We demonstrate an improvement in the photoresponse characteristics of ultraviolet (UV) photodetectors (PDs) using the N2O plasma-treated ZnO nanorod (NR) gated AlGaN/GaN high electron mobility transistor (HEMT) structure. The PDs fabricated with ZnO NRs plasma-treated for 6 min show superior performance in terms of responsivity (∼1.54×10 5 A/W), specific detectivity (∼ 4.7×1013 cm·Hz-1/2/W), and on/off current ratio (∼40). These improved performance parameters are the best among those from HEMT-based PDs reported to date. Photoluminescence analysis shows a significant enhancement in near band edge emission due to the effective suppression of native defects near the surface of ZnO NRs after plasma treatment. As our X-ray photoelectron spectroscopy reveals a very high O/Zn ratio of ∼0.96 from the NR samples plasma-treated for 6 min, the N2O plasma radicals also show a clear impact on ZnO stoichiometry. From our X-ray diffraction analysis, the plasma-treated ZnO NRs show much greater improvement in (002) peak intensity and degree of (002) orientation (∼0.996) than those of as-grown NRs. This significant enhancement in (002) degree of orientation and stoichiometry in ZnO nano-crystals contribute to the enhancement in photoresponse characteristics of the PDs.