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In this letter, we demonstrate the record small-signal performance from N-polar GaN-based metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) by using a GaN spacer structure with an AlN barrier to reduce alloy scattering. High Si doping in GaN without excessive surface roughening has been achieved using a digital doping scheme. A low ohmic contact resistance of 0.16 ?? ?? mm was measured. Submicrometer gates were fabricated by electron-beam lithography using a triple-layer resist process. f T and f MAX of 47 and 81 GHz, respectively, were obtained for the 150-nm-gate-length device. Further analysis has been done to understand the effect of access resistance on the high-frequency performance, defining a pathway for getting a higher gain and thus achieving a better high-frequency performance from N-polar GaN-based HEMTs.