Why is an Nwell guard ring needed with deep nwell?

[Artist27]

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?

 

[dick_freebird]

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.

 

[lazybear]

this cross section is how DNW commonly look, DNW need NW for body connection.

 

[RodrigoSP]

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