Weak n-type characteristics or poor p-type characteristics are limiting the applications of binary semiconductors based on ambipolar field-effect-transistors (FETs). In this work, ternary alloy of In0.2Ga0.8As nanowires (NWs) are successfully prepared by using the catalyst of Ni firstly during typical solid source chemical vapor deposition process for balancing the weak n-type conduction behavior in ambipolar GaAs NWFETs and poor p-type conduction behavior in ambipolar InAs NWFETs. The ambipolar transport is confirmed by the constructed back-gated NWFETs, contributing from the native oxide shell and body defects of as-prepared In0.2Ga0.8As NWs. As demonstrated by photoluminescence, the bandgap of as-prepared In0.2Ga0.8As NWs is 1.28 eV, promising the application of near-infrared (NIR) photodetection. Under an 850 nm laser illumination, the as-fabricated ambiploar NWFETs show extremely low dark currents of 50 pA and 0.5 pA by applying positive and negative gate voltages respectively. All of the results demonstrate the careful design of surface oxide layer and body defects of NWs for next-generation optoelectronic devices. © 2020 IOP Publishing Ltd.