In a commercial scale low pressure chemical vapor deposition (CVD) reactor, we analyzed the elementary reaction of silane based CVD with the doping gas of phosphine. The variation of conductivity in the radial direction over a monitor wafer became significant as the phosphine gas partial pressure decreased, and it reached up to about 30% at 6.5x 10(-4) Pa. The radial distribution of phosphorus fraction in a film, however, was smaller than 7%. On the basis of the diffusion model of chemical species into the wafer gaps, we found that the sticking probability of phosphine is 2-5 x 10(-5), which is 1 order of magnitude larger than the one of silane. The reaction kinetics of both silane and phosphine seems almost linear, but slight nonlinearity at high phosphine concentration range was also indicated. (C) 2004 American Vacuum Society.