Semi-insulating GaAs substrates doped with chromium were implanted with 120-160 ke V28Si+ ions (1012-1013 cm-2 at room temperature. After thermal annealing, the carrier concentration and mobility profiles, residual damage and impurity levels of implanted wafer were determined by C-V method, back scattering technique and photo-1uminescence spectra. The results showed that the semi-insulating GaAs substrate selection and 28Si+ beam purity control are necessary to obtain reproducible and high activation implantation. Fabricated with multiple localized silicon implantation for both active and contact layer, GaAs dual-gate MES FET applicated in UHF TV tuner reached a noise figure of 0.9 dB and an associated gain of 10 dB at 1 GHz. The mobility profile and appropriate annealing temperature are discussed on the basis of the present experimental results.