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This paper investigates the influence of the silicon substrate on the ac characteristics of silicon-on-insulator (SOI) MOSFETs. It is shown for the first time that the presence of the substrate underneath the buried oxide results in two transitions (i.e., zero-pole doublets) in the frequency response of the output conductance. It is demonstrated that the appearance of these transitions, the position and amplitude of which strongly depend on the substrate doping, is caused by the variation of the potential at substrate-buried oxide interface, which we call the Floating Effective Back-Gate (FEBG) effect. A first-order small-signal equivalent circuit is proposed to support our observations.