Affords high electron mobility transistors having a high-purity channel layer and a high-resistance buffer layer. A high electron mobility transistor 11 is provided with a supporting substrate 13 composed of gallium nitride, a buffer layer 15 composed of a first gallium nitride semiconductor, a channel layer 17 composed of a second gallium nitride semiconductor, a semiconductor layer 19 composed of a third gallium nitride semiconductor, and electrode structures (a gate electrode 21, a source electrode 23 and a drain electrode 25) for the transistor 11. The band gap of the third gallium nitride semiconductor is broader than that of the second gallium nitride semiconductor. The carbon concentration NC1 of the first gallium nitride semiconductor is 4×1017 cm−3 or more. The carbon concentration NC2 of the second gallium nitride semiconductor is less than 4×1016 cm−3.