The diffusion of Phosphorus in silicon using a POCl 3 source has been considered. In the base of Fair-Tsai model of P-diffusion an empirical equation for calculation of active diffused phosphorus density (Q el ), is proposed. In this equation, a relationship between (Q el ), diffusion time, temperature and junction depth of P-diffused layer (X j ), is presented. The value of sheet resistance (R s ), which is taken from theoretical determination at 900°C, has a good agreement with experimental result.