Extensive simulation investigations on III-V nitride (Gallium Nitride, GaN) based Single Drift (p(++) n n(++)) avalanche transit-time diode clearly establishes the potential of GaN material system in Terahertz region. Further, the effects of punch through on the Terahertz behavior of the GaN IMPATT (IMPact Avalanche Transit-Time diode) are studied for the first time, through a generalized simulation technique. The computed results revealed that for a fixed bias current-density, when the space charge effect is not prominent, the optimum negative conductance as well as corresponding frequency for highest negative conductance decreases with the increase of punch through determined by doping density and the thickness of the active layer of the device. Moreover, it is observed that decrease of punch through factor leads to an increase of device negative resistance and conversion efficiency. The positive series resistance of the Terahertz IMPATT diode is further estimated through a modified simulation scheme. It is interesting to find that the decrease of punch through leads to a significant lowering in the values of series resistance of the simulated Terahertz device. The results will be important for design optimization of GaN THz IMPATT oscillators for application in Terahertz communication systems. (C) 2009 Elsevier B.V. All rights reserved.