In this paper we study the dynamics of a semiconductor microcavity containing a bulk medium, driven by an external coherent field. Thermal effects are taken into account through a temperature dependence of the band-gap energy of the semiconductor, that appears in the nonlinear susceptibility. The susceptibility contains the information about nonlinear absorption and refractive index changes and it is developed in the framework of a free-carrier theory, with a few many-body correction (Urbach tail, band-gap renormalisation) introduced phenomenologically. As a first approximation we consider Plane Wave case, eliminating diffusing or diffractive effects. We are able to obtain global dynamical effects such as thermally induced Regenerative Oscillations or switching point inversion. The results we present are preliminary to a further study of the combination of spatial and thermal effects, still in progress.