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

Gallium Nitride electronics currently represents the most promising technology for a number of extremely relevant applications, such as high efficiency, high frequency and high power communication systems, ultra wideband communication systems, radars, spaceborn systems, energy efficient power switching applications, robust low-noise frontends and high-temperature ultrascaled digital devices. The high energy gap value and the high breakdown electric field of GaN, enable AlGaN/GaN and InAlN/GaN High Electron Mobility Transistors (HEMTs) to operate at voltages substantially higher than those which can be sustained by other semiconductor devices, resulting in high power density devices and high efficiency amplifiers. At the same time these new properties open a new scenario for these devices, where unprecedented high electric field are generated within the device active area. Special device designs have been devised for controlling the electric field distribution without affecting device performances, such as source and drain field-plates and recessed gate structures; despite this, however, due to the high drain voltages generally adopted, the gate-drain electric field within submicron gate length AlGaN/GaN HEMTs is extremely high, leading to severe reliability issues. The 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 talk 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.