Long-term stability of Gallium Nitride High Electron Mobility Transistors: a reliability physics approach

Several groups have demonstrated nitride-based High Electron Mobility Transistors with excellent rf output power, with a constant increase in performances. However, despite the large efforts spent in the last few years, and the progress in MTTF (Mean Time To Failure) values, reliability of GaN HEMTs (High Electron Mobility Transistors) and MMICs (Millimeter Microwave Integrated Circuits) still has to be fully demonstrated, due to the continuous evolution of adopted processes and technologies, and to the lack of information concerning failure modes and mechanisms. The role of temperature in promoting GaN HEMT failure is controversial, and the factors accelerating degradation are largely unknown. This paper proposes a methodology for the analysis of failure modes and mechanisms of GaN HEMTs, based on the extensive characterization of deep levels using Deep Level Transient Spectroscopy (DLTS) and pulsed measurements, on the detailed analysis of electrical characteristics, and on comparison with two-dimensional device simulations. Results of failure analysis using various microscopy and spectroscopy techniques are presented and failure mechanisms observed at the high electric field values typical of the operation of these devices are reviewed.