Novel metal-oxide-semiconductor structures with very large areas have been used together with the vacuum emission and carrier separation techniques to study electron heating down to low fields (≊1 MV/cm) and out to large oxide thicknesses (5200 Å). At electric field magnitudes between 1.5 and 2.0 MV/cm, the threshold field for the onset of electron heating in silicon dioxide is observed. This onset is independent of oxide thickness and composition. Its value is consistent with all of the current theoretical calculations. At fields near threshold, a minimum average electronic energy of ≊1.0 eV is shown to be necessary to observe emission of the electrons into vacuum. Although the general trends in most of the data are approximately independent of oxide thickness out to 5200 Å, certain thick oxide samples with higher water content and lower physical density do show deviations from stabilization at higher fields, particularly in the vacuum emission experiments. Also, the data tend to appear]] noiser" as the oxides become thicker. These apparently hotter electronic distributions are discussed in relationship to the proposed formation of microscopic channels in the oxide bulk.