Improved mobility of AlGaN channel heterojunction material using an AlGaN/GaN composite buffer layer

The quality of an AlGaN channel heterojunction on a sapphire substrate is massively improved by using an AlGaN/GaN composite buffer layer. We demonstrate an Al_0.4Ga_0.6N/Al_0.18Ga_0.82N heterojunction with a state-of-the-art mobility of 815 cm²/(V·s) and a sheet resistance of 890 Ω/ under room temperature. The crystalline quality and the electrical properties of the AlGaN heterojunction material are analyzed by atomic force microscopy, high-resolution X-ray diffraction, and van der Pauw Hall and capacitance–voltage (C–V) measurements. The results indicate that the improved electrical properties should derive from the reduced surface roughness and low dislocation density.