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JFET used as a diode

Started by Piotr Wyderski June 22, 2018
I need a low current, low voltage and extremely low leakage diode.
The gate junction of MMBFJ202 seems extremely well-suited for the job.
The datasheet says its absolute maximum gate current is 50mA.
Can I operate it reliably at 20mA for years? I mean, would there
be any relevant degradation mechanism or is this junction an ordinary 
diode and I shouldn't worry about it?

	Best regards, Piotr
On Friday, June 22, 2018 at 12:02:20 AM UTC-7, Piotr Wyderski wrote:
> I need a low current, low voltage and extremely low leakage diode. > The gate junction of MMBFJ202 seems extremely well-suited
Yep, should work fine. The JFETs are tested for leakage current, one would hope lots of other diodes are just as good in reverse blocking (but would require sorting). Tested transistors, with B-C shorted, might do well - and were discussed here not long ago. The old PN4117A reverse leakage spec was one picoamp (if they were available). That's a selected part, though. 20 milliamps forward current oughtn't cause any harm.
On Friday, June 22, 2018 at 3:02:20 AM UTC-4, Piotr Wyderski wrote:
> I need a low current, low voltage and extremely low leakage diode. > The gate junction of MMBFJ202 seems extremely well-suited for the job. > The datasheet says its absolute maximum gate current is 50mA. > Can I operate it reliably at 20mA for years? I mean, would there > be any relevant degradation mechanism or is this junction an ordinary > diode and I shouldn't worry about it? > > Best regards, Piotr
What sort of leakage current do you need? And how much V_rev. ? I've used BE and BC junctions in transistors. George H.
On 06/22/2018 04:58 AM, whit3rd wrote:
> On Friday, June 22, 2018 at 12:02:20 AM UTC-7, Piotr Wyderski wrote: >> I need a low current, low voltage and extremely low leakage diode. >> The gate junction of MMBFJ202 seems extremely well-suited > > Yep, should work fine. The JFETs are tested for leakage current, > one would hope lots of other diodes are just as good in > reverse blocking (but would require sorting). Tested > transistors, with B-C shorted, might do well - and were > discussed here not long ago.
Not B-C shorted--it's the base-collector junction that has the low leakage. You just leave the emitter open. BFT25As are great for that, but of course won't take 20 mA.
> > The old PN4117A reverse leakage spec was one picoamp > (if they were available). That's a selected part, though.
Those had a lot of series resistance though. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
George Herold wrote:

> What sort of leakage current do you need?
<=100nA for T<=80C
> And how much V_rev. ?
5V Best regards, Piotr
On Fri, 22 Jun 2018 09:02:14 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

>I need a low current, low voltage and extremely low leakage diode. >The gate junction of MMBFJ202 seems extremely well-suited for the job. >The datasheet says its absolute maximum gate current is 50mA. >Can I operate it reliably at 20mA for years? I mean, would there >be any relevant degradation mechanism or is this junction an ordinary >diode and I shouldn't worry about it? > > Best regards, Piotr
Jfets make terrible diodes. Series resistance and capacitance are both high. And jfets are expensive. The c-b junction of a small transistor is a much better diode. BFT25 c-b is around 0.35 pF and has leakage that's hard to measure, below 10 fA. https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1 Most gumdrop bipolars will leak well below 1 pA. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 06/22/2018 12:04 PM, John Larkin wrote:
> On Fri, 22 Jun 2018 09:02:14 +0200, Piotr Wyderski > <peter.pan@neverland.mil> wrote: > >> I need a low current, low voltage and extremely low leakage diode. >> The gate junction of MMBFJ202 seems extremely well-suited for the job. >> The datasheet says its absolute maximum gate current is 50mA. >> Can I operate it reliably at 20mA for years? I mean, would there >> be any relevant degradation mechanism or is this junction an ordinary >> diode and I shouldn't worry about it? >> >> Best regards, Piotr > > Jfets make terrible diodes. Series resistance and capacitance are both > high. And jfets are expensive. > > The c-b junction of a small transistor is a much better diode. BFT25 > c-b is around 0.35 pF and has leakage that's hard to measure, below 10 > fA. > > https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1 > > Most gumdrop bipolars will leak well below 1 pA. > >
If the "low voltage" requirement is low enough and OP doesn't need the full reverse voltage capability the c-b junction of a particular transistor provides the b-e junction should be even better. But whaddya do with the unused terminal, though? Just leave it hanging around? Does it act like an antenna?
On Friday, June 22, 2018 at 9:05:06 AM UTC-7, John Larkin wrote:
> On Fri, 22 Jun 2018 09:02:14 +0200, Piotr Wyderski > <peter.pan@neverland.mil> wrote: > > >I need a low current, low voltage and extremely low leakage diode.
> Jfets make terrible diodes. Series resistance and capacitance are both > high. And jfets are expensive. > > The c-b junction of a small transistor is a much better diode. BFT25 > c-b is around 0.35 pF and has leakage that's hard to measure, below 10 > fA.
It's not a good part, though, for 20 mA; if you want the c-b diode, with emitter open, you want something that allows 20 mA base current.
On Friday, June 22, 2018 at 11:37:48 AM UTC-4, Piotr Wyderski wrote:
> George Herold wrote: > > > What sort of leakage current do you need? > > <=100nA for T<=80C > > > And how much V_rev. ? > > 5V > > Best regards, Piotr
Speed? The bc or be junction of a 2n3904 would probably work. B. Pease "Bounding an clamping techniques" has a nice discussion of diodes... I can't find a good copy on the web. :^( George H.
On Fri, 22 Jun 2018 12:24:48 -0400, bitrex <user@example.net> wrote:

>On 06/22/2018 12:04 PM, John Larkin wrote: >> On Fri, 22 Jun 2018 09:02:14 +0200, Piotr Wyderski >> <peter.pan@neverland.mil> wrote: >> >>> I need a low current, low voltage and extremely low leakage diode. >>> The gate junction of MMBFJ202 seems extremely well-suited for the job. >>> The datasheet says its absolute maximum gate current is 50mA. >>> Can I operate it reliably at 20mA for years? I mean, would there >>> be any relevant degradation mechanism or is this junction an ordinary >>> diode and I shouldn't worry about it? >>> >>> Best regards, Piotr >> >> Jfets make terrible diodes. Series resistance and capacitance are both >> high. And jfets are expensive. >> >> The c-b junction of a small transistor is a much better diode. BFT25 >> c-b is around 0.35 pF and has leakage that's hard to measure, below 10 >> fA. >> >> https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1 >> >> Most gumdrop bipolars will leak well below 1 pA. >> >> > >If the "low voltage" requirement is low enough and OP doesn't need the >full reverse voltage capability the c-b junction of a particular >transistor provides the b-e junction should be even better. > >But whaddya do with the unused terminal, though? Just leave it hanging >around? Does it act like an antenna?
If all you need is a few volts reverse, connect the base to the collector. Then the forward drop is reduced by the transistor beta effect, 100 mV or so. For more reverse voltage, leave the emitter open, or connect it to the base; doesn't matter. https://www.dropbox.com/s/egpghnool9ql1h2/BCX70_as_Diode.JPG?raw=1 -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com