III-nitride high electron mobility transistors (HEMTs) are a strong candidate for high-power radio frequency (RF) applications due to their ability to support high breakdown fields, while maintaining a high electron mobility and a high density of the two-dimensional electron gas (2DEG). Analytic device models that are physically motivated and mathematically robust are preferred for circuit simulations. Most prior works have focused on compact models for 111-nitride HEMTs that operate in the drift-diffusive limit, while Ref.  presents a threshold-voltage-based compact model for quasi-ballistic gallium nitride (GaN) HEMTs using an empirical channel charge model. In this paper, we present a unified and self-consistent current-voltage (I-V) and capacitance-voltage (C-V) model of III-nitride HEMTs that is valid for quasi-ballistic transport. The terminal charges are used to calculate inter-nodal capacitances ensuring charge conservation in the device. Both I-V and C-V model are validated against numerical TCAD simulations and experimental data of short-channel GaN HEMTs. Effects of fringing charge resulting from bulk and surface traps are also incorporated in the model. Key equations of the dynamic model are presented in Table 1. Details of the static I-V model of III-nitride HEMTs are presented elsewhere by the authors , and equations are omitted for brevity.