The 2005 International Technology Roadmap for Semiconductors predicts ultrashallow junctions less than 7 nm deep will be required for silicon transistors to be manufactured in 2010. To meet this depth requirement, it is necessary to have a better understanding of the dopant transient enhanced diffusion (TED) behavior that undermines its achievement. As and P are the two most commonly used n-type dopants used in junction formation. In this paper, density functional theory (DFT) within the generalized gradient approximation is used to develop an understanding of n-type dopant TED mediated by silicon interstitials in crystalline silicon. We find that As-Si_i and P-Si_i pairs in the neutral and negative charge states diffuse via a mechanism in which the dopant is bond-centered at energy minima and threefold coordinated at the high energy saddle point during dopant migration. For both As-Si_i and P-Si_i pairs, we conclude that the neutral pairs will dominate under intrinsic conditions while the neutral and negatively charged pairs will both contribute under heavily doped extrinsic conditions.