Narrow gap IV-VI (e.g. Pb1-xSnxSe and PbTe) layers grown epitaxially on Si(111)-substrates by MBE exhibit high quality despite the large lattice and thermal expansion mismatch. A CaF2 buffer layer is employed for compatibility. Due to easy glide of misfit dislocations in the IV-VI layers, thermal strains relax even at cryogenic temperatures and after many temperature cyclings. This is partly due to the NaCl-structure of the IV-VI materials and at variance to the zinkblende-type semiconductors. In addition, the high permittivities of the IV-VIs effectively shield the electric fields from charged defects. This makes the materials rather forgiving, higher quality devices are obtained from lower quality material, again at variance to Hg1-xCdxTe or InSb and related compounds. We describe ways to further improve device performance by lowering the dislocation densities in the lattice mismatched layers. This is achieved by temperature rampings, which drive out the threading dislocations from the active parts of the sensors. Presently, densities of 1 X 106 cm-2 in layers of a few micrometer thickness are obtained. These densities are sufficiently low in order not to dominate the leakage currents in real devices even at 77 K. Photovoltaic p-n or Schottky- barrier sensor arrays are delineated by using photolithography. At low temperatures, the ultimate sensitivities are presently limited by defects, mainly dislocations. At higher temperatures, the ultimate theoretical sensitivity was obtained with Schottky barrier devices, this despite the large mismatch and only 3 micrometer thickness of the layers. Due to the rather low temperatures used during the MBE and delineation (below 450 degrees Celsius), sensor arrays are obtained by postprocessing even on active Si-substrates.