BJT query

[Badar]

Well i am not sure whether i've asked this question or not but i still don't know the answer athough i've searched it.
The question is in a BJT we have the sizes of the regions as follows.
Collector>Emitter>Base.
And the Doping as Follows
Emitter>Collector>Base
Why is this so.
The things i know are Emitter has to provide charge carriers so its size is smaller and highest doping.And in Base region We want least carriers (e.g. electrons) to meet or combine with the others (holes).
But what is the reason behind the largest collector size and low doping

 

[Russlk]

I am no expert but here is my input: the collector is doped first and is the largest because the lower doping has higher voltage rating and the collector disapates more power. The base is doped next, then the emitter. Since the emitter has the highest doping, it has a low voltage rating. I believe this is the epitaxial process. Other processes may be different.