# PN junction, transistor

1. Feb 17, 2012

### DanielHuber

I am having a hard time to find answers to 2 simple questions regarding semiconductors.

Firts, at a PN junction diffusion creates a potential step. Why can this voltage not been measured from the outside. The schoolbook answer is, that there are diffusion potential at the wire connections, that exactly cancels the PN voltage. Can anybody explain what causes this diffusion potential and why it exactly cancels the PN voltage.

Second, I think I understand how a transistor works as long as Uce > Ube. However, I have a problem when Uce < Ube. In a reisistor network, the current would reverse. Not so in a transistor. If Ucb changes sign (for small Ucb), the current becomes smaller, but does not reverse sign yet. That means that the charge carriers need additional energy to flow from the basis to the collector. Where does this energy come from? Thermal motion?

cheers, Daniel

2. Feb 17, 2012