Drastic Mobility Enhancement of GaN MOSFETs with Graded AlGaN Buried-Channel Formed by Aluminum Thermal Diffusion

GaN MOSFETs with the graded AlGaN layer of several nano-meter thicknesses in the channel region exhibited drastic enhancement in channel mobility comparable to bulk mobility. The graded AlGaN layer formed by the aluminum thermal diffusion into the GaN surface was expected to realize a buried channel from the MOS interface and to reduce the surface scattering. STEM-HAADF, EDX, and EELS revealed that the graded AlGaN layer with 2~3 nm thickness was formed at the GaN surface after AlN capping annealing. Effective channel mobility for MOSFETs with the graded AlGaN layer reached 80 % of the bulk mobility of GaN crystal at 175°C. In addition, threshold voltage could be controlled by Mg concentration in the $p-\text{type}$ body layer.