The n-type dopants phosphorus and nitrogen as well as the impurity-related defects are investigated by an ab initio method. The defect equilibrium and solubility are analysed. The electrical activation of nitrogen is found to saturate above a critical nitrogen concentration, due to the enhanced formation of nitrogen-vacancy complexes. The phosphorus activation is not affected by self-compensation or passivation. The calculated phosphorus solubility and implantation experiments suggest that the low value found in PVT-growth is due to the growth kinetics.