1. A model for standard cells to perform static timing analysis and sensitivity analysis of an integrated circuit design under process variations, wherein the model is stored as software in a storage device for execution with a computer, the model comprising: a voltage waveform transfer function to receive an input voltage waveform and transform it into an intermediate voltage waveform, wherein the voltage waveform transfer function captures the non-linear waveform shape change from the input voltage waveform to the intermediate voltage waveform in response to a standard cell being modeled; a voltage dependent current source to generate an output current in response to the intermediate voltage waveform; and an output capacitor coupled in parallel to the voltage dependent current source.