The noncontact transient grating technique under constant electron‐hole pumping to the density 4×1019cm−3was used to characterize the ambipolar diffusion coefficientDain Si:Al, Si:In, and Si:Sb molecular‐beam‐epitaxy‐grown layers.Dawas found to be almost constant at a value of ≊8 cm2/s up to an equilibrium carrier density of 2×1019cm−3in the layer and was independent of the doping type. At higher doping density, evidence for a sharp increase inDawas observed. For example,Daincreased to a value of 20 cm2/s at a doping density of about 1020cm−3. TheDabehavior is in reasonable agreement with results of the high‐density ambipolar diffusion theory of Young and van Driel and is incompatible with majority‐carriers diffusion coefficients according to the formulaDa= 2DnDp/(Dn+Dp). An explanation for this behavior is given.