This letter presents time-dependent gate-capacitance transient measurements (C–t) to determine the oxide trapped charges (N ot) in Al 2 O 3 films deposited on recessed AlGaN/GaN heterostructures. The C–t transients acquired at different temperatures under strong accumulation allowed to accurately monitor the gradual electron trapping, while hindering the re-emission by fast traps that may affect conventional C–V hysteresis measurements. Using this method, an increase of N ot from 2 to 6× 10 12 cm− 2 was estimated between 25 and 150 C. The electron trapping is ruled by an Arrhenius dependence with an activation energy of 0.12 eV which was associated to points defects present in the Al 2 O 3 films.