A near infrared germanium waveguide photodetector for high speed operation is grown monolithically integrated on Si with molecular beam epitaxy (MBE). With a special virtual substrate concept the lattice mismatch between Si and Ge is accommodated. For high speed operation the active device consists of a Ge p-i-n structure with very high-doped contact layers. The challenges on the MBE growth are the abrupt doping transitions over more than 4 orders of magnitudes among p-contact, intrinsic region and the n-contact. (C) 2008 Elsevier B.V. All rights reserved.