anybody who knows the purpose of epitaxial layer?
Why not make transisters in p-sub directly?

Epi substrates usually have lower resistivity compared to plain processes. For some reason, it is frequently used in bipolar and BiCMOS processes, I believe.

Due to the lower resistivity, it minimizes the risk of latch-up. It seems to me it used to be very popular during a couple of process generations for digital circuit design (maybe somebody else can confirm this?). It is however terrible in terms of substrate noise, because the noise almost propagates everywhere.

Paul

Wouldn't the collectors of all the PNPs be shorted together, if the PNPs were made in the p-sub layer?

River,

  Epitaxial  layers are used because they are more lightly
doped than the layers below them.  Growing an epitiaxial
layer is the only method to create a more lightly doped
layer on top of a more highly doped layer. For bipolars, an
n-epitaxial is used. This layer allows npn transistors to
have an N+ buried subcollector. The buried subcollector
suppresses the beta of the vertical pnp inherent in the
npn structure. For CMOS transistors, a p+ wafer is used
with a p-epitaxial layer to suprress the beta of the
parasitic bipolar that causes latch-up.

  Lower doped regions are preferred because the
breakdown voltage is higher, the junction capacitances
are lower, device output resistances are higher, device
threshold voltages are higher, ... This is a fundamental
challenge for our industry. Think about what happens as
processes are scaled down from 0.35um --> 90nm. The  
doping concentration of the substrate increases meaning
that the transistors have lower breakdown voltage,
less output resistance, ...

 The p+ substrates are doped near to degeneracy,
that is, they are more like "conductors" then
"semiconductors". You can not build devices in transistors
in a p+ substrate.

  Lateral pnp devices are built in n-epitaxial bipolar
processes so the devices are isolated. In CMOS
processes it is more typical to build vertical pnp
transistors using n-well as a base. These device
do not have an isolated collectors.  

                                             Best Regards,

                                                 Sheldon