Typically one use higher doping concentrations and lower thermal budgets for semiconductor fabrication. It was observed that Fick's laws are not accurate enough for the explaining the anomalous diffusion behavior. Therefore it is necessary to focus on atomistic level to get insight on the interaction of dopants with lattice atoms. It is established that diffusion of impurities can only occur within the crystal lattice due to the presence of point-defects.

Diffusion by point defects

Diffusion occurs in silicon primarily at native point-defects and impurity-related point defects. Typically native defects such as point defects, line defects and surface defects are present in an otherwise perfect crystal lattice. At room temperature point defect concentration is low that practically no diffusion occurs. However, an increase of point defects will cause an increase in the diffusion. The concentration of point defects will increase for example if the crystal is heated or irradiated by ion. Both situations are likely to occur during the fabrication of devices. Impurity related defects arise from the introduction of group- III elements or group-V elements into the silicon lattice.