Arsenic-doped Si(001) layers with concentrations CAs up to 5×1018 cm−3 were grown on Si(001)2×1 at temperatures Ts = 575–900 °C by gas-source molecular-beam epitaxy (GS-MBE) using Si2H6 and AsH3. This is almost an order of magnitude higher than the initially reported “maximum attainable” saturated CAs value for GS-MBE from hydride precursors. At constant JAsH3/JSi2H6, CAs decreases, while the film growth rate RSi increases, with Ts. Temperature programmed desorption measurements show that As segregates strongly to the growth surface and that the observed decrease in CAs at high film growth temperatures is primarily due to increasingly rapid arsenic desorption from the segregated layer. Decreasing Ts enhances As incorporation. However, it also results in lower film growth rates due to higher steady-state As surface coverages which, because of the lone-pair electrons associated with each As adatom, decrease the total dangling bond coverage and, hence, the Si2H6 adsorption rate. At constant Ts, CAs increases, while RSi decreases, with increasing JAsH3/JSi2H6. All incorporated As resides at substitutional electrically active sites for concentrations up to 3.8×1018 cm−3, the highest value yet reported for Si(001):As growth from hydride source gases, and temperature-dependent electron mobilities are equal to those of the best bulk Si:As. © 1999 American Institute of Physics.