Max current for a JFET connected as diode

Hi;
I've been using N channel Jfets connected as diodes for input protection for a long time. Everything seems to just work. Recently an application came up where the low leakage of a diode connected FET would be useful but when forward biased it could pass tens to a few hundred mA. Does anyone know what characteristic of a JFET would define the max current through the Diode connected device where the Anode is the shorted Drain-Source and the Cathode is the Gate? Would it be the Jfets  Id Max?

On Wed, 13 Apr 2022 13:15:16 -0700 (PDT), "neo5...@gmail.com"
<neo5bass@gmail.com> wrote:

>Hi;
>I've been using N channel Jfets connected as diodes for input protection for a long time. Everything seems to just work. Recently an application came up where the low leakage of a diode connected FET would be useful but when forward biased it could pass tens to a few hundred mA. Does anyone know what characteristic of a JFET would define the max current through the Diode connected device where the Anode is the shorted Drain-Source and the Cathode is the Gate? Would it be the Jfets  Id Max?

Jfets make pretty terrible diodes. The PAD1 series of picoamp leakage
diodes are actually jfets inside, just expensive. Jfet diodes have a
lot of equivalent series resistance.

https://www.mouser.com/ProductDetail/InterFET/PAD1?qs=OxRSArmBDfzNntVAJAN1dw%3D%3D

Idmax could well blow out the gate. Try it.

A transistor c-b junction can be a far better diode. Some of the
smaller transistors leak femtoamps but still behave like PN diodes.

 


-- 

I yam what I yam - Popeye

On Thursday, April 14, 2022 at 12:47:47 AM UTC, jla...@highlandsniptechnology.com wrote:
> On Wed, 13 Apr 2022 13:15:16 -0700 (PDT), "neo5...@gmail.com" 
> <neo5...@gmail.com> wrote: 
> 
> >Hi; 
> >I've been using N channel Jfets connected as diodes for input protection for a long time. Everything seems to just work. Recently an application came up where the low leakage of a diode connected FET would be useful but when forward biased it could pass tens to a few hundred mA. Does anyone know what characteristic of a JFET would define the max current through the Diode connected device where the Anode is the shorted Drain-Source and the Cathode is the Gate? Would it be the Jfets Id Max?
> Jfets make pretty terrible diodes. The PAD1 series of picoamp leakage 
> diodes are actually jfets inside, just expensive. Jfet diodes have a 
> lot of equivalent series resistance. 
> 
> https://www.mouser.com/ProductDetail/InterFET/PAD1?qs=OxRSArmBDfzNntVAJAN1dw%3D%3D 
> 
> Idmax could well blow out the gate. Try it. 
> 
> A transistor c-b junction can be a far better diode. Some of the 
> smaller transistors leak femtoamps but still behave like PN diodes. 
> 

Hello Neo
It depends on the manufacturer.

1986 Siliconix FET Databook, pg. 1-3,
the term is "Gate Forward Current" with symbol
    I_G(f)
where the "_" means the G(f) part is subscripted. 

On Semiconductor = "Gate Current",
NXP = "forward gate current"; both use
   I_G (without the 'f') 

Fairchild = "Forward Gate Current": I_GF

The specs seem to range from 10mA to 50mA (J111 family)

cheers, Rich S.

On Wednesday, April 13, 2022 at 1:15:20 PM UTC-7, neo5...@gmail.com wrote:

> I've been using N channel Jfets connected as diodes for input protection ....it could pass tens to a few hundred mA. Does anyone know what characteristic of a JFET would define the max current

It's likely the package.   Whatever the max current is for the FET, is likely the fusing limit for the bond
wire connection, and will dominate a gate-current scenario.   The solder-bump packages should
have highest current capability, IMHO.

So which bjt's have such low leakage? 
Since Im thinking of it how much current can the collector base junction take without failing?
- D
> 
> A transistor c-b junction can be a far better diode. Some of the 
> smaller transistors leak femtoamps but still behave like PN diodes. 

On Thursday, April 14, 2022 at 12:44:43 PM UTC, neo5...@gmail.com wrote:
> So which bjt's have such low leakage? 
> Since Im thinking of it how much current can the collector base junction take without failing? 
> - D
> > 
> > A transistor c-b junction can be a far better diode. Some of the 
> > smaller transistors leak femtoamps but still behave like PN diodes.

in AoE3, chapter 5, H&H like the PN4117 JFET.
John's advice has been immortalized , p.294.
footnote 4. "BFT25".

On Thu, 14 Apr 2022 07:23:47 -0700 (PDT), Rich S
<richsulinengineer@gmail.com> wrote:

>On Thursday, April 14, 2022 at 12:44:43 PM UTC, neo5...@gmail.com wrote:
>> So which bjt's have such low leakage? 
>> Since Im thinking of it how much current can the collector base junction take without failing? 
>> - D
>> > 
>> > A transistor c-b junction can be a far better diode. Some of the 
>> > smaller transistors leak femtoamps but still behave like PN diodes.
>
>in AoE3, chapter 5, H&H like the PN4117 JFET.
>John's advice has been immortalized , p.294.
>footnote 4. "BFT25".

Even dumb bipolar gumdrops can have pA leakage. PNPs seem to be better
for some reason. I recall that 2N3906 is pretty good c-b. But it could
vary by manufacturer.

I missed that AoE reference. Cool. I'm on p 360 too!





-- 

I yam what I yam - Popeye

In article <3c1fd9b6-b713-4923-977c-dde93f56a582n@googlegroups.com>,
neo5...@gmail.com <neo5bass@gmail.com> wrote:

>So which bjt's have such low leakage?

In "Troubleshooting Analog Circuits", Bob Pease recommended the 2N390
and 2N3707 for this application.  Gold-doped diodes and transistors
are apparently leakier.

On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote:
> So which bjt's have such low leakage? 
> Since Im thinking of it how much current can the collector base junction take without failing? 

It's not an easy question, low-leakage isn't a high test priority for BJTs.
At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively
high base doping.

How about a zener diode?   Should be good for high currents, certainly specified for 'em.

On Thu, 14 Apr 2022 12:20:31 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote:
>> So which bjt's have such low leakage? 
>> Since Im thinking of it how much current can the collector base junction take without failing? 
>
>It's not an easy question, low-leakage isn't a high test priority for BJTs.
>At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively
>high base doping.

Small geometry RF transistors seem to have fA reverse currents when
used as diodes. I'd expect audio parts to be much bigger.

https://www.dropbox.com/sh/cuoy2p93jkmyxtz/AAANclqWKW4e5xHfRn3LlPUDa?dl=0

Note that my diode connection exploits reverse beta to keep the base
current down, so it should be good for rated max Ic. That will limit
max reverse voltage.





-- 

If a man will begin with certainties, he shall end with doubts, 
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon