Understanding P-channel Mosfet Datasheet Plots

Thread Starter

topher217

Joined Oct 17, 2019
22
I'm working with this p-channel mosfet and got confused by the plots on page 3. In particular the top right plot showing the Drain Current over V_GS (voltage from gate to source). As a p-channel mosfet, I would expect the drain current to go towards infinity as V_GS approached the Gate-Source Threshold Voltage of -2.0V (see page 2). The plot on the other hand shows what I'd expect from an N-channel mosfet (i.e. the current going towards infinity at a positive V_GS = V_GS_ThresholdVoltage). It seems the plot should be backwards or state V_SG rather than V_GS. Is there a better explanation for this than a typo? Is this a standard plot that I am misinterpreting?

Furthermore, after looking at a TI datasheet for a different p-channel mosfet, I see a plot showing what I'd expect, i.e. the drain current going towards infinity as V_GS becomes more negative than the negative threshold voltage.
 

Thread Starter

topher217

Joined Oct 17, 2019
22
It is hard to tell if you are making a VERY funny sarcastic joke :D , or serious ... but just in case you are serious, I'd say that theory is invalidated by the temperature plot...unless this part was meant for deep space exploration XD.

1601458185926.png
 

ericgibbs

Joined Jan 29, 2010
10,850
As a p-channel mosfet, I would expect the drain current to go towards infinity as V_GS approached the Gate-Source Threshold Voltage of -2.0V
hi 217.
Would you agree that at -2V Gate to Source threshold voltage the P MOSFET is just coming into Source to Drain conduction.?

I agree the plot is ambiguous.
E
AAA 549 11.28.gif

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Thread Starter

topher217

Joined Oct 17, 2019
22
It's apparently a typo or ambiguous label. Other Vishay sheets are correct: https://www.vishay.com/docs/91078/91078.pdf Sad that it has not been corrected in 6 years.
Comparing the two, do you know why the I_D over V_GS looks logarithmic in that datasheet rather than exponential? I was under the impression they should always be exponential (or quadratic to fit most spice models as least).
 

ericgibbs

Joined Jan 29, 2010
10,850
hi 217,
It was not a joke.
The minus '-' sign is clearly shown on the plot, I agree it should be Vgs = -1 -2 etc... on the X axis.

What concerned me was your belief that
As a p-channel mosfet, I would expect the drain current to go towards infinity as V_GS approached the Gate-Source Threshold Voltage of -2.0V

Anyway hope we've sorted the problem. ;)
E
 

OBW0549

Joined Mar 2, 2015
3,551
Deficiencies in data sheets-- missing information, poorly drawn graphs, graphs with mislabeled or ambiguously labeled axes (as in this case), awkward or misleading descriptions, typos or just plain errors-- aren't frequent but do occur often enough that you do have to be prepared to occasionally step in and apply experience and common sense to compensate.

P-channel MOSFETs do indeed, as we all know, require their gates to be negative with respect to the source in order to make them conduct, and a graph which appears to suggest otherwise is, to a trained reader, obviously labeled incorrectly or at least awkwardly.

This does happen from time to time.
 
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