[en] Selenium implantation at room temperature in S-doped epitaxial GaAs layers as a means of the formation of n⁺ layers has been investigated. Doping profiles for Se-implanted layers have been examined by a C-V technique and/or a differential Hall effect method. It has been shown that n⁺ layers with a maximum carrier concentration of approx. equal to1.5 x 10¹⁸ cm^-3 can be formed by implantation followed by a 15 min annealing at 950⁰C. Contact resistance of ohmic electrodes is reduced by use of the Se-implanted n⁺ layers, resulting in the improvement on GaAs FET performance. Measured minimum noise figure of the Se-implanted GaAs FETs is 0.74 dB at 4 GHz. (orig.)