The role and interplay of basic structure parameters of arsenic precipitates on the ultrafast trapping of conduction band electrons have been studied in a series of low-temperature-grown GaAs and AlGaAs epilayers grown at various temperatures ranging from 170 up to 325°C and annealed at 600°C. Cross-sectional electron-transmission microscopy was used to determine the density and size of the precipitated arsenic clusters as a function of the growth temperature. The dependence of the electron trapping times (τ) on the spacing (R) and radius (α) of the arsenic precipitates has been systematically investigated by time-resolved pump-probe transient transmission spectroscopy. The electron trapping times follow a τ∝R 3/α law which holds for low-temperature-grown GaAs and is applicable for low-temperature-grown AlGaAs as well. This trend generalizes the electron trapping mechanism for low-temperature-grown and annealed GaAs and related semiconductor alloys. © 2002 American Institute of Physics.