We report the characteristics and dark current analysis of InSb p-i-n photodetectors grown on GaAs coated Si substrates by molecular beam epitaxy. Excellent InSb crystal quality and reasonably good detector performance were obtained in spite of the large lattice mismatch between InSb and GaAs/Si. In addition to a favourably low leakage current, 400 x 80 mu m(2) detectors yielded a voltage responsivity of 1.2 x 10(4) V W-1 at 77 K and around 4.5 mu m with a zero bias differential resistance of nearly 1 x 10(4) Omega. As to our knowledge, these are the best results reported for InSb photodetectors on Si substrates, and they are encouraging for the development of monolithic large area InSb focal plane arrays on Si substrates. At 77 K, the major components of the dark current were found to be shunt and tunnelling currents. At higher temperatures (125 K), generation-recombination in the depletion region and surface diffusion become the dominant limiting mechanisms for the zero bias resistance area product (R(0)A). (C) 1998 Elsevier Science Ltd. All rights reserved.