Field effect transistors (FETs), incorporating metal-oxide nanofibers as the active conductive channel, have the potential for driving the widespread application of nanowire or nanofiber FETs-based electronics. Here we report on low voltage FETs with integrated electrospun In2O3-ZnO-ZnGa2O4 composite fiber channel layers and high-K dielectric (MgO)0.3-(Bi1.5Zn1.0Nb1.5O7)0.7 gate insulator and compare their performance against FETs utilizing conductive single phase, polycrystalline ZnO or In2O3 channel layers. The polycrystalline In2O3-ZnO-ZnGa2O4 composite fibers provide superior performance with high field effect mobility (∼7.04 cm2 V(-1) s(-1)), low subthreshold swing (390 mV/dec), and low threshold voltage (1.0 V) combined with excellent saturation, likely resulting from the effective blocking of high current-flow through the In2O3 and ZnO nanocrystallites by the insulating spinel ZnGa2O4 phase. The microstructural evolution of the individual In2O3, ZnO, and ZnGa2O4 phases in composite fibers is clearly observed by high resolution TEM. A systematic examination of channel area coverage, ranging from single fiber to over 90% coverage, demonstrates that low coverage results in relatively low current outputs and reduced reproducibility which we attribute to the difficulty in positioning fibers and fiber length control. On the other hand, those with ∼80% coverage exhibited high field effect mobility, high on/off current ratios (>10(5)), and negligible hysteresis following 15 sweep voltage cycles. A special feature of this work is the application of the FETs to modulate the properties of complex polycrystalline nanocomposite channels.