This work is focused on the investigation of the soft breakdown (or kink) phenomenon in AlGaAs/InGaAs HEMT down to cryogenic temperature. The kink effect in the output characteristics consists of an abrupt increase of the drain current at a given value of the drain-source voltage. This anomaly causes degradation in the performances of the transistors as it leads to a reduction in the gain and an increase in the level of noise. On the basis of our experimental data, we attribute the origin of the kink effect to trapping and subsequent field assisted detrapping mechanisms. Trap model allows us to explain the behavior of the kink effect with respect to the bias and the temperature, and also the difference between DC and RF output characteristics. By cooling the device, a negative charge accumulation, likely due to the reduction of the thermally activated electron detrapping phenomena, leads to the threshold voltage shift towards higher values. It results in a degradation of the transistor figures of merit, like the transconductance and the magnitude of the forward transmission parameter. (c) 2005 Elsevier Ltd. All rights reserved.