Types of degradation modes of AlGaN/GaN high electron mobility transistors (HEMTs) in dependenc on epitaxial design and buffer quality

DC-Step-Stress tests have been applied on wafers as a fast AlGaN/GaN HEMTs robustness screening method with different epitaxial designs. The results showed three types of early degradation which are permanent and are dependent on epitaxial design and GaN buffer quality. The criterion of critical voltage is defined for the onset of degradation when a subthreshold drain current and/or gate leakage current increase significantly in a step source-drain voltage during stress test. It has been found that AlGaN/GaN HEMT devices with GaN cap show higher critical source-drain voltages as compared to non-capped devices. Devices with low Al concentration in the AlGaN barrier layer also show higher critical source-drain voltages. Superior stability and robustness performance have been achieved from devices with AlGaN backbarrier epitaxial design grown on n-type silicon carbide substrate. It is found that high electric field under the gate at the drain side is the main cause of degradation. Consequently careful epitaxial design to reduce high electric field is required. It is also shown that epitaxial buffer quality and growth process have a great impact on device robustness.