POSSIBLE SOLUTION OF THE CONDUCTIVITY EXPONENT PUZZLE FOR THE METAL-INSULATOR-TRANSITION IN HEAVILY-DOPED UNCOMPENSATED SEMICONDUCTORS

The electrical conductivity sigma (extrapolated to T=0) of uncompensated Si:P indicates a crossover as a function of P concentration N at N(cr) slightly above the metal-insulator transition at N(c). For N > N(cr) the exponent of sigma is similar to (N - N(c))mu is mu almost-equal-to 0.64, while mu almost-equal-to 1.3 for N(c) < N < N(cr). At N(cr) dsigma/dT changes sign from negative for N > N(cr) to positive for N < N(cr). Sigma in a magnetic field also yields mu almost-equal-to 1. The apparent discrepancy between uncompensated and compensated semiconductors is traced back to a difference in the (nonuniversal) width of the critical region.