According to example embodiments, a high electron mobility transistor (HEMT) includes a channel supply layer and a channel layer. The channel layer may include an effective channel region and a high resistivity region. The effective channel region may be between the high resistivity region and the channel supply layer. The high resistivity region may be a region into which impurities are ion-implanted. According to example embodiments, a method of forming a HEMT includes forming a device unit, including a channel layer and a channel supply layer, on a first substrate; adhering a second substrate to the device unit; removing the first substrate; and forming a high resistivity region by ion-implanting impurities into at least a portion of the channel layer.