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Why are you asking this question in a general elementary electronics forum. Why don't you get a book on semiconductor fabrication and study that? I would think that in this forum, perhaps one person in a 100 even understands your question.
In cheap CMOS technologies the handle is usually P-
(so NMOS needs no well and the substrate can be tied
to ground / VSS). A lot of things follow from that choice.
But that choice is not necessarily -the- choice, putting
your premise in question somewhat.
Your buried layer is only an ohmic contact extension
of the collector, which lies at the bottom of the structure
yet has to make it back to the top. It should be obvious
that it needs to be the same species as the collector,
which in your NPN is... drum roll....
Higher performance can be had by adding a sinker as well
to connect the buried layer to the frontside contact.
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