Using a novel time-resolved optical photoluminescence imaging technique, analogous to the electrical Haynes-Shockley experiment, we have measured room-temperature minority-carrier transport in a series of "surface- free" GaAs/Al0.3Ga0.7As double heterostructures. These measurements are only possible in "surface-free" samples in which the band-to-band radiative recombination lifetimes are long-here up to 2.5 μs. We find minority-carrier transport to be "diffusive", with diffusion lengths of up to ∼140 μm. We also find transport in thick (≳1 μm) structures to be mediated by hole-dominated ambipolar diffusion, whereas for thinner structures a transition from ambipolar to free-electron-dominated diffusion is observed. These results demonstrate that our heterostructures become effectively modulation doped for GaAs thicknesses ≲1 μm.