This paper reports improvements of the compact model for double-gate field-effect transistors (DG MOSFETs). The improvements make the model more accurate for wider range of device dimensions and device operation conditions. Carrier density calculation at the source-end is modified to the model applicable for wide range of gate voltages. By this modification, iteration counts for Newton's method are also drastically reduced. Two approaches to construct the transport equations, unified and separate current methods, in the channel are compared. Conventional charge-sheet model leads to too thin carrier density at the drain-end with unphysical high carrier velocity. Transport equation with velocity saturation and explicit drain electric field is proposed. Simulation result based on this equation is demonstrated.