
 

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