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 work, we demonstrate Al0.23Ga0.77N channel HFETs on bulk AlN with a maximum drain current density > 300 mA/mm and a specific Ron = 4 mΩ·cm2. A buffer electric breakdown field >10 MV/cm was measured. A high voltage robustness comparison of Al0.23Ga0.77N channel HFETs and thin GaN HFETs close to their respective hard breakdown voltages is also concluded, which reveals the superior reliable operation of Al0.23Ga0.77N channel HFETs up to 80% of hard breakdown voltages along with a consistent ION/IOFF ratio subsequent to 2000V voltage sweep.