We report RF small signal analysis of ultra-wide bandgap AlGaN channel MESFETs with Al composition of 70%. Ohmic contacts are achieved using a linearly-graded and heavily-doped AlGaN layer grown by metal organic chemical vapor deposition. The device with a gate length of 250 nm shows a drain current density (IDSS) of 370 mA/mm when the gate is shorted to source, unity current gain cutoff frequency (fT) of 8.8 GHz, and maximum oscillating frequency (fMAX) of 15 GHz. Bias-dependent device parameters are extracted from measured S-parameters based on a small-signal equivalent circuit model. The peak effective electron velocity of 2.9 × 106 cm/s is obtained at VGS = − 7 V. We show that the average electron velocity (ve) under the gate limits the device intrinsic RF performance while the high source access resistance (Rs) and drain access resistance (Rd) limits the device extrinsic RF performance.