Microstructural Degradation Investigations of OFF-State Stressed 0.15 um RF AlGaN/GaN HEMTs: Failure Mode related Breakdown

This paper reports two failure modes and degradation mechanism of normally-on AlGaN/GaN HEMTs of 0.15 mu m gate length with Pt/Au metallization scheme providing stable Schottky contact. Results describe the device performance and final degradation under reverse bias off-state stressing at high V-DS (up to 145 V) and stressing at elevated temperatures (up to 125 degrees C). Electrical stressing combined with microstructural diagnostic characterization confirms two microstructural failure modes. The first failure mode is observed as a local breakdown between gate head and source ohmic contact due to electrical overstress (EOS) causing electro-thermally induced material degradation. The second failure mode, seen also under different stress conditions, consists in pit formation at both gate edges, which is caused by stress induced local oxidation of the semiconductor surface. The first failure mode is related to a local breakdown of the nitride / oxide insulator in correlation with sudden increase of the device currents. The second failure mode causes local distortion of the 2-dimentional electron gas (2DEG) and partial degradation of the Schottky contact.