TCAD simulation was performed to investigate the material properties of an AlGaN/GaN structure in Deep Acceptor (DA)-rich and Deep Donor (DD)-rich GaN cases. DD-rich semi-insulating GaN generated a positively charged area thereof to prevent the electron concentration in 2DEG from decreasing, while a DA-rich counterpart caused electron depletion, which was the origin of the current collapse in AlGaN/GaN HFETs. These simulation results were well verified experimentally using three nitride samples including buffer-GaN layers with carbon concentration ([C]) of 5×10¹⁷, 5×10¹⁸, and 4×10¹⁹ cm^-3. DD-rich behaviors were observed for the sample with [C]=4×10¹⁹ cm^-3, and DD energy level E_DD=0.6 eV was estimated by the Arrhenius plot of temperature-dependent I_DS. This E_DD value coincided with the previously estimated E_DD. The backgate experiments revealed that these DD-rich semi-insulating GaN suppressed both current collapse and buffer leakage, thus providing characteristics desirable for practical usage.