This is stricly way off topic, but understanding how semiconductors work
is a big help when using them to switch power, so,



> >   conductivity = n*e*(mu_e + mu_h)        (6.35)
> >
> > The number of carriers is proportional to,
> >
> >   n = f(T) * exp( -Eg / (2*kb*T) )        (6.36)
> >
> > and f(T) is a function that depends only weakly on T, e.g., a polynomial.
> >
> > The end result is stated as, "Thus, conductivity increases exponentially
> > with temperature because of the exponential factor in (6.36)." pg. 275.


Equation 6.36 is only for an intrinsic semiconductor.  Actual devices are doped
heavily n or p to a degree much greater than the intrinsic level ( # of free
carriers >> thermally generated carriers(i.e. from 6.36) ).  Mobility
dominates TC, leading to a positive TC for resistance.  This explains MOSFETs.

BTW high and low voltage MOSFETs have greatly differing TC's due to the doping
changes needed to fabricate them and the location of the physical resistance
in the structure.

Bipolar transistors are not resistors, and resistive TC argument does not hold.



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