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A quantitative theory of 1/f type noise is derived from the distribution of trapping times for charges in interface states. The distribution of trapping times has been recently explained quantitatively by means of a random distribution of surface potential caused by a random distribution over the plane of the interface of fixed charges located in the oxide. This model, which agrees with the interface state time constant dispersion measured by the MIS conductance technique, leads to a noise spectrum which is independent of frequency at very low frequencies, tends towards a 1/f2dependence at high frequencies, and has an extended 1/f frequency dependence at intermediate frequencies. The mechanism for time constant dispersion is independent of temperature and silicon resistivity; it depends only on the majority carrier density at the silicon surface, the interface state density, and the density of fixed oxide charges. The dependence of open circuit mean square noise voltage on these parameters and frequency are illustrated for an MOS capacitor.