Continuously increasing energy costs lead to a rising demand for alternative energy sources as well as for new technologies for their efficient usage. A promising candidate for the near future is the direct conversion from heat to electricity using thermoelectric devices, which is already established in special space and in remote environments. Thermoelectric generators are often realized using semiconductor devices, where the rapid progress during the last decades has opened numerous possibilities, both in the choice of the material system and in the design of the device geometry and doping profiles. The influence of graded alloys is being investigated. Additional driving forces due to the effective mass gradient are being taken into account. The material properties of the single materials are used at certain areas in the device to optimize the electrical conductivity and the generation rate, as well as to minimize the thermal conductivity, in order to minimize the internal lattice heat flux and therefore maximize the global figure of merit. To intensify these effects, the geometrical structure is optimized by an appropriate local doping profile using MINIMOS-NT in connection with the optimization framework SIESTA. Several material systems, including Si, SiGe, SiC as well as III-V compound semiconductors are being investigated and examined for their applicability for different thermal environments. Therefore, the accuracy of the model sets has to be established and verified, thermodynamically rigorous coupling mechanisms between the electrical and the thermo-mechanical subsystem are required. Since areas of high temperature are expected, the state-of-the-art models and material parameters have to be re-examined and extended, if necessary, to extend their validity to high temperature ranges. The single parameter values will be extracted from full-band Monte Carlo simulations and from data available in the literature, as well as from measurement data.