High power electronics using wide bandgap materials are maturing rapidly, and significantmarket growth is expected in a near future. Ultra wide bandgap materials, which have an even largerbandgap than GaN (3.4 eV), represent an attractive choice of materials to further push the performancelimits of power devices. In this work, we report on the fabrication of AlN/AlGaN/AlN high-electronmobility transistors (HEMTs) using 50% Al-content on the AlGaN channel, which has a much widerbandgap than the commonly used GaN channel. The structure was grown by metalorganic chemicalvapor deposition (MOCVD) on AlN/sapphire templates. A buffer breakdown field as high as5.5 MV/cm was reported for short contact distances. Furthermore, transistors have been successfullyfabricated on this heterostructure, with low leakage current and low on-resistance. A remarkablethree-terminal breakdown voltage above 4 kV with an off-state leakage current below 1μA/mm wasachieved. A regrown ohmic contact was used to reduce the source/drain ohmic contact resistance,yielding a drain current density of about 0.1