An annealing study of the AlGaN/GaN 2DEG structure for HEMTs with or without SiN surface passivation films was conducted with the AlGaN layer thickness dependence taken into consideration. Without SiN, the sheet resistance of the samples with thin AlGaN layers increased significantly upon annealing at 900°C. In contrast, samples with SiN were thermally stable after annealing at up to 900°C even when the AlGaN layer was as thin as 14 nm. Even in the region where the sheet resistance had increased due to the annealing, depositing SiN recovered the sheet resistance to the original values.