The performance of novel AlInN/GaN HEMTs for high power / high temperature applications is discussed. With 0.25 mu m gate length the highest maximum output current density of more than 2 A/mm at room temperature and more than 3 A/mm at 77 K have been obtained even with sapphire substrates. Cut-off frequencies were f(T) = 50 GHz and f(MAX) = 60 GHz for 0.15 pm gate length without T-gate. Pulsed measurements reveal a less instable surface than in the case of AlGaN/GaN structures. Although limited by buffer layer leakage, with field plates a maximum drain bias of 100 V has been reached with these devices. The high chemical stability of this unstrained heterostructure and its surface has been demonstrated with successful operation at 1000 degrees C in vacuum. sapphire substrate [8]. The studied structure consists of 2 mu m thick GaN buffer, 1 nm thick AlN spacer layer and 13 ran thick AlInN barrier layer containing 81% A]. Hall Effect measurements at room temperature and at 77 K give a sheet carrier density Ns = 2.5x 1013 cm(-2), a sheet resistance of 210 Omega/square (RT) and 80 Omega/square (77 K) with a record electron mobility of 1170 cm 2 Ns and 3170 cm Ns respectively. NIESA isolation has been performed by dry etching in Argon plasma. For the ohmic contacts, a Ti/Al/Ni/Au metal sequence annealed at 890'C for 60 sec has been used. A contact resistance Rc = 0.7 n.mm has been obtained, as measured by TLM. Tle DrainSource distance is 2.5 pm. Ni/Au Schottky gates have been defined by e-beam lithography