Influence of GaN cap on robustness of AlGaN/GaN HEMTs

DC-Step-Stress-Tests of GaN HEMTs have been performed on wafers with and without GaN-cap. The tests consist of a step ramping of drain-source voltage V_DS by 5 V every two hours at off-state. The irreversible evolution of leakage current starting at a certain drain voltage has been taken as a criterion for the onset of device degradation. It has been stated that there is a stability limit for V_DS depending on the epitaxial design. It has been found that wafers with GaN cap show much higher critical voltages as compared to non-capped epitaxial designs. Electroluminescence measurements have been performed to localize defects after DC-Step-Stress-Tests up to 80 V for wafer without GaN cap and 120 V for wafer with GaN cap.