The effects of AlxGa1-xN aluminum fraction x and SiC surface pre-treatment on AlGaN/4H-SiC heterojunction interfaces are experimentally investigated. From capacitance vs. voltage measurements, the conduction band offsets are found to be Delta E-C approximate to 0.30 for x approximate to 0.3 and Delta E-C approximate to 0.56 for x approximate to 0.5. Forward bias ideality factors are reasonable at 3.3 for Al0.3Ga0.7N diodes, but > 9 for Al0.5Ga0.5N diodes, suggesting a higher level of interface charge related to the higher aluminum fraction. Reverse bias leakage is acceptably low, with breakdown occurring at V-A > 200 V reverse bias for all tested devices. The effect of 1500 degrees C hydrogen etching of the SiC substrate prior to AlxGa1-xN growth is also investigated, and found to have little effect for x = 0. 3 but a beneficial effect for x = 0.5. (c) 2006 Elsevier Ltd. All rights reserved.