Extendedn‐type layers have been produced in semi‐insulating GaAs by the activation of Si implanted to atomic concentrations as low as 6×1016cm−3. Active layers were obtained in two types of undoped (no intentional dopants introduced during the growth process) semi‐insulating GaAs. These were bulk GaAs grown by the liquid encapsulated Czochralski method and epitaxial layers grown by chemical vapor deposition. Three terminal transferred‐electron devices were fabricated in a completely planar geometry. Gunn domain triggering by the gate resulted in dc negative resistance current dropback between 20% and 40%. High‐frequency negative resistance was observed in the 2–7 GHz range.