When arsenic is used as the emitter diffusant in a n‐p‐n silicon transistor, it is well known that the emitter dip effect is not observed. However, it was found that the diffusion of boron underneath the arsenic emitter was accelerated during heat‐treatment at 500°–800°C. This phenomenon is related to abnormal behavior of arsenic in silicon. The diffusion of arsenic is enhanced by such treatment. The anomalous diffusion of arsenic depends on treatment time, and is remarkable in the beginning of the treatment. The diffusivity is extremely large in comparison with the normal one extrapolated from high temperature data. It is considered that this anomaly is caused by the generation of excess vacancies during such treatment. The emitter dip effect is also caused by these excess vacancies during heat‐treatment at relatively low temperature.