In this paper, a closed-form physics-based surface potential approach is proposed to derive a compact model for current in high-frequency and high-power LDMOS transistors. For this purpose, we have modeled the drift region with three variable resistors. Effects of velocity saturation in the channel as well as the quasi-saturation due to the existence of the N-Drift region have been taken into account. Furthermore, self-heating phenomena has been taken into account by setting proper model parameters. The proposed analytical expression for the transistor current provides simplicity and increased accuracy in numerical simulations. Comparison between the results obtained via modeling and those obtained via experimental analysis and process simulation confirms the consistency and validity of the proposed model.