1. L. V. Keldysh Proc. Int. Conf. Phys. Semicond., 9th, Moscow p. 1303. Nauka, Leningrad, 1968 A historical note: It is generally recognized that the decisive event in the development of the subject was Keldysh's conjecture that excitons might condense into metallic e-h droplets, an idea stimulated by the report by