The origin of the defects associated with the nitridation of the interface layer between Si and HfO₂ is investigated. The electronic properties change upon nitridation which impact severely the gate capacitance and gate leakage current. We modeled the temperature-dependent leakage current in SiON/HfO₂ gate dielectrics for positive and negative gate voltages by means of a multi-phonon trap-assisted tunneling scheme to extract the trap distribution. The results are supported by charge pumping measurements and simulation. To clarify the origin of the additional traps in the SiON interface we performed ab-initio calculation and correlated the results with the gate leakage current measurements. Finally, we shed new light on the relation between stress-induced leakage current and positive bias temperature instability.