The dependences of diffusion coefficient of As,DAs, upon total As concentration,CT, and the relationships betweenCTand resistivity &rgr; for several diffusion temperatures are experimentally obtained for the As diffusion intop‐type Si from doped polycrystalline‐Si sources in the temperature range 850–1050 °C. It is found that the relationship betweenCTand &rgr; is dependent upon diffusion temperature forCTabove 1020cm−3. The relationship betweenCTand electrically active As concentration at diffusion temperature,CA, i.e.,CT=CA+(3.2×10−6/n3i)C4Acan be obtained, assuming that only electrically active As is mobile and is diffused by the single‐level vacancy mechanism, and using the experimental result that the dependence ofDAs/Di(Diis the intrinsic diffusion coefficient of As) uponCT/ni(niis the intrinsic electron concentration) is not influenced by diffusion temperature. Using the above equation, it is found that the relationship betweenCAand &rgr; is independent of diffusion temperature, even forCAabove 1020cm−3, and can be approximated forCAabove 1020cm−3by an empirical equation: &rgr;=8.7×104C−2/5A.