Phosphorus doping of polycrystalline silicon by direct diffusion of PH3 at 685 and 785 °C followed by rapid thermal annealing (RTA) has been investigated. Unlike at 785 °C, the dopant profile is not uniform at 685 °C but becomes so after RTA at 1050 °C for 10 s, as revealed by secondary ion mass spectroscopy (SIMS). In the absence of any protection layer, RTA in N2 causes an out‐diffusion of ∼50% of P from the sample diffused at 785 °C, although it activates ∼100% of the remaining P atoms as indicated by Hall and SIMS measurements. In contrast, RTA in O2 results in 95% activation of the incorporated P from the same sample, accompanying an oxidation of ∼7 nm polycrystalline Si. As regards the sample diffused at 685 °C, RTA in N2 does not cause any out‐diffusion but activates less than 80% of the incorporated P atoms. This difference in behavior is attributed to the tendency to attain equilibrium concentrations of P in the crystallites and at the grain boundaries at the RTA temperature. The highest active P concentration achieved is 2.0×1020 cm-3 at 785 °C followed by RTA in O2 yielding a resistivity as low as 9×10-4 Ω cm. The simplicity of this relatively low temperature process is attractive for a single‐wafer multisequence fabrication scheme.