The electronic properties of undoped semi-insulating GaAs were studied by measurements of current injection under a planar configuration which is sensitive to the near-surface region. The I-V characteristics were measured as a function of temperature in the range of 295-360 K for the as-received state and also following a heat treatment of 850 °C /20 min using a Si 3N4 cap. In order to understand the observed characteristics, a computer program was developed to simulate the current both in the ohmic and space-charge-limited regimes. The combination of the electrical measurements and simulation enabled the determination of the parameters controlling the resistivity of the semi-insulating samples, and shed light on the trap distributions and concentrations. It was found that the normally assumed three-level compensation model is inadequate to explain the observed behavior. Two other deep donors lying at 0.56 and 0.40 eV below the conduction band are suggested by the data (in addition to EL2); these two donor levels control the resistivity. In some instances, the influence of a deep acceptor is also observed.