  We use a random gap model to describe a metal-insulator transition in
three-dimensional semiconductors due to doping and find a conventional phase
transition, where the effective scattering rate is the order parameter.
Spontaneous symmetry breaking results in metallic behavior, whereas the
insulating regime is characterized by the absence of spontaneous symmetry
breaking. The transition is continuous for the average conductivity with
critical exponent equal to 1. Away from the critical point the exponent is
roughly 0.6, which may explain experimental observations of a crossover of the
exponent from 1 to 0.5 by going away from the critical point.
