Studies of the photoelectrical and optical properties were done on hydrogenated amorphous silicon (a-Si:H) films in order to examine the possibility for construction. of an efficient sensor for He-Ne (632 nm) laser light detection. Films with various thicknesses were prepared by plasma-enhanced chemical vapor deposition (PECVD) on substrates of glass, molybdenum (Mo)-coated glass, and silicon (Si) wafer. Relationships between the a-Si:H film thickness and the optical properties, i.e. visible (VIS) and Fourier transforms infrared (FTIR) spectra, were given. The film's absorption coefficient at 632 nm was deduced from its visible light transmission spectra. The quantum size effect, i.e. an increase of the light absorption efficiency with the decrease of the film thickness, was observed. The optical absorption could be related to the Tauc mechanism rather than the Cubic mechanism. The band gap (E-g) and photoconductivity of the film were dependent on the thickness. The latter was also influenced by the substrate material. The a-Si:H/substrate interface is an important factor for the variation of photoelectrical properties. The He-Ne laser illumination appeared to induce more damages than the white light illumination based on the same amount of input energy. The optimal a-Si:H film thickness for the application was calculated. In summary, the photoelectric and optical properties of the PECVD a-Si:H film are adequate for the fabrication of the photosensor for the He-Ne laser light detection. (C) 2003 Elsevier B.V. All rights reserved.