Abstract It is generally found that the optimum growth of a-Si:H and a-(Si,Ge):H films and devices depends critically upon the particular growth technique, and the particular growth parameters used to grow the film. Different techniques give very different results, which are sometimes contradictory. The standard model for growth assumes that the Si surface is mostly covered with H bonds, and that growth takes place primarily from silyl radicals. The model assumes that excess surface H is eliminated by a silyl radical, and that the adjacent or wing bonded H atoms are eliminated by a spontaneous bond breaking and H 2 formation. In this paper, we show that this model is thermodynamically incorrect, and that it does not explain many of the experimental data, such as why bombardment with inert gases reduces H content, and why material grown at higher growth rates is more unstable. Rather, we suggest that the fundamental limitation to the growth of a-Si:H and a-(Si,Ge):H is the elimination of excess H, both from the surface and from the bulk. The excess H is not eliminated by spontaneous reactions, nor by interactions with the silyl molecule. Rather, it is eliminated by interactions with free H radicals and ions. Inert ions, such as He and Ar, can accelerate the desorption of H from the surface. Atomic and ionic H can diffuse into the material, and also remove subsurface excess H, reforming the Si microstructure. We also show that the influence of ion bombardment is critical for growing high quality a-(Si,Ge):H alloys, and that deposition conditions that lead to low ion bombardment flux can produce poor materials.