This paper focuses on determining an optimized value of carbon-doping level in the buffer and corresponding channel thickness to improve the performance of GaN HEMTs in terms of subthreshold slope (SS), breakdown voltage (VBD) and transit frequency (ft). With the increase in carbon-doping, we observe improvements in SS and VBD while the ft is reduced. However, as the channel thickness increases above a certain thickness, no significant impact of carbon-doping is observed on the characteristics. TCAD simulation is calibrated using experimental data for a device with carbon-doping level of 3 × 10 18 cm −3 in the buffer with channel thickness of 500 nm. Using the calibrated device in TCAD, the carbon-doping level and channel thickness are varied to see the effects on different parameters. We observed that an optimized channel thickness of 200 nm with carbon-doping level of 1 × 10 19 cm −3 in the buffer yields the best results in terms of VBD and ft.