  We report on ultra-wide bandgap (UWBG) Al0.75Ga0.25N channel
metal-insulator-semiconductor field-effect transistors (MISFET) with
heterostructure engineered low-resistance ohmic contacts. The low intrinsic
electron affinity of AlN (0.6 eV) leads to large Schottky barriers at
metal-AlGaN interface, resulting in highly resistive ohmic contacts. In this
work, we use reverse compositional graded n++ AlGaN contact layer to achieve
upward electron affinity grading, leading to a low specific contact resistance
of 1.9x10-6 ohm.cm2 to n-Al0.75Ga0.25N channels (bandgap ~ 5.3 eV) with
non-alloyed contacts. We also demonstrate UWBG Al0.75Ga0.25N channel MISFET
device operation employing compositional graded n++ ohmic contact layer and 20
nm atomic layer deposited Al2O3 as the gate-dielectric.
