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The characteristics of ultrathin-body (UTB) SOI MOSFETs, whose SOI-channel thickness T/sub SOI/ is thinner than the inversion-layer thickness of bulk MOSFETs, are investigated. It is found for the first time that at low temperatures (<50 K) the mobility of the UTB MOSFETs coincides with that of thicker body SOI MOSFETs in spite of the fact that at room temperature the mobility of UTB MOSFETs decreases as T/sub SOI/ decreases. It is experimentally demonstrated for the first time that the gate-channel capacitance of the UTB MOSFETs increases as T/sub SOI/ decreases. In addition, it is demonstrated that the physical origins of the threshold voltage increase in UTB MOSFETs can be categorized as mobility degradation and a subband energy level increase. All these results are consistently explained in terms of quanturn-mechanical effects.