Lattice matched 0.25-mu m gatelength InAlN/GaN high electron mobility transistors are realized in an ultrathin body mesa technology (50-nm AlN nucleation layer/50-nm GaN buffer) on sapphire. At room temperature, the maximum output current density is I-DS = 0.4 A/mm, the threshold voltage V-th = -1.4 V with an associated subthreshold voltage swing of 73 mV/dec and a leakage current approximate to 1 pA (for W-G = 50 mu m) and thus a current on/off ratio of 10(10). At 600 degrees C, the maximum drain current, threshold voltage, and transconductance are nearly unchanged. The current on/off ratio is still approximately 10(6). First 1-MHz class A measurements with +/- 2.0 V peak-to-peak signal amplitude have resulted in 109-mW/mm output power at V-DS = 8.75 V.