Infrared absorption spectra of molecular beam epitaxial grown GaInAsSb alloys, GaInAsSbp‐i‐nandp‐nphotodiode structures are reported. It is found that the absorption edge of theN‐type GaInAsSb has a blue shift compared with that of the unintentionally doped GaInAsSb, which is thought to be the Moss–Burstein shift. On the other hand, the absorption edges of thep‐i‐nandp‐nphotodiode structures have red shifts compared to the respective layers, which results from the built‐in field induced Franz–Keldysh effect. By using the WKB method, we have calculated the absorption spectra for photons of energy less than the energy gap. The theoretically predicted red shift is in good agreement with the experimental results. ©1995 American Institute of Physics.