In this work, we investigated the effect of crystal defects on reverse I-V characteristics of avalanche photodiodes (APDs) using electron-beam induced current (EBIC) mode of SEM. Two types of SiC APD structures (I and II) were fabricated using 2 inch p-doped substrates with n-doped epilayers and 3 inch n-doped substrates with p- and n- epilayers. Areas of the formed diodes were approximately 1 mm2. The devices without any kind of electrically active 3-D structural defects demonstrated breakdown voltages close to theoretical values, ~500 V for the APD Type I and ~1200 V for Type II APD. Stability of Type I devices was tested by applying a short pulse of high voltage (~800 V). EBIC images, taken prior to and after the failure test, showed new defects in the dislocation free area, which, presumably, were caused by thermal breakdown.