A comparative study between GaN/SiC heterojunction and 6H-SiC homojunction diodes has been performed to understand the physical properties of n-p GaN/SiC heterojunction interface. The 6H-SiC homojunction diodes presented typical I-V characteristics with satisfactory breakdown voltage (congruent to -800 V), however, the GaN/SiC heterojunction diodes showed an abnormal low forward turn-on voltage (congruent to1.8 V). The presence of a deep-level 1 eV below the p-SiC conduction band, is probably responsible for a tunneling- assisted current at low forward voltages, due to a high concentration of interface defects.