Reliability in GaN based devices still motivates numerous studies because the involved degradation mechanisms are different from that in III-V narrow bandgap devices. Direct investigations on high electron mobility transistors (HEMT) are performed with low frequency noise (LFN) measurements, pulsed electrical characterization and deep level transient spectroscopy. The first part of this thesis deals with generalities on AlGaN/GaN High electron mobility transistors and their technological particularities. The second part deals with the presentation of the diagnostic tools used in this study. A low frequency noise bench developed in LAAS-CNRS allowing measurements from few hertz up to 1 MHz is described, an original method of electrical pulsed characterization has and current deep level spectroscopy bench. In the third part of this study, low frequency noise is used to assess effects of deuterium (H+ ions) in diffusion condition on the robustness of 0.25 *2*75 μm² gate area AlGaN/GaN high electron mobility transistors (HEMT) grown on Si substrate. H+ Ions are diffused from the above AlGaN/GaN layer through the AlGaN/GaN interface and GaN layer, notably under the gated channel where the defects are located. In the fourth part, undoped AlGaN/GaN devices grown on silicon substrate have been stressed at a junction temperature of 175°C. Gate-lag and drain-lag measurements method have been performed versus different quiescent bias points and under different pulse conditions. This method allows the discrimination of each lag phenomenon as well as the thermal contribution. Thus it is possible to track and model the trapping mechanisms versus bias conditions. This electrical modeling is completed with LFN measurements and deep level transient spectroscopy, which is largely used for reliability investigations.