The intentional doping of lateral GaN power high electron mobility transistors (HEMTs) with carbon (C) impurities is a common technique to reduce buffer conductivity and increase breakdown voltage. However, this comes at the cost of increased intrinsic defects together with degraded dynamic on-resistance (Ron) and current-collapse effects. The aim of this project is compare the performance of HEMTs devices containing different quantities of extrinsic defects (such as C atoms) and intrinsic defects (such as dislocations), as a function of growths conditions to guide toward optimized buffer structure with good dynamic Ron and low vertical leakage simultaneously.