Hi!
I was wondering what happens with Base current direction if NPN BJT works in reverse active mode? In forward active and in saturation mode currents are like this:



Collector and emitter currents in reverse active mode would be in opposite direction:

 

If the device were constructed symmetrically, then it would behave just like the same NPN with the collector and emitter swapped. Most MOSFETs implemented on an IC are this way and the "drain" and "source" are interchangeable. But most BJT transistors, even on an IC, are not symmetric and are designed to allow high reverse bias voltages on the base-collector junction. A consequence of this is that the reverse beta tends to be very low. It IS an NPN transistor even when reversed, just a pretty crappy one by the metrics we generally use.

 

The only advantage that the reverse transistor has is it's very low Vce. This trick was used long ago to minimize offset.