Compact models for two-dimensional field-effect transistors (2D FETs), predominantly adapted from conventional unipolar Si metal-oxide-semiconductor FET (MOSFET) frameworks, frequently fail to capture essential physics such as ambipolar conduction, threshold voltage roll-off, and output current upswing in saturation. While models based on Landauer formalism and the Pao-Sah approach offer partial solutions, their reliance on curve fitting and neglect of defects obscure the link to underlying device physics. Here, we introduce a physics-grounded carrier-phase-space (CPS) framework for 2D FET compact modeling. By employing the quasiequilibrium approximation and formulating the drain-source current as a path integral within the CPS, our model accurately describes threshold voltage roll-off, saturation current upswing, and ambipolar behavior. Crucially, analytical expressions for key electrical turning points across all operating regimes, including threshold voltage, saturation voltage, pinch-off voltage, and p-n-junction onset voltage, are derived for the first time through analysis of the bias-dependent integration path in CPS. Furthermore, the model incorporates the effects of ionized defects and is rigorously validated against experimental data from both ambipolar and unipolar FETs, demonstrating excellent electrostatic gating control and uniform thermal dissipation. This work establishes a comprehensive physics-based CPS compact model for 2D FETs, significantly advancing the foundation for their practical circuit design and manufacturing.