A high electron mobility transistor includes first, second and third compound semiconductor layers. The second compound semiconductor layer has a first interface with the first compound semiconductor layer. The third compound semiconductor layer is disposed over the first compound semiconductor layer. The third compound semiconductor layer has at least one of lower crystallinity and relaxed crystal structure as compared to the second compound semiconductor layer. The gate electrode is disposed over the third compound semiconductor layer. Source and drain electrodes are disposed over the second compound semiconductor layer. The two-dimensional carrier gas layer is generated in the first compound semiconductor layer. The two-dimensional carrier gas layer is adjacent to the first interface. The two-dimensional carrier gas layer either is absent under the third compound semiconductor layer or is reduced in at least one of thickens and carrier gas concentration under the third compound semiconductor layer.