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 620 and 800°C. In contrast, samples with SiN were thermally stable after annealing at up to 800°C even when the AlGaN layer was as thin as 152Å. TEM observations revealed that neither surface roughness nor interfacial diffusion at the SiN/AlGaN interface occurred due to annealing. The SiN layer is very effective for passivating an AlGaN surface and improving the thermal stability of a thin-AlGaN 2DEG channel.