B diffuses in crystalline Si by reacting with Si self-interstitial (I) with a frequency g, forming a BI complex that can migrate for an average length λ. We experimentally measured both g and λ as a function of the hole concentration p by means of iso-concentration experiments on B delta-layers both under p- and n-doping conditions. On the basis of these data, we propose a comprehensive model that fixes the interplay among free charge, I and BI charge states that determines the B diffusion. Pairing effect with donors was also considered.