Peculiar failure mechanisms in GaN power transistors

Commercial GaN power amplifiers for RF applications, made of a pair of discrete transistors for operation in Doherty configuration, failed during the HAST tests. Failure Analysis pointed out a layout-specific issue related to thermal expansion at the level of the field plates. Anyway, the search for initial degradation stages using Optical Beam Induced Resistance Change and Photon Emission Microscopy revealed a subtle second mechanism, involving Ga interdiffusion into the gate metal lines, coming from hollow pipes in GaN. Both mechanisms are discussed.