Possible solution of the conductivity exponent puzzle for the metal-insulator transition in heavily doped uncompensated semiconductors

The electrical conductivity {σ} (extrapolated to T=0) of uncompensated Si:P indicates a crossover as a function of P concentration N at Ncr slightly above the metal-insulator transition at Nc. For N>Ncr the exponent of {σ}{~}(N-Nc{)}^{{{μ}}}is μ0.64, while μ1.3 for{N}cN{N}cr$. At ${N}crdσ/dT changes sign from negative for N>{N}cr$ to positive for N${N}cr$. $sigma--- in a magnetic field also yields {μ}{}1. The apparent discrepancy between uncompensated and compensated semiconductors is traced back to a difference in the (nonuniversal) width of the critical region.