This paper examines Schottky barrier contact structures based on III-nitrides. Using the concept of discrete charge distribution of electroactive defects in the semiconductor depletion region, inhomogeneities in the Schottky barrier height have been analyzed, taking into account the mirror image forces. Characteristic quantities and scales of effective barrier changes in the contact were determined, caused by fluctuations in the electric fields of charged impurity atoms and threading misfit dislocations. The average inhomogeneities in the effective Schottky barrier height were shown to reach values of approximately 100 meV while strong semiconductor compensation due to carrier trapping at dislocation states (i.e., the self-compensation effect) took place. The possibility of minimizing these inhomogeneities by selecting the semiconductor doping level was demonstrated.