GaN and SiC materials and their device technology have matured and become commerciallyavailable. Fundamental material properties will soon limit the performance. Consequently, a novelbreakthrough in power electronics performance requires a new generation of materials. In this frame,the so-called ultra-wide-bandgap (UWBG) materials that have energy bandgaps larger than that of SiCand GaN (EG > 3.4 eV) are very promising to enable the next leap forward in power electronicsperformance. AlN-based material system has a unique advantage due to its prominent spontaneousand piezoelectric polarization effects and flexibility in inserting appropriate heterojunctions thusdramatically broadening the device design space. Furthermore, AlN material represents the ideal backbarrier for high voltage HEMT applications due to its large electrical breakdown field and high thermalconductivity. In turn, the AlN buffer can potentially not only increase the electron confinement intransistor channel but also help to boost the breakdown voltage owing to its wider bandgap whilebenefiting from an enhanced thermal dissipation as compared to GaN-based devices.In this paper, we present the fabrication and the DC / high voltage characteristics of a thinchannel AlGaN/GaN HEMT on AlN buffer grown on AlN/sapphire template. Lateral buffer breakdownvoltage assessment reveals a remarkable breakdown field of 5 MV/cm for short contact distances,which is far beyond that of GaN-based material system. Furthermore, fabricated transistors are fullyfunctional with low leakage current and low on-resistance.A 190 nm AlN layer has been grown by ammonia-MBE on a 6 µm thick AlN template onsapphire substrate followed by a thin GaN channel of 8 nm and a high aluminum content (90%) AlGaNbarrier layer capped with 5 nm in-situ SiN as confirmed by HRTEM. The 2DEG properties have beenobtained on fat FET devices with a charge density up to 1.9x1013 cm-2 and a drift mobility of 340cm²/V.s. The rather low mobility can be attributed to the thin channel, the high Al content into thebarrier layer and/or the regrown interface that can still be optimized with a large room forimprovement.Significant lateral buffer breakdown voltage up to 10 kV has been measured on isolated ohmiccontacts for large distances above 90 µm. This shows that the heterostructure does not suffer fromany parasitic conduction as full depletion down to the sapphire substrate occurs. Interestingly, aremarkable breakdown field of 5 MV/cm is observed for short distances with for instance more than1000 V for a 2 µm contact distance. It appears that the breakdown mechanism is not limited by theGaN channel when using a sub-10 nm thickness enabling to benefit from the AlN bandgap for shortcontact distances (i.e. under high electric field).Electrical characterizations were carried out on devices with gate width/length = 50µm/1.5µm andgate-to-drain spacing (LGD) varied from 2 to 40µm. For the various transistor designs, the off-stateleakage current at VDS = 10 V is below 1 µA/mm and the static on-resistance RON scales as expected withLGD to reach values below 15 mΩ.cm2for a 5 µm distance