Measurements of transconductance, low-field Hall voltage, and Shubnikov de Haas oscillations (SdH) have been performed in n-type Si inversion layers at low temperatures over a wide range of interfacial oxide charge densities (6.0×1011Nox4.0×1012 cm-2). The threshold voltages found by these three techniques are in agreement only at the lowest measured values of Nox. As the amount of oxide charge at the interface is increased, the SdH thresholds become independent of Nox, while the Hall thresholds continue to shift in the expected direction. The evolution and behavior of the complicated structure in the transconductance with Nox are also analyzed and compared with the SdH and Hall data. The discrepancy between the thresholds at a given Nox appears to be due to the agglomeration of sodium ions into patches of high concentration. In addition, the Hall constant is observed to vary with the experimental parameters (e.g., magnetic field, temperature) for a fixed Nox. The study of this effect shows that it is caused by electron-electron interactions in the presence of disorder. © 1991 The American Physical Society.