Magneto-optical analysis of prominent photoluminescence lines from GaAs FET structures has been performed. Fifteen samples were investigated. Each consisted of a sulfur doped active layer on a high resistivity buffer layer (both epitaxially grown films) on a chromium doped GaAs substrate. The active layers were generally 2μm thick or less, except for two thicker layers (4 and 5μm) grown especially for this study. Buffer layer thicknesses ranged from 1.5 to 26μm. A model based on carrier diffusion through the active layer has been used to interpret the spectra as originating from the active-buffer interface region. All spectra contain strong-evidence of two donorbound exciton complexes associated with sulfur (1.51417eV) and silicon (1.51412eV). Other sharp spectral features included up to six lines associated with more complicated complexes. Linear Zeeman and quadratic diamagnetic behavior of the lines in applied magnetic fields are discussed.