An InGaP/GaAs composite channel has been proposed in order to improve the electron transport properties of InGaP for high power device applications. The electron mobility and velocity are increased due to the contribution of high mobility GaAs. Although the composite channel FET shows higher transconductance and drain current than those of the InGaP single channel FET, the breakdown voltage is nearly the same. The composite channel FET delivered output power of 0.6 W/mm with power added efficiency of 46.2% under 17 V operation at 1.9 GHz.