In this study, we present a detailed analysis of trapping characteristics at the AlxGa1−xN/GaN interface of AlxGa1−xN/GaN high-electron-mobility transistors (HEMTs) with reliability assessments, demonstrating how the composition of the Al in the AlxGa1−xN barrier impacts the performance of the device. Reliability instability assessment in two different AlxGa1−xN/GaN HEMTs [x = 0.25, 0.45] using a single-pulseID–VDcharacterization technique revealed higher drain-current degradation (∆ID) with pulse time for Al0.45Ga0.55N/GaN devices which correlates to the fast-transient charge-trapping in the defect sites near the interface of AlxGa1−xN/GaN. Constant voltage stress (CVS) measurement was used to analyze the charge-trapping phenomena of the channel carriers for long-term reliability testing. Al0.45Ga0.55N/GaN devices exhibited higher-threshold voltage shifting (∆VT) caused by stress electric fields, verifying the interfacial deterioration phenomenon. Defect sites near the interface of the AlGaN barrier responded to the stress electric fields and captured channel electrons—resulting in these charging effects that could be partially reversed using recovery voltages. The quantitative extraction of volume trap density (Nt) using 1/flow-frequency noise characterizations unveiled a 40% reducedNtfor the Al0.25Ga0.75N/GaN device, further verifying the higher trapping phenomena in the Al0.45Ga0.55N barrier caused by the rougher Al0.45Ga0.55N/GaN interface.