The rapidly increasing power demand, downsizing of power electronics and material specific limitation of silicon has led to development of AlGaN/GaN heterostructures. Commercial GaN power devices are best available for radio frequency (RF) and high voltage switching applications. Emerging AlxGa1-xN channel based heterostructures are promising to enhance the limits of next generation high voltage GaN power switching devices. AlxGa1-xN heterojunction FETs (HFETs) have been an eye catcher for high voltage power electronics with its potential to outperform the predecessors by virtue of high critical breakdown field of the material, which can be tuned by varying Al mole-fraction. In this presentation, we will report on electrical performances of AlGaN channel HEMTs on both bulk AlN and silicon substrate using various Al content. The fabricated devices exhibited outstanding buffer breakdown electric field above 2.5 MV/cm considering the submicron thin heterostructures grown on silicon substrate. Furthermore, we also experimentally demonstrate high temperature operation of fabricated AlGaN channel HEMTs. AlGaN channel HFETs on bulk AlN with a maximum drain current density > 300 mA/mm and a specific Ron = 4 mΩ.cm 2. A buffer electric breakdown field >10 MV/cm was measured. A high voltage robustness comparison of AlGaN channel HFETs and thin GaN HFETs close to their respective hard breakdown voltages is also investigated, which reveals the superior reliable operation of AlGaN channel HFETs up to 80% of hard breakdown voltages along with a consistent ION/IOFF ratio subsequent to 2000V voltage sweep.