  We report a photoluminescence study of electron-hole complexes in specially
designed semiconductor heterostructures. Placing a remote dilute layer of
donors at different distances \itshape d \normalfont from the quantum well
leads to the transformation of luminescence spectra of neutral ($X$) and
negatively charged ($X^{-}$) excitons. The onset of an additional spectral line
and its energy dependence on \itshape d \normalfont allows us to unambiguously
relate the so-called $X^{-}$ trion state with charged excitons bound on charged
donors in a barrier. The results indicate the overestimation in free-trion
binding energies from previous studies of GaAs/Al$_{0.3}$Ga$_{0.7}$As quantum
wells, and give their corrected values for QWs of width 200 and 300 \AA \space
in the limiting case of infinitely distant donors.
