In this work, we experimentally investigate the effects of the extension of depletion regions in a GaAs MESFET on the frequency variation of the output conductance, gd, as well as the maximal relative variation, Δgdmax, at different polarisations of drain-source, Vds, and gate-source, Vgs. It is found that, for weak depleted regions, the values of gd(f) are very small with a Δgdmax which remains negligible. However, for large extensions, the values of gd(f) are very significant; they could reach 371.4 Ω-1 together with a Δgdmax high value of about 2.7 dB at Vds = 1.5V and │Vgs│ = 0.2V. Moreover, Δgdmax values undergo an increase when Vds increases. Hence, maximal variation of the dispersion gets higher when the regions become more depleted. Therefore, the widening of space charge region introduces frequency dispersion of gd that may limit potential GaAs MESFET applications in several fields in Micro- and nano-devices.