Correlation of threshold voltage of implanted field‐effect transistors and carbon in GaAs substrates

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 of VT on the carbon concentration. For depletion‐mode FET’s, ‖VT‖ decreases linearly as the carbon concentration increases, with a variation of 185 mV in VT for a 1×1016 cm−3 change of carbon concentration, consistent with theory. Our results also show that the radial carbon concentration across crystals is uniform to within about ±6×1014 cm−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.