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Missing VGS(th) / VT max. characterististic from a MOSFET's datasheet.


I've been looking at the datasheet for an n-channel enhancement MOSFET, p/n Diodes Inc ZXMN6A08G (https://www.diodes.com/assets/Datasheets/ZXMN6A08G.pdf) and only the minimum threshold voltage (VGS(th)) is specified, and not the maximum. I would think the maximum value is needed to design with to ensure that the part is turning on. Am I missing something about why the maximum has not been given?


Hah, that is weird.  Kind of a dick move, yeah.

On the upside, it's certainly no more than, somewhat less than 4.5V -- Rds(on) is given for this condition, and it's well saturated by then.

Offhand, I would guess typ +/- 1V, being a fairly common range.  Logic level parts like this tend to have even narrower ranges (say 0.8-2.0V).

You can see from Transfer Characteristics, it's less than 2.5V (plots are typical data, by the way), mind, at higher Vds (10V) and current (graph cuts off at 10mA).  Note the linear slope on the semilog plot -- this is in the exponential subthreshold* / cutoff region: we can extrapolate down to lower currents from here, with reasonable confidence.  With a slope of about 10x per 0.35V, 0.25mA should be 0.56V lower, or Vgs(th)(typ) ~ 1.89V.

*Different meaning of "threshold".  Vgs(th) is just measured at whatever low current.  The device characteristic transitions from Id ~ exp(Vgs) at low currents, to Id ~ Vgs^2 at high; the transition between these characteristics being this meaning of "threshold".  I suppose for small (RF?, IC) MOSFETs, such a current might be close to this characteristic, but they just keep using the same current for ever-larger power transistors, so, obviously they're going to fall lower on the curve, relatively speaking.

So, still no (max), other than what can be inferred from Rds(on), and similar shifting-about arguments using the other typical plots.


Seems like it's intended for use as a digital switch.  The threshold is telling you it's guaranteed-off below 1V, and the RDSon tells you it has a healthy channel by 4.5V.  If you want to guarantee it's on, you need to give it 4.5V.  A maximum threshold would only tell you where you've definitely entered the linear region by.  But that's probably not very useful for a switch.

Without the figure I feel it's not possible to judge if the device can be controlled (as a switch) with a 3.3V MCU GPIO. Diodes inc have responded that this device has VGS(th) max = 3V, and the reason a figure was not included in the datasheets originally could be because this limit is not tested during final production test.

I am searching for an alternative for an Onsemi NDT3055L.


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