We have investigated the effect of residual carbon concentration on the threshold voltage (VT) of field‐effect transistors (FET’s) fabricated by direct ion implantation with undoped semi‐insulating liquid encapsulated Czochralski GaAs. The results show the direct dependence ofVTon the carbon concentration. For depletion‐mode FET’s, ‖VT‖ decreases linearly as the carbon concentration increases, with a variation of 185 mV inVTfor a 1×1016cm−3change of carbon concentration, consistent with theory. Our results also show that the radial carbon concentration across crystals is uniform to within about ±6×1014cm−3; however, the carbon concentration invariably decreases toward the tail of the crystals by 40% or more. As a result, ‖VT‖ tends to increase toward the tail of the crystals.