An Al2O3 layer is inserted between the InAlAs layer and the metal gate in InAs/AlSb HEMTs to suppress the leakage current. The transport mechanisms of leakage current in Al2O3/InAlAs metal-oxide-semicon ductor (MOS) capacitors at both positive and negative biases at different temperatures ranging from 10 degrees C to 70 degrees C are investigated. For positive bias, the leakage current is dominated by Schottky emission. Based on the fitted straight lines, the relative dielectric constant of Al2O3 and the barrier height between Al2O3 and InAlAs are extracted. However, for negative bias, the leakage current is dominated by Frenkel-Poole (F-P) emission and the depth of the trap energy level from the conduction band (phi(t)) is extracted. Furthermore, authors explain the reason why the dominating mechanisms at positive and negative biases are different. (C) 2016 Elsevier Ltd. All rights reserved.