The role and interplay of basic structure parameters of arsenic precipitates on the ultrafast trapping of conduction band electrons has been studied in a series of low-temperature-grown GaAs epilayers grown at various temperatures ranging from 170 up to 325 °C and annealed at 600 °C. Cross sectional electron-transmission characterization was used to determine the density and size of the precipitated arsenic clusters with growth temperature. The dependence of the electron trapping times (τ) on the spacing (R) and radius (α) of the arsenic precipitates has been systematically studied by time-resolved pump-probe transient transmission spectroscopy. The present work demonstrates that the electron trapping time follows a τ∝R3/α law. © 2001 American Institute of Physics.