Wats is the purpose of putting epitaxial layer on substrate

[Somashekhar]

Epitaxial layer.

Wats is the main purpose of putting the epitaxial layer on the substrate??
When exactly its used??

 

[dick_freebird]

Epitaxial layer.

That is not so much about what the epi is, bur what it is not.

You would put an epi layer on top of a heavily doped handle
wafer to get a stiff substrate shunt, yet have a device layer
that can be sensibly doped and counter-doped for CMOS
transistors. The light doping needed for those, if the whole
wafer thickness was that, would be difficult to keep from
latchup without very dense ties.