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Datasheets specify the Rds(on) of a MOSFET at some specific gate-source voltage, e.g. 100mΩ @ Vgs=4V. Up until now, I assumed that the test gate-source voltage was essentially arbitrary and chosen depending on the intended application.

However, after recently digging into more details about the distinction between saturation and triode regions in MOSFETs, it occurred to me that it would make sense for the Rds(on) test voltage to be equal to (or at least close to) the typical Vgs at the boundary between those two regions, where Vds ≈ Vgs - Vgs(th). This would make sense because the majority of the variation in Rds occurs in the saturation region, and once Vgs has risen (or fallen, in the case of a PFET) enough to put the MOSFET into the triode region Rds is close to approaching its lower limit.

Is this generally the case? Do manufacturers pick an Rds(on) test voltage fairly close to the saturation/triode region boundary, or is it essentially arbitrary?

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MOSFETs that are intended to be used as a switch are typically specified under conditions that are sensible and representative for use as a switch. So Rds(on) will be fairly constant with the specified current or less at the specified Vgs. That means it's in the triode region and well away from saturation where Vds > Vgs + Vth.

For example, the IRF530 is specified as max 0.16Ω at Vgs = 10V and Id = 8.4A. That means Vds < 1.3V. So if you are using it at 4A then Vds < 0.64V (ignoring temperature effects, which you cannot do in a real evaluation).

As another example, the AO3401A is specified under 4 separate operating conditions (left column is typical/right is maximum):

enter image description here

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No. It's usually at VDS = 0.1 V, and VGS at max. allowed continuous value (e.g. 5 V).

For very low RDSON devices it may be at even lower VDS because the current would be too high at 100 mV. Or it may be measured at lower VDS, but specified at 100 mV.

RDSON appears 'best' at low VDS.

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  • \$\begingroup\$ RF MOSFETs are often defined at fixed Vds; power MOSFETs, at given Id. \$\endgroup\$ Sep 20, 2022 at 16:07
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The conditions are always (usually??) given in the description/notes. Power MOSFETs typically at given Vgs and Id. RF MOSFETs often seem to give Vgs, Vds for whatever reason.

Vds = Vgs ("diode strapped" mode) will give a higher value, and dissipate quite a lot of power in the process -- perhaps destructively so for some power MOSFETs (this could be 100s of A!). Granted, not that pulsed tests are unusual, either.

Consider the plots usually provided: Rds(on) vs. Vgs with Id as parameter, for example. And consider typical applications. For power switching application, you simply need to know Rds(on) at whatever Id you intend to use it at, and provide enough Vgs to get there. For RF amplification, you want to avoid the saturation* (triode/transition) region anyway, and Rds(on) merely gives you a baseline for how much output voltage you can expect at clipping.

*That is, voltage saturation. Note: traditionally, the Vds ≥ Vgs range has been called "saturation" (i.e., current saturation), and below, "linear" (resistive / triode region). I would suggest people move away from this meaning as it's inconsistent with BJTs, and no one cares about the "linearity" of the triode region anymore (no one makes (or at least, should..) multipliers, variable resistors, etc. with discrete FETs anymore).

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