  Phase-resolved transient grating spectroscopy in semiconductor quantum wells
has been shown to be a powerful technique for measuring the electron-hole drag
resistivity $\rho_{eh}$, which depends on the Coulomb interaction between the
carriers. In this paper we develop the interacting drift-diffusion theory, from
which $\rho_{eh}$ can be determined, given the measured mobility of an
electron-hole grating. From this theory we predict a cross-over from a
high-excitation-density regime, in which the mobility has the "normal" positive
value, to a low-density regime, in which Coulomb-drag dominates and the
mobility becomes negative. At the crossover point, the mobility of the grating
vanishes.
