In this paper, we present an atomistic kinetic Monte Carlo (KMC) model for boron diffusion in amorphous silicon (a-Si). Boron diffusion is assumed to be indirect, mediated by dangling bonds (DBs) present in a-Si. The model shows a transient character due to a-Si relaxation modeled by DB annihilation. In addition, B produces clusters at higher concentrations, leading to an immobile B part. The model, when calibrated, has been successfully applied to experiments of thermal anneals of amorphized B marker layers and B implantation into preamorphized Si. In addition, we studied the contribution of B diffusion in a-Si in ultrashallow junction (USJ) formation of advanced technologies. We have also used the model to demonstrate how the contribution to diffusion of B in a-Si in current and future technologies can be higher than in c-Si. (C) 2010 Elsevier Ltd. All rights reserved.