Shallow impurity interactions and the metal-insulator transition

A review is given of our current understanding of the interactions between shallow impurities in semiconductors at intermediate concentrations, particularly as seen through optical (far-infrared), dielectric and magnetic properties. Interpreted in terms of models appropriate for the non-metallic phase, these results point out the importance of randomness in the dopant distribution. Connection with the recent scaling approach to the metal-insulator transition, based on perturbative expansions about the high density metallic (weak-coupling) limit, is discussed.