We present a detailed analysis of carrier-induced modifications on the spatial band diagrams of In-Ga-As/In-Ga-As-P separate-confinement multiple-quantum-well structures and its consequences on optical properties. In the high-carrier-density regime, we observe experimentally a second peak in the spectra of the optical gain. This can be understood by a buildup of a space charge and band bending which is caused by a spatial separation between electrons and holes due to the different density of states of the conduction and valence bands. This band bending causes a localization of barrier conduction band states in the quantum-well region and an enhancement of the overlap integral of those states with quantized heavy hole states giving rise to substantial modifications in the optical matrix elements. These modified matrix elements are the origin of the new class of optical transitions observed in the optical gain spectra.