AbstractSilane pyrolysis at polysilicon low‐pressure‐deposition conditions is analyzed along the reactor‐tube axis with respect to the existence of competing decomposition reactions leading to the formation of silicon‐containing gaseous byproducts. The presence of such a parallel reaction is indicated by a difference between silane decomposition efficiency ηSiH4and silicon deposition efficiency ηsi. The rate of such a competing parallel reaction is influenced by the variation of deposition temperature and/or substrate area of silicon deposition. It is drastically accellerated by the presence of phosphine in the gas phase. For the latter case the competing parallel reaction is shown to occur as a homogeneous