Solid‐state interface reactions between metal thin films and (100) GaAs substrates at elevated temperatures are studied by conventional and heavy‐ion Rutherford backscattering spectrometry, x‐ray diffraction, and transmission electron microscopy. Metals investigated in this study include Pt, Pd, Ni, Co, Rh, and W. Electrical properties of the metal/n‐GaAs diodes undergoing annealing treatments at various temperatures were also measured with the current‐voltage dependence. Optimum diodes with maximum barrier heights as well as minimum leakage currents are obtained for diodes annealed at temperatures at which a uniform thin layer of reacted phase is observable at the interface. The barrier heights of the optimum diodes show a linear dependence on the work functions of the various metals. The range of these barrier heights is limited by nonstoichiometry related defects as suggested by a recently proposed amphoteric native defect model.