We have successfully grown GaAs/AlGaAs MODFETs directly on silicon substrates by molecular beam epitaxy. We have found that an orientation slightly off (100) is well suited for the growth of GaAs/AlGaAs on silicon substrates. MODFETs fabricated from layers grown on Si had transconductances of 170 mS/mm at room temperature and exhibited no looping. When cooled to 77 K, the transconductance rose to 275 mS/mm. Hall mobilities of 51000 and 38000 cm2/Vs were obtained at 10 and 77 K, respectively, with sheet electron densities of 8.30 × 1011 cm−2 in both cases. These results clearly demonstrate that device quality GaAs/AlGaAs is obtainable directly on Si substrates which has great implications with regard to the monolithic integration of III–V and Si technology.