AllnN/GaN high electron mobility transistors (HEMTs) on sapphire substrate have yielded a maximum drain current density of 1.26 A/mm with a current gain cutoff and maximum oscillation frequencies about 26 and 40 GHz, respectively, for a 0.25 mu m gate length device. Pulsed characterisations indicate absence of the virtual gate effect and reveal that the drain current dispersion is mainly due to thermal effects. Temperature stress experiments up to 800 degrees C indicate that surface and hetero-interface are inherently stable. The reasons for the behaviour are discussed.