Simulation investigation is carried out on the single drift region and Quasi-Read type hexagonal GaN based IMPATT devices for Terahertz frequency operation. It is observed that Quasi-Read GaN IMPATT may generate a RF power density of ∼43×1010 Wm-2 with an efficiency of 20%, whereas its flat profile counterpart is capable of delivering a power density of 31×1010 Wm-2 with an efficiency of 17%. The total parasitic series resistance, including the effects of ohmic contact resistances, has been found to be a major problem that reduces the RF power output of the THz IMPATTs significantly. The study reveals that the value of Rs decreases by 30% as the doping profile of the diode changes from flat to Quasi-Read type with the incorporation of the charge bump. This study establishes the advantages of Quasi-Read type IMPATT over its flat profile counterpart to realize a high-power source in the THz regime. © 2010 IEEE.