The effect of phosphine/monosilane flow ratio value gamma in the range of 0-0.02 on the apparent activation energy E(a) for an in situ phosphorus-doped polysilicon deposition process has been investigated. A strong dependence of E(a) on the gamma value has been found. The marginal value of gamma has been determined whereby the apparent activation energy is "saturated" and becomes practically independent on the phosphine/monosilane flow ratio. An explanation of the obtained results on the basis of two reaction paths existing for the phosphorus-doped polysilicon deposition process in the phosphine/monosilane system has been proposed.