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 &mgr;s. We find minority‐carrier transport to be ‘‘diffusive,’’ with diffusion lengths of up to ∼140 &mgr;m. We also find transport in thick (≳1 &mgr;m) 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 becomeeffectivelymodulationdopedfor GaAs thicknesses ≲1 &mgr;m.