This paper presents a systematic study of Al_0.23Ga_0.77N/GaN/Al_xGa_1−xN double-heterojunction high-electron-mobility transistors (DH-HEMTs) with a boron-doped P+ GaN cap layer under the gate. The boron-doped GaN cap layer shows great potential to form a high-bandgap Schottky gate in DH-HEMT devices to increase the resistivity of the GaN cap with excellent structural characteristics. Thus, the polarization-induced field in the GaN cap layer can be used to raise the conductive band of the device in the normally OFF operation. In this paper, these AlGaN/GaN power-switching devices with freewheeling Schottky barrier diodes are examined in their working states. In comparison with conventional HEMT power devices, the HEMT with a B-doped GaN cap offers the lowest switching charges, area-specific ON-state resistance, and energy losses. Therefore, this study clearly shows the advantage of GaN transistors for power electronics applications.