The influence of dilution of silane with Ar, He and H2 on the mechanism of radical generation in radio frequency discharges used for the deposition of a-Si:H has been examined. The density of SiH radicals in the discharge volume and near the deposition surface was examined by laser-induced fluorescence. The observed phenomena were attributed to the enhancement of the contribution of sheath-related electrons under certain dilution conditions. This explanation was confirmed by numerical simulation using Monte Carlo and probabilistic models that permitted the calculation of the total collision cross section of silane.