Effects of gate shaping and consequent process changes on AlGaN/GaN HEMT reliability

The effect of gate shape and its necessary fabrication process on the reliability of AlGaN/GaN high electron mobility transistors (HEMT) was studied on devices fabricated on the same wafer, using DC and pulsed HEMT analysis. Simulations were used to determine the difference in electric field on the surface and in the barrier for the three gate shapes studied, I-shaped, slanted, and recessed slanted. Prior to each electrical characterization during stress, devices were exposed to ultraviolet illumination to probe only newly generated traps rather than the filling of preexisting electronic traps. Degradation was seen to increase with electric field strength; in each device type traps were generated by device stress at the location of the peak electric field. It was found that HEMTs with I-shaped gates showed larger degradation under the same stress conditions than devices with slanted gates. This was due to a higher electric field at the interface between the SiNx passivation and the AlGaN barrier layer resulting in higher surface trap generation. HEMTs with slanted recessed gates showed indications of barrier trapping while surface trapping effects played a smaller role. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim