Abstract: Planar diffusion sources are provided wherein the source is a wafer of inert material, preferably silicon or silicon dioxide and wherein the wafer acts as a substrate for a surface coating comprising a salt, preferably the oxide, of the dopant element. An inert oxide such as aluminum oxide or silicon dioxide may also be included in the coating. When applied to the substrate as a paste or slurry and fired to suitable temperatures, the dopant oxide coating tightly adheres to the substrate wafer. The coated diffusion source is placed alongside semi-conductor wafers in a diffusion furnace; where, at diffusion temperatures, the dopant element volatilizes and diffuses into the surface of the semi-conductor material. The diffusion source can be reused numerous times.