In most semiconductors, increased temperature bumps more carriers up
     into the conduction band from the valence band leading to lower
     resistance.  Higher temps also lead to more lattice vibrations called
     phonon scattering which increase resisitance.  In metals, which
     already have saturated conduction bands, this effect dominates.  Until
     a semiconductor becomes degenerate by heavy doping or very high
     temperatures (> ~200C), lower resistance due to greater carrier
     concentration dominates.

     -Kurt


______________________________ Reply Separator _________________________________
Subject: Re: H-Bridge - Warning!
Author:  Ben L Wirz <blw2@CEC.WUSTL.EDU> at Internet_Exchange
Date:    12/7/96 12:18 AM


Hello Everyone,

On Tue, 3 Dec 1996, Martin Nilsson wrote:

> The reason is as follows:
>
> Bipolar transistors have negative temperature coefficient,
> i.e. resistance goes down as they heat up. For two paralleled outputs,
> the one with lower on-resistance will take more current, and will heat
> up more, resistance becomes even lower, etc. The positive feedback
> quickly causes current imbalance.

        I've been thinking about this for a week now and I'm not sure I
understand.  I thought resistance always increased with increasing
temperature.  The basic idea being that as you heat something up the
mocules move around faster making it that much harder for the electrons
to move through the conductor which is in effect increasing the resitance.
Take super conductivity for example, you decrease the temperature to the
point where the mocules are moving very slowly which makes the resistance
very low.

        Now I can believe that increasing the temperature affects the
transitors Beta, but I find it hard to believe that it decreases the
resitance.

        The total emitter current is given by:

        Ie=Ieo(e^(Vbe/nVt)-1)

        Vt=kT/q

        As T increase, the (Vbe/nVt) term will decrease.  Raising the
exponent to a smaller power results in a smaller Ie.  So it looks to me
like increasing the temperature results in smaller current flow through
the eimitter which will cause a smaller current in the collector.

        The one thing I may be overlooking is that if the Beta does change
with temperature in a real part.  Does anyone have some insight for me?

Thanks,

Ben

P.S.  Sorry for the Non-PIC related post.