A physics-based model for the surface potential and drain current for monolayer transition metal dichalcogenide (TMD) field-effect transistor is presented. Taking into account the two-dimensional (2D) density-of-states of the atomic layer thick TMD and its impact on the quantum capacitance, a model for the surface potential is presented. Next, considering a drift-diffusion mechanism for the carrier transport along the monolayer TMD, an explicit expression for the drain current has been derived. The model has been benchmarked with a measured prototype transistor. Based on the proposed model, the device design window targeting low-power applications is discussed. © 2012 American Institute of Physics.