MBE-Grown AlGaN/GaN HEMTs on SiC

Abstract

We report on the development of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on SiC using plasma-assisted molecular beam epitaxy (MBE). In this work, we show that performance comparable to state-of-the-art AlGaN/GaN HEMTs can be achieved using MBE-grown material. Buffer leakage was an important limiting factor for these devices. The use of either carbon-doped buffers, or low Al/N ratio in the nucleation layer growth were effective in reducing buffer leakage. Studies varying the thickness and concentration of the carbon doping were carried out to determine the effect of different carbon doping profiles on the insulating and dispersive properties of buffers, On devices without field plates, at 4 GHz an output power density of 12 W/mm was obtained with a power-added efficiency (PAE) of 46 % and gain of 14 dB. 15.6 W/mm with PAE of 56 % was obtained from these devices after field-plating. Two-tone linearity measurements of these devices were also carried out. At a C/I 3 level of 30 dBc, the devices measured had an output power of 1.9 W/mm with a PAE of 53 %. The effect of the Al/N ratio in the AlN nucleation layer on buffer leakage was studied. N -rich conditions yielded highly insulating GaN buffers without carbon doping. At 4 GHz, devices without field plates delivered 4.8 W/mm with a PAE of 62 %. At a higher drain bias (50 V), 8.1 W/mm with a PAE of 38 % was achieved.