Normally-off nitride semiconductor transistor device

A nitride semiconductor transistor device is disclosed. The device includes a first nitride semiconductor layer disposed over a substrate, and a second nitride semiconductor layer with a band gap larger than the first nitride semiconductor disposed over the first nitride semiconductor layer. Over the second nitride semiconductor layer, a first insulating film, a charge-storing gate electrode, a second insulating film, and a second gate electrode are formed in order thereon. A source electrode and a drain electrode are disposed over the second nitride semiconductor layer interposing the charge-storing gate electrode in a plane direction. The device further includes a first gate electrode capacitively coupling with the charge-storing gate electrode with an insulating film therebetween forming a first capacitor, and the charge-storing gate electrode is charged by an electron injection from the first gate electrode through the first capacitor.