in the SHQW is approximately 4 × 10 10 cm − $^{2}$. There is a large uncertainty in calculating and measuring the actual exciton density in a QW with a 2DEG-exciton system. Nevertheless, the observed appearance of high-intensity PL at a point 25 µ m from the edge of the excitation region less than 0 . 1 ns after excitation is a possible manifestation of a long range transport of exciton excitation energy. Further research is needed to understand this exciton excitation energy transport mechanism and its potential application to optoelectronic devices. 7 The experimental results obtained from measurement of the exciton dephasing rate show that electron-exciton scattering is twice as efficient as exciton-exciton scattering. It is important to note that the enhanced electron-exciton scattering also leads to efficient relaxation of exciton energy in agreement with previous theoretical results [24]. Efficient exciton energy relaxation involves collective plasmon-phonon interactions in the exciton/two-dimensional electron gas (exciton/2DEG) system which could result in the formation of the second sound branch. Experimental results [13] clearly show that the collective screening response of the exciton/2DEG system to a photo-induced potential results in fundamental changes in exciton characteristics including the possibility of condensation and transport of excitons. The 2DEG density in our experiment is 2 × 10 11 cm − $^{2}$. Negatively charged excitons (trions), which consist of two electrons bound to a single hole with a theoretical [26] and observed [27–29] binding energy relative to the exciton of approximately 1 meV have been observed in modulation- doped GaAs QWs. A photoluminescence peak from trions has been observed from the exciton 2DEG system in our structure around 1 meV below the exciton PL peak only when the exciton linewidth has been minimized using slight variations of the applied gate voltage [30]. In the present experiments, broadening of the PL linewidths at V$_{g}$ = − 36 V and the short exciton lifetime of 0 . 5 ns at V$_{g}$ = 0 precluded the resolution of a PL peak from trions. In conclusion, calculations of the superfluid density and the K-T transition temperature for the 2DEG-exciton system show that the K-T transition temperature increases with increasing exciton density and support the possible occurrence of a fast transport of exciton excitation energy. The superfluid density and the K-T transition temperature for dipole excitons in CQWs in the presence of a 2DEG in one of the wells increases with increasing inter-well separation D . The superfluid density and the K-T transition temperature in CQWs increase when the phenomenological screening parameters β ( n, n$_{e}$ ) for a single well or θ ( n, n$_{e}$, D ) for CQWs in the presence of 2DEG increase. Acknowledgments: O. L. B. would like to acknowledge the support by PSC CUNY grant 621360040, GG would like