A high electron mobility transistor includes a T-type gate electrode disposed on a substrate between source and drain electrodes and insulating layers disposed between the substrate and the T-type gate electrode. The insulating layers include first, second, and third insulating layers. The third insulating layer is disposed between the substrate and a head portion of the T-type gate electrode such that a portion of the third insulating layer is in contact with a foot portion of the T-type gate electrode. The second insulating layer is disposed between the substrate and the head portion of the T-type gate electrode to be in contact with the third insulating layer. The first insulating layer and another portion of the third insulating layer are sequentially stacked between the substrate and the head portion of the T-type gate electrode to be in contact with the second insulating layer.