Gas-phase reactions are a critical parameter for the formation of hydrogenated amorphous silicon (a-Si:H) and microcrystalline (mu c-Si:H) silicon films. On the example of the two important radicals SiH2 and SiH3, produced by different deposition methods, photo chemical vapour deposition employing a CO2 laser, and the hydrogen abstraction method, the role of the film precursors in the deposition mechanism of a-Si:H is discussed. The transition from a-Si:H to mu c-Si:H material at different experimental parameters is correlated with the characteristic pressure p* and is explained on the basis of SiH3 reactions.