The theory of internal photoemission in semiconductor heterojunctions has been investigated and the existing model has been extended by incorporating the effect of different effective masses in the active region and the substrate, nonspherical-nonparabolic bands, and the energy loss per collision. Photoresponse measurements on Si1-xGex/Si heterojunction internal photoemission (HIP) infrared photodetectors (IP) have shown that they are fit well by the theory. Qualitative model describing the mechanisms of photocurrent generation in our structures are presented. We also study the effect of a double barrier on the photoresponse spectrum of a SiGe/Si HIP IP. It has been shown that the performance of our devices depends significantly on the applied bias and the operating temperature; therefore, their cut-off wavelengths can be tuned to the desired region by changing the potential difference across the device and/or changing the device temperature. The barrier heights (correspondingly the cut-off wavelengths) of the samples have been determined from their IP spectra by using the extended model which has the wavelength and doping concentration dependent free carrier absorption parameters. Crown Copyright (c) 2005 Published by Elsevier B.V. All rights reserved.