Al0.26Ga0.74N/AlN/GaN heterostructures with 1-nm-thick AlN interfacial layers were grown on 100-mm-diam epitaxial AlN/sapphire templates and sapphire substrates by metalorganic vapor phase epitaxy. It was found that AlN/sapphire templates significantly enhanced the electron mobility of the two-dimensional electron gas (2DEG) confined at the GaN channel. This can be explained by the high-crystal-quality GaN channel realized by the use of epitaxial AlN/sapphire templates as substrates. The very high Hall mobilities of approximately 2100 cm2/V s at room temperature and approximately 17 000 cm2/V s at 77 K with a 2DEG density of approximately 1×1013/cm2 were uniformly obtained for AlGaN/AlN/GaN heterostructures on 100-mm-diam epitaxial AlN/sapphire templates. The Hall mobility of AlGaN/AlN/GaN heterostructures on epitaxial AlN/sapphire templates reached a very high value of 25 500 cm2/V s at 15 K.