High-frequency dispersion of the AlGaN/GaN HEMT output resistance has been investigated. Using the electron momentum balance equation, we demonstrated that the deviation of the dynamic drift velocity from its static value could be considered as the main reason for the high-frequency dispersion of the transistor output resistance. Moreover, a new intrinsic small-signal equivalent circuit has been proposed to consider the output resistance dispersion. Comparison with measured s-parameters have validated the proposed approach.