GaP/Si heterojunction diodes have been fabricated on (1 1 1) Si. The diodes were electrically and optically characterized. The conduction band offset has been measured and analyzed. The heterostructures. showed good diode IV characteristics with a low leakage current. The crystalline quality of the Gap is expected to increase with a lower step density on the Si substrate, suggesting the importance of establishing atomically smooth surfaces on which GaP is nucleated. This was investigated with cathodoluminescence.