We report on ultra-wide bandgap (UWBG) Al0.75Ga0.25N channel metal-insulator-semiconductor field-effect transistors (MISFETs) with heterostructure engineered low-resistance ohmic contacts. The low intrinsic electron affinity of AlN (0.6 eV) leads to large Schottky barriers at the metal-AlGaN interface, resulting in highly resistive ohmic contacts. In this work, we use a reverse compositional graded n(++) AlGaN contact layer to achieve upward electron affinity grading, leading to a low specific contact resistance (rho(sp)) of 1.9 x 10(-6) x cm(2) to n-Al0.75Ga0.25N channels (bandgap similar to 5.3 eV) with non-alloyed contacts. We also demonstrate UWBG Al0.75Ga0.25N channel MISFET device operation employing the compositional graded n(++) ohmic contact layer and 20 nm atomic layer deposited Al2O3 as the gate-dielectric. Published by AIP Publishing.