We report the small-signal high-frequency performance of novel enhancement-mode N-polar GaN metal–insulator–semiconductor field effect transistors with a peak short-circuit current gain cutoff frequency ( f t ) of 120 GHz for a 70-nm gate length device. The device has an 8-nm GaN channel with AlN back barrier and a 5-nm SiN x gate dielectric. These devices show a peak drain current of 0.74 A/mm and peak transconductance of 260 mS/mm at a drain bias of 3.0 V. This is the first demonstration of high-frequency operation of N-polar enhancement-mode GaN devices.