Drain current trapping characteristics is shown to be able to distinguish between bulk and surface traps. This is illustrated on stressed and unstressed AlGaN/GaN HEMTs with different surface oxygen content. A trapping characteristic with positive amplitude is shown to be related to surface traps located in the transistor access region close to the gate edge. In contrast, current trapping characteristic with negative amplitude is related to emission from AlGaN bulk traps located underneath the gate. This method enables easy distinguishing between bulk and surface traps generated during reliability testing.