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In this paper, a surface-potential-based (SP-based) model for AlGaN/GaN modulation-doped field-effect transistors (MODFETs) is built for the first time. First, a closed-form analytical approximation for the Fermi potential EF relative to the bottom of the conduction band at the AlGaN/GaN interface is presented and verified to be accurate enough under different biases and temperatures. Then, the potential of the bottom of the conduction band at the AlGaN/GaN interface φs is defined as the SP, and the value of φs relative to ground is calculated. The development of SP-based compact dc model is achieved based on this calculation. Velocity saturation, channel length modulation, drain-induced barrier lower effect, and self-heating effect are included in the presented model. Compared with the Vth-based model developed by our group, this SP-based model provides a symmetric and more accurate but simpler description for AlGaN/GaN MODFETs. The calculated dc characteristics and transconductance for devices with different lengths are in excellent agreement with the experimental data over the full range of applied gate and drain biases and under different temperatures.