AW: Re: FET Transistors

[Haible_Juergen#Tel2743]

>     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?

Sorry that I can't give the long and comprehensive answer you want,
but anyway: A bipolar transistor *does* work in the reverse direction.
Parameters are very different then. Beta for example becomes very
small, but if memory serves the saturation voltage is even better (smaller)
in reverse operation. (?) So if you want to switch an audio signal
off by shorting it to gnd (often used to avoid a thumping noise at 
power-on),
a transistor with C and E connections interchanged is the way
to go.
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.
Unlike a fet (where the resistance is controlled by electric fields), the
bipolar transistor is controlled by *diffusion* effects. Therefore the
*thickness* of the different layers is very important.

JH.