Approximations to two-step diffusion process by Prony's method

A simple two-term approximation that is accurate over a wide range of drive-in ratios is given for the two-step diffusion process. Comparison with earlier approximations are graphically given. The final approximation is simple and convenient in evaluating the device performance. The Prony's method adopted to approximate erfc(x) is useful in approximating other profiles as well, such as anomalous diffusion profile of phosphorus, ion-implanted profile, etc.