AlGaN/InGaN heterostructure field effect transistors were grown on sapphire by metalorganic chemical vapor deposition. Transmission electron microscopy shows that there are no additional dislocations induced by inserting the InGaN channel while a variation of strain field across the channel is observed. The transistors exhibit good pinch-off characteristics with a threshold voltage of about −2.9 V and a saturation current density of 0.55 A/mm. At room temperature, a peak transconductance of 132 (mS/mm) was obtained for a 1.0 μm-device. Current gain cutoff frequency fT of 9.4 GHz and maximum oscillation frequency fmax of 28.2 GHz were measured for the 1.0 μm-device. As the temperature is increased to 300 °C, the transconductance decreases to 50 mS/mm accompanied by a reduction of saturation current density of 0.24 A/mm due to the enhanced carrier scattering, gate leakage, and drain-source resistance.