Insight on the deposition mechanism of hydrogenated amorphous silicon (a-Si:H) can be gained by inspecting the consumption rates of SiH4by various species in the plasma since essentially all the silicon in the silane consumed is incorporated in the solid. Chemical chain reactions eliminating H2are thermodynamically allowed as the replacement of two Si-H bonds with H-H and Si-Si bonds is thermoneutral or exothermic. In the case where the chain carriers are positive ions, the chain reaction is known to proceed at collisional rates. The relative rate of SiH4removal by electron impact induced fragmentation can be estimated for various plasma conditions by making appropriate assumptions for the distribution function, mean electron energy, and fragmentation cross section. It is found that under most conditions used for the deposition of a-Si:H, the species arising from chain reactions are more important in silicon transport than fragments directly formed in electron impact. © 1983, American Vacuum Society. All rights reserved.