Solid-state gas sensors were fabricated using diamond films, and the sensing characteristics of the sensors upon exposure to semiconductors doping gases (PH3, B2H6, and AsH3) were examined. The sensors consisted of a double layer of undoped and B-doped polycrystalline diamond films and a pair of Pt electrodes. The electrical resistance between the electrodes sharply increased with time upon exposure to the gases, and then saturated with an increase in the gas concentration. The sensitivity of the sensors S was defined as S = (R-g - R-air)/R-air, where R-g and R-air are the resistances between the Pt electrodes before and after exposure to the gases, and between 10 and 100% for 0.2 ppm PH3. Characteristics of the sensors were interpreted based on the Langmuir isotherm in conjunction with a formation of a depletion layer at the sensor surface due to gas adsorption. (C) 2001 The Electrochemical Society. All rights reserved.