The effects of $H_2$ dilution on hydrogenated amorphous silicon (a-Si:H) films prepared by microwave-PECVD have been studied. Photoconductivity of $~10^4$ $(\Omega cm)^{-1}$ and photosensitivity of $10^6~10^4$ under $100 mW/cm^2$ white-light illumination were obtained when the ratios of gas flow rate of $H_2$ to $SiH_4$ were 10 and 25. With increasing the $H_2$ dilution ratio, photoconductivity was decreased, and Infrared absorption spectra of ($SiH_2$)n and/or $SiH_2$ stretching mode at $2100cm^{-1}$ were increased. It may be concluded that a-Si:H films prepared by microwave-PECVD become poorer as the dilution ratio of $H_2$ to $SiH_4$ is increased.
