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well i suppose what u meant by your question is why do we always use a buried n well inside a p well and not the other way round..
well when u bury an p well(p sub) on an n-well,it s doping should be high ie Na>2Nd,,
this will decrease the mobility of the p sub which is already less compared to the n sub,hence this is not advantageous compared when u bury an n well into a p well,,
and sizing also becomes a problem when an p well is buried into an n well..(sizing is done to match the mobilities of n and p device)