In this problem we are asked to compute the intrinsic carrier concentration for PbTe at room temperature.
Since the conductivity and both electron and hole mobilities are provided in the problem statement, all we need do is
solve for n and p
Thus,
n=s/e(me+ mh)
n = 500 / (1.602 x10-19) ( 0.16 + 0.075)
= 1.33 ´ 10^22