In this work, we examine the effect of NH3 plasma post-treatment on the ultraviolet (UV) photo-response transient characteristics of zinc oxide (ZnO) nanorod (NR)-gated AlGaN/GaN high electron mobility transistor (HEMT) photodetectors. The recovery time of the detectors (180 ms) was significantly reduced to 80 ms by the NH3 plasma-treatment for 180 s, while no significant reduction in response time upon UV illumination is measured from the plasma-treated devices. The photoluminescence and X-ray diffraction reveals that continuous improvement in crystalline quality of NRs is observed with the plasma duration up to 180 s. The X-ray photoelectron spectroscopy measurement shows that the surface band bending of ZnO crystals is lowered with the corresponding plasma treatment.