Temperature Coefficient of Resistivity in Amorphous Semiconductors
  By invoking the microscopic response method in conjunction with a reasonable set of approximations, we obtain new explicit expressions for the electrical conductivity and temperature coefficient of resistivity (TCR) in amorphous semiconductors, especially a-Si:H and a-Ge:H. The predicted TCR for n-doped a-Si:H and a-Ge:H is in agreement with experiments. The conductivity from the transitions from a localized state to an extended state (LE) is comparable to that from the transitions between two localized states (LL). This resolves a long-standing anomaly, a "kink" in the experimental $\log_{10}\sigma $ vs. T$^{-1}$ curve. 
Ming-Liang Zhang and David A. Drabold
cond-mat.stat-mech cond-mat.mtrl-sci