# Understanding P-channel Mosfet Datasheet Plots

#### 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.

#### ericgibbs

Joined Jan 29, 2010
16,747
hi 217,
It does say on the plot '-' .
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#### topher217

Joined Oct 17, 2019
22
It is hard to tell if you are making a VERY funny sarcastic joke , 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.

#### Alec_t

Joined Sep 17, 2013
13,223
Looks like typos on the Vishay datasheet. The capacitance v Vds graph is in error too.

#### topher217

Joined Oct 17, 2019
22
Looks like typos on the Vishay datasheet. The capacitance v Vds graph is in error too.
Alright. Thanks for making me feel sane again

#### ericgibbs

Joined Jan 29, 2010
16,747
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.
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#### 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).

#### topher217

Joined Oct 17, 2019
22
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.
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View attachment 218389

View attachment 218388
Yep, I agree that if the horizontal axis were indeed negative, this would look normal to me.

#### ericgibbs

Joined Jan 29, 2010
16,747
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.
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#### OBW0549

Joined Mar 2, 2015
3,566
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.