Improved high power thick-GaN-capped AlGaN/GaN HEMTs without surface passivation

A record high power density on sapphire without passivation was achieved using an epitaxial approach to dispersion reduction. SiN passivation has been employed to reduce DC-to-RF dispersion of GaN-based HEMTs, but is sensitive to surface and deposition conditions. A proposed epitaxial solution is to grow a thick GaN cap on top of the conventional HEMT to increase the distance between surface and channel, reducing the effect of surface potential fluctuations on device performance. Initial results from a gate-recessed device structure showed that dispersion was reduced greatly without surface passivation. Nevertheless, high gate leakage and low breakdown limited the output power. We investigate the cause of these leakage and breakdown issues, propose solutions, and discuss the results. As a consequence, 8.5 W/mm with a PAE of 57% was achieved at 50 V at 4 GHz from unpassivated HEMTs on sapphire, the highest power density reported.