Compression strains are generated at an interface between the cap layer and the barrier layer and an interface between the channel layer and the buffer layer and a tensile strain is generated at an interface between the barrier layer and the channel layer. Therefore, negative charge is higher than positive charge at the interface between the cap layer and the barrier layer and the interface between the channel layer and the buffer layer, while positive charge is higher than negative charge at the interface between the barrier layer and the channel. The channel layer has a stacked layer structure of a first layer, a second layer, and a third layer. The second layer has a higher electron affinity than those of the first layer and the third layer.