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The direct current characteristics of AlGaN/GaN double-channel HEMTs (DC-HEMTs) are investigated by using 2-D numerical simulations. The output characteristics have been predicted with the drift-diffusion, thermodynamic, hydrodynamic, and hot-electron models, respectively. The prediction by the hydrodynamic model is in good agreement with the experiment. It is demonstrated that the hot-electron effect makes a negligible contribution to the negative differential conductance (NDC) of an AlGaN/GaN DC-HEMT; instead, the NDC effect is caused by the self-heating effect. The transfer and transconductance characteristics of an AlGaN/GaN DC-HEMT are also discussed in detail. Finally, a new In0.18Al0.82N /GaN/AlGaN/GaN DC-HEMT structure is proposed for optimizing AlGaN/GaN DC-HEMTs.