This paper presents a design-oriented charge-based model for dc operation of AlGaAs/GaAs and AlGaN/GaN-based high-mobility field-effect transistors. The intrinsic model is physics-based and does not introduce any empirical parameter. The central concept is based on the linear approximation of the channel charge density with respect to the surface potential, leading to explicit and continuous expressions for charges and current in all the regions of operation, including subthreshold. In addition, an effective circuit design methodology based on the pinchoff surface potential, the pinchoff voltage and the key concept of inversion coefficient (IC) is proposed, likewise for silicon MOSFET circuits.