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MOSFET gate_source leakage current at high temperature?

FvM

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100 nA maximum leakage is a standard spec of Vishay MOSFET, saying this particular transistor "has" 100 nA misses the point. Typical leakage is a only small fraction of the value. But if you want extrapolate the worst case spec, consider doubling every 10 K.
 
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danadakk

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You can contact Vishay FAE or their local rep or direct sales office and ask
for characterization data. Typically thats available on request.


Regards, Dana.
 
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treez

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Thanks,


I take it that the leakage current is due to the "intrinsic" gate_source zener rather than the gate_source junction itself?....

As such, It seems strange that the "intrinsic" gate_source zener (in a FET) should have potentially high leakage current. -Becuase discrete zeners dont have high leakage current...
For example, we have a UDZV8.2 (8.2V) zener on the fet gate, and its datasheet says it only has a maximum of 1nA of reverse current...even at 125degc.

UDZV8.2 Zener diode datasheet
https://fscdn.rohm.com/en/products/databook/datasheet/discrete/diode/zener/udzvte-178.2b-e.pdf

I wonder why the "Intrinsic" gate_source zener found in mosfets is said to be so leaky of current?
 

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Hi,
Becuase discrete zeners dont have high leakage current...
How do you come to this conclusion?

MOSFET leakage current is given at -8V .... +12V

Compare this range with the zener. --> it will burn because of high current!

Don't compare apples and pears.

If you don't want the whole range at the zener you should also limit the voltage range at the Mosfet.

As far as I can see the leakage current of the zener will be much higher than the Mosfet.

Klaus
 
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danadakk

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treez

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Thanks,
Actually i notice that the UDZV8.2B (8V2 Zener) is said in its datasheet page 2 to have a Reverse Leakage current of 500nA at VR=5V and 25degC. However, its datasheet on page 4 states that it has a Reverse Leakage current of <10 picoAmps at VR=5V and temp = 25degC.

Do you know which one is correct?

UDZV8.2 zener


--__--__--__--__---
Also,
Please can you state what is the overall leakage current through these attached back to back zeners?
One is a 5V1 zener, and the other an 8V2 zener.

UDZV8.2 (8.2V Zener)

UDZV5.1 (5.1V Zener)

(BTW Page 4 and page 2 of their datasheets contradict each other over leakage current)
--- Updated ---

Also, do you think the “modified back to back zeners” circuit, (also attached) with a MMBD4148 diode in series with each Zener, will reduce the amount of leakage current whilst having the same “back to back” Zener effect?

MMBD4148 diode datasheet
--- Updated ---

Also, does the attached BJT_zener circuit give the lowest overall leakage current?
 

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KlausST

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Hi,
Actually i notice that the UDZV8.2B (8V2 Zener) is said in its datasheet page 2 to have a Reverse Leakage current of 500nA at VR=5V and 25degC. However, its datasheet on page 4 states that it has a Reverse Leakage current of <10 picoAmps at VR=5V and temp = 25degC.

Do you know which one is correct?
I have to admit, that (@post#5) I wrongly read "uA" instead of "nA". So they are much better than I expected.

Page2 value is marked with "max"
Page4 value is not marked "max", thus one may assume it is the typical current.


(BTW Page 4 and page 2 of their datasheets contradict each other over leakage current)
In which case?
Page2: max 2uA @ 1.5V
page4: below 1nA typ at 1.5V

Also, do you think the “modified back to back zeners” circuit, (also attached) with a MMBD4148 diode in series with each Zener, will reduce the amount of leakage current whilst having the same “back to back” Zener effect?
The leakage current will be smaller, but then they don't protect against negative voltage anymore.

*****
Still unclear: your expected voltage range.
You can´t compare the leakage current @5V of the Zener with the MOSFET leakage current at 12V.

Klaus
 

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Thanks the fet gates (and zeners) will have 7V on them.

On page 4 of the UDZVTE zener datasheet, it says that leakage current at 125degc is sub 1 nA.

...Surely you agree, this cannot be possible.?....no semiconductor could have leakage current that low at 125degc?
 

dick_freebird

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It's all about doping and junction area / depletion
volume. Small protection devices can stay sub-nA
if their doping is high enough to keep depletion
region volume low, so the generation current at
high temp is minimized. HV junctions encompass
more silicon volume within which e-h pairs that
are thermally generated, get swept to create
leakage current. A low voltage zener will be fairly
highly doped. It's likely to be much, much smaller
than the drain-body junction area as an ohm or
ten of clamp series resistance will suffice for HBM
ESD protection of a 20V gate oxide, while Rds(on)
is desired to be milliohms.

I've tested JFET-input op amps at 300C and seen
input leakage below 1uA, there. 1nA at 125C is not
out of the question for small signal devices. What
the clamp construction is, who knows?
 
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KlausST

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Hi,
Thanks the fet gates (and zeners) will have 7V on them.

On page 4 of the UDZVTE zener datasheet, it says that leakage current at 125degc is sub 1 nA.

...Surely you agree, this cannot be possible.?....no semiconductor could have leakage current that low at 125degc?
It does not state "sub 1nA" @ 7V. With 7V I'd say you are too close to the be abke to predict the current.

Please tell us about your application. Why is the leakage current that important?
What is the signal frequency at the gate? And is an "active circuit" a posible solution?
Why an 8V zener when the Mosfet can stand 12V?

Klaus
 
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treez

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Why an 8V zener when the Mosfet can stand 12V?
The drive signal is 7V, and we want to be just above that........the Gate is very sensitive and if we choose 11v, then a little overvoltage spike may still kill the gate.

I've tested JFET-input op amps at 300C and seen
input leakage below 1uA, there. 1nA at 125C is not
out of the question for small signal devices.
...thanks, though an input of an opamp would, as you know, obviously be designed and doped to be low in leakage current...a zener is not likely to be designed for low leakage current.

It does not state "sub 1nA" @ 7V. With 7V I'd say you are too close to the be abke to predict the current.
Thanks, and we would be interested in what the leakage really is at 7V?
 

KlausST

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Hi,

The drive signal is 7V
I guess the drive signal is not always 7V. Instead it has a range from xxx Volts to xx Volts.

the Gate is very sensitive and if we choose 11v, then a little overvoltage spike may still kill the gate.
Yes. But why don´t you use a 9.1V or 10V zener. Then the knee voltage is far away from the 7V and the expected current is more low and can be better determined.

Where do you expect the overvoltage to come from? Is it an open circuit that can be touched and cause an ESD pulse?

What is the problem with the leakage current? Most power MOSFET circuits work happily even with a leakage current of 1mA. Why is your requirement that critical?

Klaus
 
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