In this study, we investigated the effect of gate length and surface area of the ZnO ultraviolet (UV) absorbing structure on the transient characteristics of AlGaN/GaN HEMT based UV photodetectors. The gate-areas (2, 6 and 18 μm lengths with same width of 100 μm) of AlGaN/GaN HEMTs are covered with ZnO thin film and nanorods (NR) separately. The fabricated devices show enhancement in response speed with the reduction of gate length or the increase of ZnO surface area. The best response speed of ∼10 ms response time and ∼190 ms recovery time is measured from the NR-gated device with a gate length of 2 μm. A model for the oxygen desorption kinetics is proposed for the first time which theoretically shows that the response speed is dependent on two key parameters; light absorbing surface area and gate length. From our model analysis, it is shown that predicted response time is a strong function of these two device parameters, and the calculations show a good agreement with the experimental measurements.