A novel device technology has been developed to enable the fabrication of high performance mm-wave GaN high electron mobility transistors (HEMTs) for solid-state power amplifiers operating in W-band. By utilising the reverse polarisation of N-polar GaN, an in-situ GaN cap layer added in the device access regions acts to improve the channel conductivity and reduces the impact of surface states on device performance. A low dispersion HEMT with 149 GHz peak f T and 285 GHz peak f max demonstrated simultaneous high efficiency and output power density with 34.2% peak power-added efficiency and an associated 2.5 W/mm output power at 87 GHz.