Failure mechanisms in AlGaN/GaN HEMT RF power amplifiers implemented on silicon substrate and envisaging radar operating conditions are investigated in this paper. Several power amplifier prototypes have been designed, fabricated, characterized, and tested. Ageing tests were performed in conditions as close as possible to real applications. Ageing under increased drain voltage allowed studying both thermal and electrical aspects. Characterization performed before and after ageing tests proved that ageing process is mainly due to thermal stress for the irreversible part and to trapping effects for the reversible one, respectively. This was confirmed by additional physical analysis, the results revealing strong changes of the Schottky contact. Photons emission microscopy associated with X radiography gives spatial correlation between gate degradation and evolution of the brazing interface between the transistor chip and its packaging. It also illustrates the appearance of overheating points and acceleration of the Schottky contact degradation.

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