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In this paper, we study the effect of the drain voltage on the threshold voltage extraction in long-channel MOSFETs by the transconductance change and transconductance-to-current ratio change methods, using analytical modeling and experimental data obtained on advanced UTB SOI MOSFETs. It is shown that, although these two methods have the same physical background, they feature radically different behaviors with respect to the drain voltage effect. In particular, the transconductance change method yields a threshold voltage value, which regularly increases with drain voltage, and interpretation, as well as analytical expression for this dependence, is provided. In contrast, for the transconductance-to-current ratio change method, the increase of the extracted threshold voltage value with drain voltage is smaller and rapidly saturates; hence, the threshold voltage extraction is more stable and reliable. Modeling derivations are found to be in excellent agreement with measurements on long-channel UTB SOI MOSFETs as well as 2-D simulations.