The theory of internal photoemission in semiconductor heterojunctions has been reviewed and the existing model has been extended by incorporating the effects of the difference in the effective masses in the active region and the substrate, non spherical-nonparabolic bands, and the energy loss per collisions. This complete model has been applied to describe the experimental results obtained from Si1-xGex/Si heterojunction infrared photodetectors. The barrier heights (correspondingly the cut-off wavelengths) of SiGe/Si samples have been determined from their internal photoemission spectra by using the extended model which has the wavelength and doping concentration dependent free carrier absorption parameters. Fowler analysis showed that the model is in good agreement with the experiments for the entire spectrum. Crown Copyright (c) 2004 Published by Elsevier B.V. All rights reserved.