  We study the negatively $T^{-}$ and positively $T^{+}$ charged trions in bulk
materials in the effective mass approximation within the framework of a
potential model. The binding energies of trions in various semiconductors are
calculated by employing Faddeev equation in configuration space. Results of
calculations of the binding energies for $T^{-}$ are consistent with previous
computational studies and are in reasonable agreement with experimental
measurements, while the $T^{+}$ is unbound for all considered cases. The
mechanism of formation of the binding energy of trions is analysed by comparing
contributions of a mass-polarization term related to kinetic energy operators
and a term related to the Coulomb repulsion of identical particles.
