1. A method of modeling integrated circuits, the method comprising: generating a set of circuit parameters of a circuit using a random mean, wherein the set of circuit parameters reflects a statistic distribution of measured circuit parameters after a negative-bias thermal instability (NBTI) stress is performed on the circuit, with the set of circuit parameters being a function of a stress time and a function of a stress temperature; and generating a target performance parameter of the circuit by assigning the set of circuit parameters into a circuit simulator.