Shallown+‐pjunctions were formed by utilizing aninsitudoped thin polycrystalline silicon layer as a diffusion source. Theinsituarsenic‐doped polycrystalline silicon films were deposited by rapid thermal processing chemical vapor deposition. The dopant pileup phenomena were observed at both the polycrystalline silicon/silicon interface and at the surface. The dopant concentrations were higher when the deposition temperatures were lower. The observed pileup phenomena at the polycrystalline silicon/silicon interface were temperature dependent and mainly due to the segregation of arsenic at the grain boundary. The dopant distribution was mainly due to the grain boundary diffusion and grain growth mechanisms. Extremely shallown+‐pjunctions were achieved and laterally uniform delineated junctions were observed. The dopant concentration in the Si substrate drops two orders of magnitude in less than 500 A˚.