Vitaliy wrote:
> Perhaps I'm not phrasing the question correctly?
>
> I'm not asking how to use a transistor. I understand that exceeding or
> even approaching the maximum ratings is a no-no. I understand that when
> the transistor is in saturation, the power dissipated (assuming Imax)
> is lower than when it's operating in the linear region, etc, etc.
>
> I'm just trying to understand why in a saturated transistor the product
> of the C-E voltage and maximum allowed current through the transistor is
> considerably less than the maximum dissipated power.

Because heating isn't the only limiting factor on transistor operation.  The
holes and electrons in the semiconductor material can only move so fast and
there can only be so many of them.  Most of the useful operating modes of
semiconductor devices comes from "starvation" or careful control of the
minority carriers.  Another way to put it is that if you have too many holes
and electrons banging around in a transistor, you cease to have a
transistor.

Other secondary (usually) limitations come from other parts of the overall
package.  The dissipation in the bond wires and the junctions between the
bond wires and the semiconductor material is a function of current thru the
device, not on how much the semiconductor device dissipates itself.  In
practise beefing up the bond wires and connections is relatively cheap
compared to beefing up the transistor, so in most real cases the limits they
impose are higher than the limits of the semiconductor device itself.

There are various independent physical processes going on, and each impose
their own limits.  The allowed operating region is the union of the
operating regions imposed by each of the separate limits.  In some parts of
the operating range you will bump up against power dissipation, in other
parts against inherent maximum current carrying capability, and voltage
stresses in other parts.

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