Analysis of Time Dependent Electric Field Degradation in AlGaN/GaN HEMTs

The authors report on an electrical and optical analysis of AlGaN/GaN HEMTs stressed under high electric field conditions into a state of permanent degradation, evidenced by an increase in OFF-state leakage current and a reduction in breakdown voltage. A method of stress testing AlGaN/GaN HEMTs to voltages close to breakdown while protecting the device from catastrophic failure is presented. Using this stress method, a detailed study was performed to observe device degradation that limits safe operation in the OFF-state. Electrical analysis reveals that quantitatively the Schottky properties of the gate diode are degraded by the stress and suggests a localized defect. An optical analysis confirms localized degradation via electroluminescence (EL) spots on the stressed side of the gate finger. It is shown that the dominant EL site in the degraded device may be observed prior to the application of stress. Finally, it is confirmed that the localized EL emission of the stressed device is the dominant gate leakage path via thermal imaging. These results suggest a method for identifying and understanding the failure mechanisms that limit the safe operating area of GaN HEMTs.