To improve the breakdown characteristics of an AlGaN/GaN based high electron mobility transistor (HEMT) for high voltage applications, AlGaN/GaN/Al xGa 1-xN double heterostructure (DH-HEMTs) were designed and fabricated by replacing the semi-insulating GaN buffer with content graded Al xGa 1-xN (x=x 1 → x 2, x 1 > x 2), in turn linearly lowering the Al content x from x 1=90% to x 2=5% toward the front side GaN channel on a high temperature AlN buffer layer. The use of a highly resistive Al xGa 1-xN epilayer suppresses the parasitic conduction in the GaN buffer, and the band edge discontinuity limits the channel electrons spillover, thereby reducing leakage current and drain current collapse. In comparison with the conventional HEMT that use a semi-insulating GaN buffer, the fabricated DH-HEMT device with the same size presents a remarkable enhancement of the breakdown voltage.