On the redox origin of surface trapping in AlGaN/GaN high electron mobility transistors

Water-related redox couples in ambient air are identified as an important source of the surface trapping states, dynamic on-resistance, and drain current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). Through in-situ X-ray photoelectron spectroscopy (XPS), direct signature of the water-related species-hydroxyl groups (OH) was found at the AlGaN surface at room temperature. It was also found that these species, as well as the current collapse, can be thermally removed above 200 degrees C in vacuum conditions. An electron trapping mechanism based on the H2O/H-2 and H2O/O-2 redox couples is proposed to explain the 0.5 eV energy level commonly attributed to the surface trapping states. Finally, the role of silicon nitride passivation in successfully removing current collapse in these devices is explained by blocking the water molecules away from the AlGaN surface. (C) 2014 AIP Publishing LLC.