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GaN high-electron-mobility transistor (HEMT) has superior features of wide band gap, high electron mobility and very high electric field strength due to its material advantages. By using the GaN HEMT, switching frequency can be enhanced up to megahertz with extremely high efficiency. Unfortunately, GaN HEMTs accomplished by undesirable switching oscillations and voltage spikes due to extremely fast switching frequencies with very high <inline-formula> <tex-math notation="LaTeX">${dv}/{dt}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">${di}/{dt}$ </tex-math></inline-formula> and parasitic parameters. In this paper, RLC equivalent circuit models are developed for turn on and turn off conditions, including all parasitic components. In addition, the relative effect of each parasitic parameter is analyzed and estimated. Moreover, simple mathematical model is developed for theoretical analysis of switching oscillation phenomenon and, for guidance of snubber or damping circuit design. To validate these simple equivalent circuit models, both circuit simulation and experimental measurements are employed.