AW: Re: FET Transistors

[Paolo Predonzani]

> >     Like - okay what's the real difference between an
> >      emitter and a collector in a transistor? In an NPN they're both "N"
> >      with a "P" between them, but you can't swap them in a circuit. Why
> >      not?
> 
> As I said, I can't give the full physical details, but the geometry of
> the three layers is very unsymmetrical, in order to achive a large
> beta in one direction and don't care for the beta in the reverse direction.

The emitter is doped more than the collector. This has 2 effects:
-larger beta in normal active region than in inverse region.
-breakdown voltage of BE junction smaller than breakdown voltage of BC junction.

There should be a difference in stored charge, too. This can affect the
freq. response of the BJT.

Some power BJTs have a npnn structure where the collector is doped in
two differnt ways to improve its breakdown voltage. I never had time to
study how they work.

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| Paolo Predonzani  |  email: predo at dist.dist.unige.it |
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