A reaction engineering model has been developed to describe the mercury-sensitized photochemical vapor deposition of hydrogenated amorphous silicon (a-Si:H) semiconductor thin films. Model equations governing the gas-phase generation, transport, and surface reactions of SiH, and H film precursor radicals are solved to predict film growth rate and bonded hydrogen content. Behavior of the model has been studied as a function of deposition conditions (pressure, temperature, feed composition, and flow rates) and has been verified by comparison with experimental results.