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

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... , a " kink" in the experimental is predicted by the theory and attributed to localized to extended transitions, which have not been properly handled in earlier theories.