Diffusion of phosphorus in silicon in an ambient of pure N-2, pure NH3, and mixtures of NH3 and N-2 has been investigated to study the effect of the direct nitridation reaction on the diffusivity. The diffusion coefficient of phosphorus can be expressed as a function of the partial pressure of NH3 and temperature as D-P(NRD)= 0.145 exp (-3.26 eV/kT) - 1.26 x 10(2) exp (-4.11 eV/kT)P(NH3)cm(2)s(-1). In addition, the ratio of the interstitial concentration under nitridation conditions to the concentration under inert conditions is also determined as C-I(NRD)/C-I*= 1 - 8.651 x 10(2) exp (-0.841 eV/kT)P-NH3.