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The effect of high gate-leakage current on the accuracy of mobility evaluation was investigated. This investigation showed that a high gate leakage current makes it difficult to measure the mobility accurately in the case of using a conventional equivalent circuit with lumped circuit elements. To measure the mobility accurately, the authors therefore used a transmission-line model. Its validity was experimentally confirmed by using the capacitance-frequency characteristic of the gate of MOSFETs. The transmission-line model shows that a high gate-leakage current induces a voltage distribution in the channel, which causes a serious error in the mobility evaluation. Accordingly, a precision parameter, which clarifies the relation between channel length and measurement error, was defined. This parameter was then used to define a criterion for channel length for accurately measuring mobility. The channel-length criterion was used to successfully evaluate the mobility of n-MOSFETs with gate dielectrics of 1.4-nm-thick oxynitride (SiON).