In this work, the robustness of three different AlN/GaN structures targeting high frequency applications with ultra-thin 3 nm AlN barrier and devices with different gate-to-drain spacing are evaluated. Devices were step-stress tested in off-state, semi-on-state, and on-state conditions. Different failure mechanisms were identified. Devices without AlGaN back-barrier show a fast increase of gate leakage current, leading to a significant degradation of electrical parameters such as the threshold voltage Vth, extrinsic transconductance GM, drain saturation current IDSsat and on-resistance Ron. This study reveals the role of the AlGaN back-barrier in the improvement of the device robustness presenting a good electrical parameters stability. The presence of an AlGaN back-barrier on low carbon- doped buffer leads to better gate leakage current stability, low trapping-related and self-heating mechanisms. The AlGaN back-barrier shows an extension of the operational Safe Operating Area for AlN/GaN technology.