  In this letter, we design and demonstrate an improved MOCVD grown reverse
Al-composition graded contact layer to achieve low resistance contact to MOCVD
grown ultra-wide bandgap (UWBG) Al0.70Ga0.30N channel metal semiconductor
field-effect transistors (MESFETs). Increasing the thickness of the reverse
graded layer was found to improve contact layer resistance significantly,
leading to contact resistance of 3.3x10^-5 Ohm.cm2. Devices with gate length,
LG, of 0.6 microns and source-drain spacing, LSD, of 1.5 microns displayed a
maximum current density, IDSMAX, of 635 mA/mm with an applied gate voltage,
VGS, of +2 V. Breakdown measurements on transistors with gate to drain spacing,
LGD, of 770 nm had breakdown voltage greater than 220 , corresponding to
minimum breakdown field of 2.86 MV/cm. This work provides a framework for the
design of low resistance contacts to MOCVD grown high Al-content AlxGa1-xN
channel transistors.
