The doping of semiconductors leads to free charge carriers which can be investigated by IR spectroscopy. The response of free carriers to oscillating electric fields can be described to a good approximation by the simple Drude model. The parameters of that model relate the concentration of the charge carriers and their mobility to properties of the dielectric function. After a model parameter fit of the simulated spectrum to measured data the carrier concentration and the mobility or the resistivity of the material can be computed.