[en] A physics-based threshold voltage and drain-source current model based on double-channel Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$N/GaN/Al${}_y$Ga${}_{1-<!--\; ???\; -->y}$N/GaN high-electron-mobility transistors (HEMTs) is proposed in this article. Electrons limited in the potential well are derived from ionized donor-like surface states of the Al${}_x$Ga${}_{1-<!--\; ???\; -->x}$N layer. Accumulated 2D electron gases (2DEGs) and positive charges left are equivalent to planar plate capacitors and further form electric fields, weakening the polarization fields in barrier layers. The physical properties of the dual-channel HEMT are considered holistically rather than two separate and independent single heterojunctions. The effect of the charges and 2DEGs distributed in lower layers on the upper channel is contained in our model as well. Subsequently, a drain-source current model is proposed through coupling the critical electric field with the threshold voltage (V${}_{TH}$). The influence of channel length modulation, short channel effect, self-heating effect, and kink effect is involved in this model. The accuracy of the proposed models is verified by comparison with technology computer-aided design simulations and the experimental results. Proposed models can greatly describe the output characteristics, the transmission characteristics, and the relationship between V${}_{TH}$ and device parameters of double-channel AlGaN/GaN HEMTs. (© 2023 Wiley‐VCH GmbH)