Hi Walter,
Their is no phase change going on. The transistor is a switch.
The negative is already their at the emitter, and by applying positive to the base, the negative is allowed to flow from the emitter to the collector.
The positive from the base is NOT being changed into negative.
The emitter, collector region is like a diode once you apply a signal to the base.
I have been refering to " place a positive to the base " only becuase I am used to using an NPN.
If you read the article I posted, you will see that all the base signal does, is reduce the insulating silicon to allow ngative to flow from emitter to the collector.
THE POLARITY IS NOT CHANGED. THE SIGNAL, IS JUST THAT, A SIGNAL TO ALLOW CURRENT TO FLOW FROM THE EMITTER OUT THE COLLECTOR.
Read this article:
http://www.amasci.com/amateur/transis.html
After reading this article if you have any troubles, write back.
I suggest you read this article it shows you what goes on inside the transistor, and therefore will help you understand how it works.
Hope this helps!
Regards
D.J.