Based on the structure of strained Si/SiGe NMOSFET, a unified charge model is presented, in which charge conservation is guaranteed by using the charge as the state variable. The model describes device characteristics from subthreshold to strong inversion as well as from the linear to the saturation operating regions using a smoothing function, and guarantees the continuities of charges and capacitances. Furthermore, capacitance models have been presented using Verilog-A, a language to describe analog behavior. Comparisons between the model and measured data show that the charge model can describe the device characteristics well. The proposed model is useful for the design and simulation of integrated circuits made of strained Si.