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Si-detectors are a demanding field to advance 3D numerical device simulations. Depending on the application, very different requirements determine the design goal: space or terrestrial applications largely shift the weights with respect to the standard wishes like low power, low noise, high spatial, time and angle resolution, high speed, ..., as well as to special requests like the maximal charge handling capacity of the detector. The result is a large set of different, 3D structured designs. The basic size of the detector is defined by the absorption length of the particles to detect, hence it is often very large. This justifies the validity of the classic drift-diffusion model only to some extent, because particle trajectories may result in very local, high density sources of electron-hole pairs. Small length scales are introduced in modern low noise matrix pixel detectors, which integrate the first amplifier stage into each pixel. This adds design and geometric complexity but does not change the basic problems. The simulations contribute to the detector design, help to verify the input assumptions for the `readout electronics', and finally they are used to improve the interpretation of the measured data in the experiments, too. Therefore, verification of the 3D simulations by special experiments is part of the process. The text illustrates the limits of the presently existing methods, the models and the corresponding numerical algorithms using the most recent 3D designs and validation experiments.