  We have analyzed the energetics of decay of the X^- trion (exciton +
electron) on the assumption that the exciton and trion are independent
excitations of a single two-dimensional semiconducting quantum well. For the
first time, it has been shown that in filling a well with electrons from a
selective donor-doped matrix, the binding energy of the trion (of the electron
with the exciton) increases linearly with a shift of the Fermi level into the
depth of the c band. This agrees with the well-known experimental data on the
low-temperature radiative decay (photoluminescence) of trions in the
heterostructures ZnSe/Zn_0.89Mg_0.11S_0.18Se_0.82 and CdTe/Cd_0.7Mg_0.3Te.
