Phosphorus is diffused into silicon at 900 °C from a phosphorus‐doped silicon‐dioxide layer. Since a profile of phosphorus concentration is expressed by a function ofx/t, wherexis the distance from a surface andtthe diffusion time, diffusion coefficients are determined by the Boltzmann‐Matano method. They are larger than the intrinsic diffusion coefficient and are dependent not only on the concentration (the concentration effect) but also on some unknown condition at a surface (the surface effect). The surface effect extends more than 20 &mgr; deep into a bulk of silicon, and is stronger than the concentration effect. All of phosphorus atoms are located at substitutional sites. Diffusion‐induced dislocations are not found. A new mechanism for the generation of excess vacancies is suggested. The new mechanism consists of the following: (i) Phosphorus diffuses by a vacancy mechanism. The diffusion of phosphorus occurs only through the diffusion ofEcenters. (ii) When phosphorus atoms enter from a surface into a bulk, they should be in a form ofEcenters. Affected by a surface, a large amount ofEcenters is formed per unit time at a surface. (iii) TheEcenters flow into a bulk. (iv) By their dissociations, excess vacancies are generated. The surface effect and the emitter dip effect are attributed to excess vacancies.