The photolysis of disilane (Si2H6) and trisilane (Si3H8) under direct excitation by light from a low pressure mercury lamp was carried out to prepare hydrogenated amorphous silicon films (a‐Si:H). The electronic and optical properties of the films were investigated as functions of preparation conditions such as partial pressure and substrate temperature. The conductivity of the films prepared from Si3H8at 300 °C was 10−10S cm−1in the dark and 10−5S cm−1under the illumination of a He‐Ne laser with a photon flux of 1015cm−2s−1. The high photoconductivity was attained when the silane gas was blown over the substrate from a slit‐type nozzle placed beside the substrate plate at a pressure of less than 20 Torr.