two types of bipolar ESD diodes.

hello.. i am currently studying esd especially bipolar. i have seen two connections for bipolar:
a) NPN transistor- emitter and base shorted to ground and collector connected to pad.
b) PNP transistor-emitter connected to ground and collector and base shorted to pad

whats the difference between the two and which is better?

There's "better" and there's "better". As in "free".

ESD diodes are often opportunistic. In an P-substrate, N-well
(N-collector) technology you'd have a free PNP but no choice
other than the collector being substrate potential. A good,
diffuse current path but poor transistor attributes, high BV.
The transistor only has to be big, tough and not leak. Any
current gain only reduces the forward voltage some.

The fancy little signal transistors are likely not so suited
to abuse.

thank you for your reply.

you mean that in a p-sub process, i only get a PNP type esd diode with collector grounded and emitter and base shorted to pad? can you recommend any books or materials that i could use.

Your collector is substrate, your base is well, your emitter is
S/D. This is a very normal CMOS situation and the only vertical
BJT in many flows, hence the only device with decently high
areal current density. It can unfortunately not serve all roles
since one terminal is pinned.

Your other diode is the N-well, P S/D "diode" which is in fact
the E-B of that PNP, so you have to be aware / sure about
what that action will be, when you consider what the "diode"
will do. It could help, or it could hurt.

It is very helpful to draw cutaway figures of the I/O devices
and ESD elements, if you have an eye for device structure
you would immediately recognize the PNP in a PMOS FET.
Less obvious are the combinations of lateral and vertical
features that make (low quality, but still troublesome)
SCRs and lateral BJTs.

I hate JI technologies myself. Not the transistors so much.
It's the in-laws.

yup, i have seen the PNP transistor.. however i am a little bit confused on how this transistor respond to ESD? if lets say esd appears at the S/D or the emitter then how can this transistor turn on to clamp this esd and sink the current associated with the esd?