An accurate doping profile extraction in devices fabricated in semi-insulating GaAs substrates is difficult. The conventional Schottky barrier diode C-V technique is prone to large errors caused by bias dependent series resistance, gate current conduction, deep-level traps, and breakdown of the depletion approximation. It is shown that using both ac admittance (G-V and C-V) and dc (I-V) measurements of Schottky barrier diode the doping profile can be extracted more accurately and the range of accurate measurements can be clearly established. The characterization procedure has been verified experimentally using devices fabricated in a commercial 1 mum GaAs process.