On Fri, 22 Jun 2018 15:58:32 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 06/22/2018 03:33 PM, Tauno Voipio wrote: >> On 22.6.18 17:14, Phil Hobbs wrote: >>> 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. >>> >> >> Since when there has been base and collector in an JFET? >> > >The referent was "tested transistors", i.e. BJTs, not JFETs. If you >short the gate of a JFET to drain or source, it stops acting like a diode. > >Cheers > >Phil HobbsPHEMTs make dynamite diodes, with the gate shorted to either source or drain. If you can still buy PHEMTS. GanFets can be used as diodes, but Vf is high. There is no actual substrate diode, just sort of. I've heard that LEDs make low leakage diodes, at least in the dark. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
JFET used as a diode
Started by ●June 22, 2018
Reply by ●June 22, 20182018-06-22
Reply by ●June 22, 20182018-06-22
On 06/22/18 16:30, John Larkin wrote:> On Fri, 22 Jun 2018 15:58:32 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 06/22/2018 03:33 PM, Tauno Voipio wrote: >>> On 22.6.18 17:14, Phil Hobbs wrote: >>>> 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. >>>> >>> >>> Since when there has been base and collector in an JFET? >>> >> >> The referent was "tested transistors", i.e. BJTs, not JFETs. If you >> short the gate of a JFET to drain or source, it stops acting like a diode. >> >> Cheers >> >> Phil Hobbs > > PHEMTs make dynamite diodes, with the gate shorted to either source or > drain. If you can still buy PHEMTS.You can still get the Skyworks ones. Broadcom did an NXP and killed off all the old Avago ones, as you know. But on the plus side, CEL has a whole bunch of new ones!> > GanFets can be used as diodes, but Vf is high. There is no actual > substrate diode, just sort of. > > I've heard that LEDs make low leakage diodes, at least in the dark.Yup. The Footprints multiplexer was built of 96 ordinary display LEDs from Chicago Miniature Lamp. Low femtoamp leakage near zero, and less than 50 fA from -5V to + 0.5V bias. The cute thing was putting them under a plastic cover (painted black on the outside and white on the inside, then illuminating them with four other LEDs with PWM drive. That gave me an adjustable 0-5 pA bias current with reasonable uniformity, so that I could use a diode MUX with an inherently AC-coupled transducer, namely PVDF pyroelectric film. Capacitance was much higher than a BFT25A's, of course. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●June 22, 20182018-06-22
On 2018-06-22, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> 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.Why not short base-emitter and expliot the reverse beta? I would expect that should reduce the base current by 5 so -- ت
Reply by ●June 22, 20182018-06-22
On 06/22/18 19:50, Jasen Betts wrote:> On 2018-06-22, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >> 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. > > Why not short base-emitter and expliot the reverse beta? > I would expect that should reduce the base current by 5 soIt depends if you care more about a 40-mV change in V_BC or about super low leakage. The CB junction is the low-leakage one. I have a production design (for a safety system) that has a TIA with a 200 Mohm feedback resistor shunted by two BFT25A CB junctions in series. That gives it excellent linearity down near the detection limit, plus the ability to sense changes in much larger signals that would otherwise just rail the amplifier. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●June 22, 20182018-06-22
On Friday, June 22, 2018 at 8:49:25 PM UTC-4, Phil Hobbs wrote:> On 06/22/18 19:50, Jasen Betts wrote: > > On 2018-06-22, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> 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. > > > > Why not short base-emitter and expliot the reverse beta? > > I would expect that should reduce the base current by 5 so > > It depends if you care more about a 40-mV change in V_BC or about super > low leakage. The CB junction is the low-leakage one. > > I have a production design (for a safety system) that has a TIA with a > 200 Mohm feedback resistor shunted by two BFT25A CB junctions in series. > That gives it excellent linearity down near the detection limit, plus > the ability to sense changes in much larger signals that would otherwise > just rail the amplifier.Huh, linear down low but then changing to logarithmic. That's a cute trick. George H.> > Cheers > > Phil Hobbs > > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC / Hobbs ElectroOptics > Optics, Electro-optics, Photonics, Analog Electronics > Briarcliff Manor NY 10510 > > http://electrooptical.net > http://hobbs-eo.com
Reply by ●June 23, 20182018-06-23
On 2018-06-23, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> On 06/22/18 19:50, Jasen Betts wrote: >> On 2018-06-22, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> 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. >> >> Why not short base-emitter and expliot the reverse beta? >> I would expect that should reduce the base current by 5 so > > It depends if you care more about a 40-mV change in V_BC or about super > low leakage. The CB junction is the low-leakage one.There's leakage from collector to emitter? I was thinking it would allow a part with 5mA I_B to pass 20mA safely. -- ت
Reply by ●June 23, 20182018-06-23
On Friday, June 22, 2018 at 5:01:18 PM UTC-7, Jasen Betts wrote:> On 2018-06-22, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > > 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... 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. > > Why not short base-emitter and exploit the reverse beta? > I would expect that should reduce the base current by 5 soThat's probably correct; the Rbb would also be slightly less troublesome. Since emitter current is zero when it's open, there's not a lot of reason not to connect it to the base (it'd float near that potential anyhow, if one were inclined to let a terminal float). Because the emitter is highly doped, and the B and E share a surface of the silicon, B-E junction leakage is higher than B-C. The prescription for C-B shorting is best at lowering the parasitic forward resistance, not the reverse current. They both give near-ideal diode behavior, but in different ways (different I-V quadrants).
Reply by ●June 23, 20182018-06-23
John Larkin wrote:> 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=1There was a book called, "What They Don't Teach You in Harvard Business School." This is like, "What They Don't Teach You in AoE."
Reply by ●June 24, 20182018-06-24
"whit3rd" <whit3rd@gmail.com> wrote in message news:5fe41e5b-3669-4aea-9265-65b1f5af4f37@googlegroups.com...> Because the emitter is highly doped, and the B and E > share a surface of the silicon, B-E junction leakage is higher than B-C. > The prescription for C-B shorting is best at lowering the parasitic > forward > resistance, not the reverse current. They both give near-ideal diode > behavior, but in different ways (different I-V quadrants).It's also faster (near zero t_rr), because it's never saturated. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: https://www.seventransistorlabs.com/
Reply by ●June 24, 20182018-06-24
