The pinch-off current leakage characteristics of AlxGa1 N- x/GaN HEMTs using semi-insulated GaN or Al0.04Ga0.96N buffer layers have been fully investigated. Their gate-drain leakage current densities are only 0.2 and 0.075 mA/mm at V-GD of 100 V respectively, which guarantees excellent reverse Schottky breakdown characteristic. Meanwhile, by introducing low-Al AlxGa1-xN high-resistivity layer, it shows not only a much sharper sub-threshold turn off characteristic with a higher transconductance peak value, but also very lower deep-depletion leakage current. And its better carrier confinement under high V-DS greatly improves both the small-signal characteristic and microwave power performance of GaN HEMT devices. In addition, electrical reliability of AlxGa1-xN/GaN HEMTs at high voltage operation has been greatly improved. (C) 2012 Elsevier Ltd. All rights reserved.