Compared with Si-based devices, TCAD model calibration for GaN-based devices have yet to be well developed. In this work, we demonstrate a modeling methodology for AlGaN/GaN power HEMT development using a Sentaurus TCAD simulation package. The study focuses on investigating and modeling the influence of interfacial charges, interfacial traps, and bulk traps on the device characteristics. Additionally, the output capacitance of the device was identified as a valuable parameter for model evaluation. Through a systematic modeling extraction methodology, the calibrated models exhibited agreement with experimental electric characteristics. Consequently, these calibrated models may effectively predict the electric performances of novel AlGaN/GaN power HEMTs.