  In this paper, we develop an approximate theory of the temperature
coefficient of resistivity (TCR) and conductivity based upon the recently
proposed Microscopic Response Method. By introducing suitable approximations
for the lattice dynamics, localized and extended electronic states, we produce
new explicit forms for the conductivity and TCR, which depend on easily
accessible material parameters. The theory is in reasonable agreement with
experiments on a-Si:H and a-Ge:H. A long-standing puzzle, a \textquotedblleft
kink\textquotedblright\ in the experimental $% \log_{10}\sigma $ vs. 1/T curve,
is predicted by the theory and attributed to localized to extended transitions,
which have not been properly handled in earlier theories.
