In this paper we consider the wavelength response of fast Ge photodetectors. The detectors are realized as germanium diodes with a p-i-n structure on a silicon-on-insulator (SOI) substrate for vertical incident light. The device structures are able to be monolithically integrated and are grown with molecular beam epitaxy. A layer stack model is developed to calculate internal and external quantum efficiencies of the detectors. Furthermore optical responsivities for detectors with different thicknesses of the absorption layer are calculated and measured to confirm the model. The absorption range of these devices goes from 700 nm to 1550 nm. (C) 2009 Elsevier Ltd. All rights reserved.