The resistivity and mobility ofp‐type andn‐type silicon films grown epitaxially on (0001) and (11¯02) sapphire have been measured as a function of substrate prefiring, growth parameters, and film postfiring. Sapphire preheated in hydrogen produced the highest mobility silicon films. Preheating the substrate in helium resulted in films with low mobility. The growth temperature at which the maximum mobility was obtained was found to be about 1200°C on the (0001) sapphire and about 1115°C on the (11¯02) sapphire. The electrical properties were found to be relatively insensitive to growth rate in the range between 0.3 &mgr;/min and 3 &mgr;/min except that slower growth rates and longer growth times lead to relatively more auto‐doping from the substrate. The auto‐doping was linked to the hydrogen reduction of sapphire (Al2O3) resulting in aluminum‐doped silicon. Postfiring of the films below 1250°C resulted in no gross crystal changes but could result in appreciable electrical changes. Postfiring in hydrogen introduced aluminum into the films and postfiring in oxygen removed aluminum. Postfiring in helium resulted in little change in the doping of the films.