Schottky-barrier-gate n channel depletion-mode field-effect transistors have been fabricated in GaAs by the use of sulphurion implantation directly into semi-insulating Cr-doped substrates to produce the channel. This technique eliminates the need for the growth of a thin epitaxial layer, as is usually done, and results in better uniformity of device characteristics over the wafer area. Performance of these devices at 1 to 12 GHz is described, and low-frequency characteristics are given.