Searching for a P mosfet with low subthreshold current.  - Page 1
Forum section: electronics / projects

author: Geoff
author_rank: Contributor
timestamp: October 09, 2020, 03:46:06 pm
content: How do you searching for a P mosfet with low subthreshold current when both Digikey and RS do not have this as a filter?

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2020, 04:20:23 pm
content: That's because it doesn't exist. Try searching for threshold voltage.

author: Geoff
author_rank: Contributor
timestamp: October 09, 2020, 08:59:21 pm
content: Quote from: Zero999 on October 09, 2020, 04:20:23 pmI 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2020, 09:33:04 pm
content: 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

author: KT88
author_rank: Frequent Contributor
timestamp: October 09, 2020, 10:05:39 pm
content: Here is an interesting paper about super cut-off and stacking approaches.https://www.ijareeie.com/upload/2015/september/19_Leakage.pdf] https://www.ijareeie.com/upload/2015/september/19_Leakage.pdfThat might help to tackle the challenge...Ideally you use a FET with minimal Ids and higher Vds.CheersAndreas

author: Geoff
author_rank: Contributor
timestamp: October 09, 2020, 10:17:22 pm
content: Quote from: Geoff on October 09, 2020, 08:59:21 pm1. 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.

author: MasterT
author_rank: Frequent Contributor
timestamp: October 09, 2020, 10:41:36 pm
content: Higher voltage, lower current & power mosfet likely would have lower leakage

author: Geoff
author_rank: Contributor
timestamp: October 09, 2020, 11:50:43 pm
content: Quote from: KT88 on October 09, 2020, 10:05:39 pmThank 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 10, 2020, 01:23:00 pm
content: 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 therearesome 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.

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2020, 09:59:13 pm
content: Quote from: Geoff on October 09, 2020, 08:59:21 pmI'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

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2020, 10:43:55 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2020, 01:32:35 pm
content: Quote from: David Hess on October 10, 2020, 10:43:55 pmIs it obsolete? I couldn't find it for sale anywhere. How about a J-FET+MOSFET cascode?

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2020, 01:58:27 am
content: Quote from: Zero999 on October 11, 2020, 01:32:35 pmIt 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.

author: cj
author_rank: Contributor
timestamp: October 12, 2020, 06:03:52 am
content: 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

author: tszaboo
author_rank: Super Contributor
timestamp: October 12, 2020, 07:03:59 am
content: 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.

author: mzzj
author_rank: Super Contributor
timestamp: October 12, 2020, 07:24:00 am
content: 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)

author: tszaboo
author_rank: Super Contributor
timestamp: October 12, 2020, 03:58:10 pm
content: Quote from: mzzj on October 12, 2020, 07:24:00 amI assume OP talks about subthreshold current + leakage current together.

author: voltsandjolts
author_rank: Unknown
timestamp: October 12, 2020, 04:35:54 pm
content: Quote from: MasterTech on October 12, 2020, 07:46:41 amNice but unfortunately the max ratings are stubbornly high in the uA level, so you can't depend on it in production without testing.

author: voltsandjolts
author_rank: Unknown
timestamp: October 12, 2020, 04:43:37 pm
content: PMZ950UPEL is rated 25nA max at 5V Vds

author: mzzj
author_rank: Super Contributor
timestamp: October 16, 2020, 09:22:52 am
content: 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 VdsHigh 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.

author: Geoff
author_rank: Contributor
timestamp: October 28, 2020, 12:17:37 am
content: Quote from: NANDBlog on October 12, 2020, 07:03:59 amThank 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.It also needs to work outside for two yearsand be robustreliablebut most of all it needs to be cheap.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.QuoteTemperature at night is not going to be too hot, around 25C max.QuoteGiven the time it spends off, it is worth putting time in to the switch.