We prepared undoped microcrystalline silicon films (mu c-Si : H) using a mercury-sensitized photochemical vapor deposition system under various film deposition conditions of SiH4/(SiH4 + H-2) gas ratio and total gas flow rate, and investigated the dependence of the film quality on the deposition conditions in order to obtain high-quality undoped microcrystalline films at a high deposition rate. Up to a SiH4/(SiH4 + H-2) gas ratio of similar to 0.1; the crystallite size of the mu c-Si:H films decreased linearly as the ratio increased, but the volume fraction of crystallites remained constant at about 80%. The dark conductivity of the films was very low, in the range of 10(-7)-10(-6) Scm(-1) under nearly all experimental conditions, and so a high photosensitivity of similar to 550 was obtained without the addition of a dopant gas such as diborane. In addition, a high deposition rate of similar to 30 Angstrom/min was obtained at a low total flow rate of 2l sccm. No degradation of the microcrystallinity was observed for the low total flow rate.