Semiconductor-superconductor transition in granular Al-Ge

The electrical resistance of granular Al-Ge specimens has been measured for various metal concentrations around the metal-insulator transition. We find that, in a narrow range of metal concentrations, the system is semiconducting in the normal state and yet undergoes a superconducting transition. It is shown that the occurrence of the semiconductor-superconductor transition is governed by the interplay between the intergrain tunneling coupling energy, the Josephson coupling energy, and the Coulomb charging energy, and is possible only for granular systems with large enough grains.