The physical electronics at low temperatures of semiconductor materials and devices will be studied. Included is a combined theoretical and experimental investigation of carrier freezeout for various semiconductor devices as a function of material, impurity type (n or p) and specie, impurity concentration and temperature. The temperature dependence of the operating characteristics of GaAs MESFET's, Si MOSFET's, and CRYOFET's will be investigated. A CRYOFET has structure much like that of a MOSFET but has substrate of the same conductivity type Source and Drain. While it is a low linear resistance at room temperature it functions much like a MOSFET at temperatures below that for carrier freezeout (e.g. 4.2 K).