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    What is the substrate modulation effect?


    The substrate modulation effect is called the body effect or the back gate effect.In the analysis of the previous section, the source and substrate are shorted together so that VBS=0.When the potentials of the source and the substrate are not equal, the performance of the MOS transistor is affected.Therefore, for an NMOS transistor, VBS ≤ 0; for a PMOS transistor, VBS ≥ 0.In some cases, VBS=0 cannot be guaranteed.At a certain V ,, the amount of charge induced on the surface of the substrate is constant, a portion of the induced charge is used to form the depletion layer, and another portion is used to form the channel.That is to say, at the original threshold voltage, since the depletion layer consumes more induced charges, the remaining charge is insufficient to form a channel, and a higher gate-source voltage is required to form a channel, so the threshold voltage is increased;For a channel formed by a certain VGS, the channel is narrowed due to the thickening of the depletion layer.


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