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A comparative analysis of metal-oxide-semiconductor ͑MOS͒ capacitors by capacitance-voltage ͑C-V͒ and current-voltage ͑I-V͒ characteristics has been employed to characterize the thickness variations of the oxide on different length scales. Ultralarge area ͑1 cm 2 ͒ ultrathin ͑ϳ5 nm oxide͒ MOS capacitors have been fabricated to investigate their functionality and the variations in oxide thickness, with the use as future electron emission devices as the goal. I-V characteristics show very low leakage current and excellent agreement to the FowlerNordheim expression for the current density. Oxide thicknesses have been extracted by fitting a model based on Fermi-Dirac statistics to the C-V characteristics. By plotting I-V characteristics in a Fowler plot, a measure of the thickness of the oxide can be extracted from the tunnel current. These apparent thicknesses show a high degree of correlation to thicknesses extracted from C-V characteristics on the same MOS capacitors, but are systematically lower in value. This offset between the thicknesses obtained by C-V characteristics and I-V characteristics is explained by an inherent variation of the oxide thickness. Comparison of MOS capacitors with different oxide areas ranging from 1 cm 2 to 10 m 2 , using the slope from Fowler-Nordheim plots of the I-V characteristics as a measure of the oxide thickness, points toward two length scales of oxide thickness variations being ϳ1 cm and ϳ10 m, respectively.