  We propose a double channel mechanism for the formation of charged excitons
(trions); they are formed through bi- and tri-molecular processes. This
directly implies that both negatively and positively charged excitons coexist
in a quantum well, even in the absence of excess carriers. The model is applied
to a time-resolved photoluminescence experiment performed on a very high
quality InGaAs quantum well sample, in which the photoluminescence
contributions at the energy of the trion, exciton and at the band edge can be
clearly separated and traced over a broad range of times and densities. The
unresolved discrepancy between the theoretical and experimental radiative decay
time of the exciton in a doped semiconductor is explained.
