Annealing effects on the electrical and optical characteristics of GaAs/InGaP/n+InGaAs:Si/GaAs epilayers grown on GaAs semi-insulating substrates by metal organic chemical vapor deposition have been investigated. This epilayer structure has been used for heterostructure metal–semiconductor field effect transistors. The carrier concentration is decreased and the mobility is increased by annealing at 700 °C or higher, suggesting the deactivation of Si donors in the n+InGaAs layers. With such annealing, the band-edge photoluminescence (PL) peak from the n+InGaAs layers becomes dramatically weak or disappears and deep level PL broad peaks around 1.2 and 1.5 μm appear instead. It is confirmed by the selectively excited PL that these peaks are from the n+InGaAs layer. The deep levels probably act as nonradiative recombination centers for the near band-edge PL and are attributed to Si complex levels created during the Si donor deactivation process. These levels have no relationship with the strain relaxation or dislocation generation. At 600 °C or lower, however, the carrier concentration, mobility, and PL spectra are hardly changed at all by annealing. © 1998 American Institute of Physics.