In CMOS, why circuits are built on p-substrate?

[dhruvabedre]

Hi,

In CMOS technology why always the circuits are built using nmos/pmos on p-substrate but not on n-substrate?

 

[dick_freebird]

I've worked in technologies built on N- epi, N+ handle
material, so forget "always". Uncommon, yes.

Since CMOS logic favors NMOS drive strength / area
making those devices not-require a well implant may
have been one "value". The "Pwell" in modern technolgies
is not a deep implant with gross side diffusion to be
added to spacings. The PWell in N-epi CMOS has only
boron to choose from which side-diffuses a lot under
process thermal budget. An NWell can be made with
As and that stays where you put it pretty much, for
less groundrule bloat (phosphorus, moves more, but at
least there's a choice).

 

[dhruvabedre]

Thanks a lot for the explanation dick_freebird!