An analytic three-terminal band-to-band tunneling current model for the gate-induced drain leakage current (GIDL) in an n-MOSFET is developed. This model considers impurity doping concentration, vertical field, lateral field, and so-induced electron momentum enhancement, as well as the surface electro-static potential in the gate-to-drain overlapped region. Based on a constant surface-potential approximation, a closed-form equation has been obtained instead of the complex integral-form in previous works. The results from this new model show good agreement with the measurement data over a wide range of gate and drain biases and device channel lengths. This work is useful for GIDL analysis in transistor design as well as in circuit simulation.