Is the Rds(on) of a MOSFET typically specified at the boundary of the saturation and triode regions?

Question

Datasheets specify the R_ds(on) of a MOSFET at some specific gate-source voltage, e.g. 100mΩ @ V_gs=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 R_ds(on) test voltage to be equal to (or at least close to) the typical V_gs at the boundary between those two regions, where V_ds ≈ V_gs - V_gs(th). This would make sense because the majority of the variation in R_ds occurs in the saturation region, and once V_gs has risen (or fallen, in the case of a PFET) enough to put the MOSFET into the triode region R_ds is close to approaching its lower limit.

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

Answer 1

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):

Answer 2

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.

Answer 3

The conditions are always (usually??) given in the description/notes. Power MOSFETs typically at given V_gs and I_d. RF MOSFETs often seem to give V_gs, V_ds for whatever reason.

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

*That is, voltage saturation. Note: traditionally, the V_ds ≥ V_gs 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).