Why was this question migrated back to the EE forum.
Granted it is about an electronics device, but the atomic level behaviour of the seimiconductor junction is definitely a physics question.
I originally posted it to both forums, understanding that the EE folks have a more behavioural explanation for the phenomenon, but the root physical description of what is going on is much less clear, to many an EE.
Indeed the accepted descriptions are, in the EE world, to the most part, accepted at face value with an air of.. "OK, I don't get it, but it explains what I can measure it doing, so let's go with that."
I'd really like so see a deeper explanation than just "oh it's because electron s magically diffuse across despite the electric field going the wrong way and without affecting the base in their travels."