A newly developed sulphur diffusion method for fabricating GaAs FETs is demonstrated. Sulphur atoms adsorbed at the surface of GaAs by the (NH(4))(2)S(x) treatment were diffused into GaAs during rapid thermal annealing. Transconductance as high as 190mS/mm was obtained for MESFETs with 1.0 mu m gate length, fabricated on the lavers diffused by this technique. This diffusion technique was found to be suitable for GaAs device fabrication.