In this paper we show the creation of electrical traps in AlGaN/GaN HEMTs during electrical stress. In fact we highlight that an ageing test carried out for V-DS = 20 V and V-GS = -5 V (OFF-state stress) or for V-DS = 20 V and V-GS = 0 V (ON-state stress) induces a decrease in the drain current and an increase of the access resistance (R-k). The degradation of these electrical performances observed after ageing tests are reversible, contrary to the majority of the results found in the literature. We have demonstrated, by using simple methods, that the observed phenomena are explained by the creation of electrical traps, which can be considered as donors and acceptors, and not by degradation of the ohmic contacts and/or of the Schottky contact and/or the appearance of cracks in the passivation layer. Moreover, this paper shows that the two ageing tests are also responsible for the creation of two kinds of electrical traps in the gate-drain region of the devices, particularly in the top of the device structure. However, the creation of electrical traps at the AlGaN/GaN buffer interface has been observed for an OFF-state stress though not for an ON-state stress. Crown Copyright (c) 2012 Published by Elsevier Ltd. All rights reserved.