nus_lin wrote on Jan 12th, 2011, 4:20am:dear guys, the bjt used in bandgap reference is actually diodes, so why don't we just use nwell-diodes, or n+p- diodes?
Why should the foundry bother to model one more device (a diode) when it already is modelling a BJT, which is anyway part of the design kit? Since a BJT may be used for other things and not just as a diode, it is also modelled much more extensively as pointed out by the others.
Anyway, you never have plain diodes in any process. There are always other parasitic diodes which you need to model anyway.
Also, the BJT layout is usually well-defined and fixed.
If you decide to use a perfectly good BJT to make a diode, then that's your decision. It is also possible to make excellent bandgaps where the BJT C-B junction is not shorted out.
Vivek