Abstract: A method and apparatus for the determination of parameters of interests of a semiconductor sample is provided. For example, the thickness of an epitaxial or implanted layer, the thickness of a transition layer and the concentration of free carriers in a substrate layer may be determined without having to destroy the semiconductor sample in the process. In an embodiment, a method is provided for determining at least one parameter of a semiconductor. The method starts by measuring an experimental reflectance spectrum of the semiconductor. An analytical model of the semiconductor having a film layer, a transition layer and a substrate layer is then constructed. Next, optical constants n and k for the film layer, transition layer and substrate layer are expressed as a function of doping level. A profile of the transition layer is determined, and if an abrupt profile exists, the transition layer is not included in the semiconductor model.