Abstract
High-crystal-quality Al 0.26 Ga 0.74 N/AlN/GaN structures with a very high mobility, such as over 2100 cm 2 /(V s) with a two-dimensional-electron gas (2DEG) density of approximately 1×10 13 /cm 2 , were grown on epitaxial AlN/sapphire templates by metalorganic vapor phase epitaxy and adopted for the fabrication of high-electron-mobility transistors (HEMTs). The fabricated devices showed good DC characteristics compared with those in conventional AlGaN/GaN HEMTs and in devices grown on sapphire substrates. Also, it was confirmed that further improvements in DC characteristics are achieved by applying recessed ohmic contacts to AlGaN/AlN/GaN HEMTs. A high extrinsic transconductance of approximately 220 mS/mm and a high drain current density of over 1 A/mm with a threshold voltage of -4.08 V were obtained for 1.5-µm-gate-length recessed ohmic AlGaN/AlN/GaN HEMTs on AlN/sapphire templates. In addition, it was found that the gate leakage current of HEMTs can be suppressed by using epitaxial AlN/sapphire templates as substrates.