Nonpolar $a$-Plane AlGaN/GaN Heterostructure Field-Effect Transistors Grown on Freestanding GaN Substrate

We fabricated and characterized nonpolar $a$-plane AlGaN/GaN heterostructure field-effect transistors (HFETs) grown on an $a$-plane freestanding GaN substrate. By optimizing the growth conditions, the unintentionally doped oxygen concentration was much reduced in the $a$-plane GaN buffer layer. As a result, the low leakage current in the buffer layer was realized without doping of deep acceptors, such as Fe and C, by which an impurity-contamination-free channel layer can be successfully grown. A maximum drain current of 220 mA/mm at a gate source voltage of +3.0 V, an on resistance of 10.4 m$\Omega$$\cdot$cm 2 , and a threshold voltage of $-1.6$ V were realized.