ZnO, a wide band gap material, has interesting features like high electron mobility, high optical transparency and costeffectiveness. The major drawback of using ZnO as the channel layer is the presence of point defects which affects the carrier concentration. Carrier concentration (n) and interface states of the thin film greatly influence its threshold voltage, thus controlling the intrinsic defects of the film and passivation of interface states is crucial to reduce the threshold voltage variation. UV-Ozone (UV-O) treatment can be used to suppress these defects by supplying reactive oxygen to the film. In this report, we have studied grain boundary, carrier concentration and interface charge effects separately to emulate the effect of UV-O treatment. We have incorporated two distinct defect levels in our model for acceptor-like and donor-like defects in the grain boundary. The electrical parameter extraction was carried out in Silvaco using the TCAD simulator. We have observed an enhancement in corresponding electron mobility from 142.9 to 149.9 cm2 /Vs, along with the positive shift in threshold voltage from -0.40 to -0.25 V, with the decrease in number of grain boundaries. With change in carrier concentration, mobility of the device was increased from 44.2 to 150 cm2 /Vs. Passivation of interface charge density from 1 x 1012 to 1 x 1010 cm-2 in our model resulted in a significant change in the threshold voltage from 1.25 V to -0.4 V. Authors would like to acknowledge Department of Science and Technology (DST), India and IIT Bombay.