Disordered Semiconductors with Controllable Properties

Techniques have been developed that provide disordered semiconducting materials having energy band tails arid electronic properties that are precisely controllable. The materials are heavily doped, closely compensated n-GaAs in which the compensation ratio can be made arbitrarily close to 1. Thus the Fermi level can be controlled within the band tail. This allows measurement of various electrical transport properties as functions of energy. There are many similarities to amorphous semiconductors but with controllability that those materials do not have. Clear evidence is found for progressively greater localization of electrons of lower energy, i.e., no sharp mobility edge. Detailed measurements show an important disagreement with theoretical predictions for the temperature dependence of the conductivity at low temperatures. A very strong dependence of the conductivity on electric field strength is found, with the nonohmic behavior extending down to a few V/cm at 4.2 K. These various properties offer applications in precision thermometry and voltage surge suppression but not as nonvolatile, bistable switches.