Electrical – MOSFET: What is considered “large voltage” when finding early voltage via experimental method


I have found that to get early voltage you can take the tangent a I vs V MOSFET characteristic graph at "large voltages" and wherever the lines intercept the negative x-axis is the early voltage (see wikipedia graph below)
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I am trying to do this with my graph, but am having trouble determining what constitutes "large voltages". At first I just added a trendline for the entire curve and realized that did not work.
At what voltage do I need to start extrapolating my curve data to get early voltage?

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Best Answer

I am not a specialist in Solid-State physics, but recall that the "Early Effect" was first identified for BJT, as a good approximation of I-V characteristics for different base currents. In MOSFET there is some resemblance (but versus Vgs), so the shapes of MOSFET I-V curves are also sometimes characterized by "Early Voltage". However, this is still an approximation, and it doesn't work well for MOSFETS.

Identification of "linear I-V" section is somewhat subjective, and the negative ("Early") voltages derived by extrapolation of I-V curves may not be identical for different Vgs curves, that's why you are having difficulties.

In other words, the dynamic impedance (RdsON) for MOSFETs is not only dependent on Vgs, but also on Vds, that's why the RdsON is so fuzzy-specified in MOSFET datasheets.