The failure mechanism of a SiC metal-oxide-semiconductor capacitor with a poly-Si gate electrode was investigated by time-dependent dielectric breakdown testing under a 200-nA constant current stress. The capacitor exhibited both hard and soft breakdowns. After dielectric breakdown in both cases, adjacent concaves were observed on the capacitor with a field-emission scanning electron microscope. Additional optical beam-induced resistance changes and photo-emission analysis of a capacitor after hard-breakdown located a failure point on the periphery of a group of adjacent concaves. Cross-sectional scanning transmission electron microscope observation revealed that a narrow, vertical defect had formed at this point on the SiC substrate.