The correlation between the microstructure and phase formation, and the electrical properties for the Co/GaAs, Co/Ge/GaAs and Ge/Co/GaAs systems have been studied. The microstructural analysis was carried out by transmission electron microscopy, and the component redistribution was determined by Auger electron spectroscopy. The electrical properties (Schottky barrier height and contact resistivity) were studied by means of current-voltage and capacitance-voltage measurements.