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.