Author Topic: Searching for a P mosfet with low subthreshold current.  (Read 978 times)

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Offline Geoff

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Searching for a P mosfet with low subthreshold current.
« on: October 09, 2020, 03:46:06 pm »
How do you searching for a P mosfet with low subthreshold current when both Digikey and RS do not have this as a filter?
 

Offline Zero999

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #1 on: October 09, 2020, 04:20:23 pm »
That's because it doesn't exist. Try searching for threshold voltage.
 

Offline Geoff

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #2 on: October 09, 2020, 08:59:21 pm »
That's because it doesn't exist. Try searching for threshold voltage.

I wish it did not exist. :)

Subthreshold current (or subthreshold conductance, leakage current, zero gate voltage drain current) is listed in datasheets, Idss, but it is not an option on Digikey or RS as a filter.

Do any other electrical characteristics tend to indicate low (or high) subthreshold current?

Or is there a more complete searchable database of contemporary mosfets?   

« Last Edit: October 09, 2020, 09:06:35 pm by Geoff »
 

Online T3sl4co1l

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #3 on: October 09, 2020, 09:33:04 pm »
Two problems:
1. Obviously, less current flows, the further below Vgs(th) you are.  Presumably you want Vgs(off) = 0 so that reverse bias isn't required?  So, higher Vgs(th) is advantageous to some extent.
2. Idss is rarely measured accurately.  For a small device, a measurement of some uA is probably a fast check.  Typical devices might do nA, but that takes more time to measure.  So you'll have to test parts yourself (or find the few specialty types that are rated for low leakage).

So, even if suppliers sorted by Idss, it might not be all that useful.

Tim
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Online KT88

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #4 on: October 09, 2020, 10:05:39 pm »
Here is an interesting paper about super cut-off and stacking approaches.
https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf] [url]https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf[/url]
That might help to tackle the challenge...
Ideally you use a FET with minimal Ids and higher Vds.

Cheers

Andreas
 
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Offline Geoff

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #5 on: October 09, 2020, 10:17:22 pm »
That's because it doesn't exist. Try searching for threshold voltage.

I wish it did not exist. :)

Subthreshold current (or subthreshold conductance, leakage current, zero gate voltage drain current) is listed in datasheets, Idss, but it is not an option on Digikey or RS as a filter.

Do any other electrical characteristics tend to indicate low (or high) subthreshold current?

Or is there a more complete searchable database of contemporary mosfets?   



1. Obviously, less current flows, the further below Vgs(th) you are.  Presumably you want Vgs(off) = 0 so that reverse bias isn't required?  So, higher Vgs(th) is advantageous to some extent.

Thank you, that is a very good point, :)

2. Idss is rarely measured accurately.  For a small device, a measurement of some uA is probably a fast check.  Typical devices might do nA, but that takes more time to measure.  So you'll have to test parts yourself (or find the few specialty types that are rated for low leakage).

Yes, sadly, all the datasheets I've looked at have a note to that effect. :( 

So, even if suppliers sorted by Idss, it might not be all that useful.

I was looking for a diligent (or an OCD) supplier. :)
 

Offline MasterT

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #6 on: October 09, 2020, 10:41:36 pm »
Higher voltage, lower current & power mosfet likely would have lower leakage
 
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Offline Geoff

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #7 on: October 09, 2020, 11:50:43 pm »
Here is an interesting paper about super cut-off and stacking approaches.
[url=https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf]https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf] [url]https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf]https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf]https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf] [url]https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf[/url]
That might help to tackle the challenge...
Ideally you use a FET with minimal Ids and higher Vds.

Cheers

Andreas

Thank you Andreas. I skimmed over the paper, that is an interest paper, and I will take a good look at it. :)

"Ideally you use a FET with minimal Ids and higher Vds."

Will do. :)
« Last Edit: October 09, 2020, 11:54:32 pm by Geoff »
 

Offline Siwastaja

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #8 on: October 10, 2020, 01:23:00 pm »
Most FET manufacturers just copy-paste the standard leakage current value of 1µA in the datasheet; this holds over the whole operating temperature range. In reality it could be two-three orders of magnitude better, but applications that are interested about this value seem to be too rare to allow the extra cost of this parameter being guaranteed and controlled.

And because it's always specified at 1µA, the distributors do not list it. Which makes me think, maybe there are some parts with lower leakage specs but I have just not seen them for the same reason; it's not available in parametric search.

What is the maximum leakage you are looking at? Maybe you could test a batch of parts for yourself, then hope the next batch is in the same order of magnitude? Or, if you can guarantee a limited junction temperature range during operation, then it's highly unlikely you'll never get close to the datasheet value.
« Last Edit: October 10, 2020, 01:24:42 pm by Siwastaja »
 
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Offline Zero999

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #9 on: October 10, 2020, 09:59:13 pm »
That's because it doesn't exist. Try searching for threshold voltage.

I wish it did not exist. :)

Subthreshold current (or subthreshold conductance, leakage current, zero gate voltage drain current) is listed in datasheets, Idss, but it is not an option on Digikey or RS as a filter.

Do any other electrical characteristics tend to indicate low (or high) subthreshold current?

Or is there a more complete searchable database of contemporary mosfets?   
I've heard of leakage current, cut-off and zero gate voltage drain current, but not the term subthreshold current. It's not listed for MOSFETs because they're designed to switch high currents, so the tiny leakage current is tiny. Choose high voltage, low current and high threshold devices and operate at lower temperature, as possible.

EDIT:
Is this any good?
https://assets.nexperia.cn/documents/application-note/AN90009.pdf
 
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Offline David Hess

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #10 on: October 10, 2020, 10:43:55 pm »
Except for small signal MOSFETs intended for low leakage applications, the manufacturer's test and specification will be high so you have to either look for low leakage parts, or qualify them yourself.  A 3N190 for instance is less than 200 picoamps.
 
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Offline Zero999

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #11 on: October 11, 2020, 01:32:35 pm »
Except for small signal MOSFETs intended for low leakage applications, the manufacturer's test and specification will be high so you have to either look for low leakage parts, or qualify them yourself.  A 3N190 for instance is less than 200 picoamps.
Is it obsolete? I couldn't find it for sale anywhere. How about a J-FET+MOSFET cascode?
 
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Offline David Hess

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #12 on: October 12, 2020, 01:58:27 am »
Except for small signal MOSFETs intended for low leakage applications, the manufacturer's test and specification will be high so you have to either look for low leakage parts, or qualify them yourself.  A 3N190 for instance is less than 200 picoamps.

Is it obsolete? I couldn't find it for sale anywhere. How about a J-FET+MOSFET cascode?

It is not obsolete that I know of, but availability was never good.  Linear Systems lists it, and Calogic probably has it or something similar.

There are circuit arrangements where the transistor inputs are bootstrapped to minimize leakage but I do not think they are really a substitute for a low leakage part.
 
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Offline cj

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #13 on: October 12, 2020, 06:03:52 am »

For low voltage and low current applications analog switches might be a solution, it solved my low leakage requirement problem.
On Digikey you can filter on 'Current - Leakage (IS(off)) (Max)' , didn't check other suppliers.

CJ
 
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Offline NANDBlog

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #14 on: October 12, 2020, 07:03:59 am »
It is very very temperature dependent, so giving a typical value for room temperature might be completely useless, as it could go 1000 times worse over the entire temperature range. You should be focusing on the rest of the circuits to make it less leaky.
Driving the gate below 0 could help. But there are other methods, just tell us what you want to achieve, so we can avoid the X-Y problem.
 
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Offline mzzj

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #15 on: October 12, 2020, 07:24:00 am »
IIRC subtreshold current falls a decade by every 100mV below treshold voltage.

IRF9540 is specified for  250ua. With 2 volts at gate. At 1 volt it should fall to approx 250 pA. At zero gate voltage real leakage currents dominate(subtreshold current would be in atto-amp range)
 
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Offline NANDBlog

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #16 on: October 12, 2020, 03:58:10 pm »
IIRC subtreshold current falls a decade by every 100mV below treshold voltage.

IRF9540 is specified for  250ua. With 2 volts at gate. At 1 volt it should fall to approx 250 pA. At zero gate voltage real leakage currents dominate(subtreshold current would be in atto-amp range)
I assume OP talks about subthreshold current + leakage current together.
 

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #17 on: October 12, 2020, 04:35:54 pm »
Once I had to search for a low Idss N-mosfet, and found the NTR4003N, that plots Idss vs Vgs, measured at a whopping 125º to be around 10nA. At 25º its probably in the low pA area.
The Vgsth is very low 0.8-1.4V but the Rdson is high (1-2Ohm).

It is also given for p parts like the NTR5105P

Nice but unfortunately the max ratings are stubbornly high in the uA level, so you can't depend on it in production without testing.
 

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #18 on: October 12, 2020, 04:43:37 pm »
PMZ950UPEL is rated 25nA max at 5V Vds
 
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Offline mzzj

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #19 on: October 16, 2020, 09:22:52 am »
Depending on exactly how low leakage you need you might be ok with measuring some % per batch and assume rest of them good.
I tested 3 pcs of each:

Onsemi MTB50P03 = 300..500pA @ 20 Vds (logic level =low threshold mosfet)
MTP23P06V = less than 30pA @ 20 Vds (below measurement limit on my setup)
30 years old IR IRF9620 = 5...6nA @20 Vds

High leakage on IRF9620 is curious but these are OLD stock from Nokia television or CRT display business with 10 years of storage in cold and damp and another 20 years of storage god knows where.
 
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Offline Geoff

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Re: Searching for a P mosfet with low subthreshold current.
« Reply #20 on: October 28, 2020, 12:17:37 am »
It is very very temperature dependent, so giving a typical value for room temperature might be completely useless, as it could go 1000 times worse over the entire temperature range. You should be focusing on the rest of the circuits to make it less leaky.
Driving the gate below 0 could help. But there are other methods, just tell us what you want to achieve, so we can avoid the X-Y problem.

Thank to all who have posted. :)

I'm really at the early stage of design and it is also a matter of curiosity re the sub-threshold current of MOSFETs.
 

  • The product only needs to be on in daylight hours.
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My best guess is to use 4 D cells, simple and more robust than a solar cell arrangement, to power an ESP8266 (as it is much cheaper than an ESP32) based device. And I only need to make 4 or 5 sensor read then transmissions a day. The rest of the time the ESP will be in deep sleep during the day.

Yes, I do know a solar cell with a supper cap is a very sexy solution but I only need 2 years. I know the senor will deteriorate but I also know it will last 2 years if it does not get physically damaged. 

And yes, the ESP32 has some nice low power in sleep mode specifications and it's brownout protection circuit will save me from the cost of rolling my own. It is a close call, but I'm still thinking the ESP8266 is a cost winner.

The ESP32 is very nice but I am not needing anything other than the low power and brownout, there is no need of any DSP'ing of the data.     

Quote
It is very very temperature dependent, so giving a typical value for room temperature might be completely useless, ...
Temperature at night is not going to be too hot, around 25C max.
Quote
You should be focusing on the rest of the circuits to make it less leaky.
Given the time it spends off, it is worth putting time in to the switch.
   
« Last Edit: October 28, 2020, 12:31:32 am by Geoff »
 


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