NPN protective zener

[Winfield Hill]

NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners, etc. For example,
the emitter-base junction of the common
2N3904 breaks down near 7.25 volts, and
has 6 to 7pF of capacitance at 0V, drops
to about 3.7pF at 5 volts. Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?


--
Thanks,
- Win

 

[Robert Baer]

Winfield Hill wrote:

NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners, etc. For example,
the emitter-base junction of the common
2N3904 breaks down near 7.25 volts, and
has 6 to 7pF of capacitance at 0V, drops
to about 3.7pF at 5 volts. Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?

I would guess, that a reasonable limit would be a ten microsecond
pulse at 10mA.

 

[TTman]

On 20/06/2019 01:09, Winfield Hill wrote:

NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners, etc. For example,
the emitter-base junction of the common
2N3904 breaks down near 7.25 volts, and
has 6 to 7pF of capacitance at 0V, drops
to about 3.7pF at 5 volts. Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?


Would that protcet the gate against ESD though ? ( body model)

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Winfield Hill]

Robert Baer wrote...

Winfield Hill wrote:
NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners, etc. For example,
the emitter-base junction of the common
2N3904 breaks down near 7.25 volts, and
has 6 to 7pF of capacitance at 0V, drops
to about 3.7pF at 5 volts. Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?


I would guess, that a reasonable limit would
be a ten microsecond pulse at 10mA.

Bah, humbug, I'll bet it can do amps.
Hope so anyway.


--
Thanks,
- Win

 

[piglet]

On 20/06/2019 10:25 am, Winfield Hill wrote:

Robert Baer wrote...

Winfield Hill wrote:
NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners, etc. For example,
the emitter-base junction of the common
2N3904 breaks down near 7.25 volts, and
has 6 to 7pF of capacitance at 0V, drops
to about 3.7pF at 5 volts. Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?


I would guess, that a reasonable limit would
be a ten microsecond pulse at 10mA.

Bah, humbug, I'll bet it can do amps.
Hope so anyway.

I have used BJT e-b as gate protection (combined with duty as an emitter
follower) and brief 1A reverse pulses were clamped nicely.

I may get some bench time next week to try higher currents.

piglet

 

[John Larkin]

On 20 Jun 2019 02:25:24 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Robert Baer wrote...

Winfield Hill wrote:
NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners, etc. For example,
the emitter-base junction of the common
2N3904 breaks down near 7.25 volts, and
has 6 to 7pF of capacitance at 0V, drops
to about 3.7pF at 5 volts. Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?


I would guess, that a reasonable limit would
be a ten microsecond pulse at 10mA.

Bah, humbug, I'll bet it can do amps.
Hope so anyway.

It's limited by Rbb. I have a book around here somewhere that has a
table of transistor Rbb's.

An alternate would be to put a schottky diode in series with a
charged-up zener diode maybe paralleled with a cap.

There might be a FOM for schottkies, which would be Cjo * Rs. That's a
time constant, and we want it to be low. It would be interesting to
scatter-plot that. BAT15 is about 1 picosecond. 1N5819 is around 6.

Zeners could have thye same FOM.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Winfield Hill]

John Larkin wrote...

Winfield Hill wrote:
Robert Baer wrote...
Winfield Hill wrote:
NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners ... ... Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?

I would guess, that a reasonable limit
would be a ten microsecond pulse at 10mA.

Bah, humbug, I'll bet it can do amps.
Hope so anyway.

It's limited by Rbb. I have a book around here
somewhere that has a table of transistor Rbb's.

Yes, Rbb' Aha, good point, Table 8.1a, page 501.
We measured 110 ohms for the 2N3904, but that'd
be an extra 1.1V at 10mA, now I gotta go check.

The table reveals much better choices, low Rbb'
with low capacitance. Time to do some browsing.


--
Thanks,
- Win

 

[Tim Williams]

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
newse2ngele7mh91kmcuo32jc8kogqvuoh17v@4ax.com...

An alternate would be to put a schottky diode in series with a
charged-up zener diode maybe paralleled with a cap.

There might be a FOM for schottkies, which would be Cjo * Rs. That's a
time constant, and we want it to be low. It would be interesting to
scatter-plot that. BAT15 is about 1 picosecond. 1N5819 is around 6.

Schottkys are bad, by their own admission so to speak. Under surge
conditions, the guard ring activates, and you're just using another dumb PN
diode. I don't think the schottky barrier itself actually gains you
anything.

But, BAT54S and such continue to be used (including by myself), so it seems
a diode of sufficient size isn't bothered by it, regardless which part of it
is doing the work.

Wideband ESD devices (for protecting PCIe and HDMI for instance) use a PN
diode as small as possible, into a common rail with a TVS onboard.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[John Larkin]

On Thu, 20 Jun 2019 09:04:53 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
newse2ngele7mh91kmcuo32jc8kogqvuoh17v@4ax.com...
An alternate would be to put a schottky diode in series with a
charged-up zener diode maybe paralleled with a cap.

There might be a FOM for schottkies, which would be Cjo * Rs. That's a
time constant, and we want it to be low. It would be interesting to
scatter-plot that. BAT15 is about 1 picosecond. 1N5819 is around 6.

Schottkys are bad, by their own admission so to speak. Under surge
conditions, the guard ring activates, and you're just using another dumb PN
diode. I don't think the schottky barrier itself actually gains you
anything.

If we're trying to protect a mosfet gate from a spike, activating the
guard ring doesn't hurt... unless we want to recover fast.

I wonder if the really fast low-barrier schottkies, SMS7621 and BAT15
and such, have guard rings. They are only rated for 2 or 4 volts
reverse.

But, BAT54S and such continue to be used (including by myself), so it seems
a diode of sufficient size isn't bothered by it, regardless which part of it
is doing the work.

Wideband ESD devices (for protecting PCIe and HDMI for instance) use a PN
diode as small as possible, into a common rail with a TVS onboard.

I guess that an initial spike would have to charge up the TVS
capacitance. One could bleed in a little current to keep it charged,
on a multi-section part.

Tim

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Tim Williams]

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
newsi6ngetlfbpgtl3dapco62a8ib4tthts7a@4ax.com...

Wideband ESD devices (for protecting PCIe and HDMI for instance) use a PN
diode as small as possible, into a common rail with a TVS onboard.

I guess that an initial spike would have to charge up the TVS
capacitance. One could bleed in a little current to keep it charged,
on a multi-section part.

The ones for Ethernet are precharged by the signal itself, kind of annoying
I suppose but not destructive for that kind of data. The others are usually
used with supply bias (e.g., super easy for USB), but can be alone as well.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[Winfield Hill]

Winfield Hill wrote...

John Larkin wrote...
Winfield Hill wrote:
Robert Baer wrote...
Winfield Hill wrote:
NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners ... ... Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?

I would guess, that a reasonable limit
would be a ten microsecond pulse at 10mA.

Bah, humbug, I'll bet it can do amps.
Hope so anyway.

It's limited by Rbb. I have a book around here
somewhere that has a table of transistor Rbb's.

Yes, Rbb' Aha, good point, Table 8.1a, page 501.
We measured 110 ohms for the 2N3904, but that'd
be an extra 1.1V at 10mA, now I gotta go check.

OK, it appears Rbb' isn't an issue, at least the
parameter we measured with our noise measurements.
I measure ON-Semi and Fairchild parts, with both
C-B tied together, and with B alone.

I found, first, the V_EB breakdown is very sharp,
changing by only about 5mV/decade from 1uA up,
increasing some above 1mA. E.g., 7.413 volts at
1uA, 7.447V at 1mA, and 7.555V at 10mA. No extra
Rbb' voltage visible at 10mA.

Then it rises faster, to 7.985V at 50mA, 8.39V at
100mA, and 9.23V at 500mA. That's Rs = 3 ohms.
I was quickly switching the power on and off,
grabbing a reading. At 1A for 1 or 2 seconds, it
shorted. Sorry, I didn't setup my SMU machine
for sub-ms pulsing. After the book is in, I'll
take some 10A measurements at 20us.

The E-to-B and E-to-CB connections gave the same
answer, SFAICT, except for 100mA and up, where
the CB form had lower voltages. Maybe it was a
little inverted-mode gain going on in that case?
At any rate, that's the way I like to


--
Thanks,
- Win

 

[John Larkin]

On 20 Jun 2019 07:15:10 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

John Larkin wrote...

Winfield Hill wrote:
Robert Baer wrote...
Winfield Hill wrote:
NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners ... ... Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?

I would guess, that a reasonable limit
would be a ten microsecond pulse at 10mA.

Bah, humbug, I'll bet it can do amps.
Hope so anyway.

It's limited by Rbb. I have a book around here
somewhere that has a table of transistor Rbb's.

Yes, Rbb' Aha, good point, Table 8.1a, page 501.
We measured 110 ohms for the 2N3904, but that'd
be an extra 1.1V at 10mA, now I gotta go check.

The table reveals much better choices, low Rbb'
with low capacitance. Time to do some browsing.

Also please reconsider the injustice you did to my favorite gumdrop,
the BCX70. It came in dead last.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[whit3rd]

On Thursday, June 20, 2019 at 6:39:52 AM UTC-7, John Larkin wrote:

On 20 Jun 2019 02:25:24 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Robert Baer wrote...

Winfield Hill wrote:
NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners, etc.

I would guess, that a reasonable limit would
be a ten microsecond pulse at 10mA.

Bah, humbug, I'll bet it can do amps.
Hope so anyway.

It's limited by Rbb. I have a book around here somewhere that has a
table of transistor Rbb's.

There's package limits, too; I've seen the emitter wire explode on a TO-18 transistor
where the plastic-package version just kept going. Surges can generate
hypersonic stresses, and wires don't just fail by thermal shocks.

Plug-package rectifiers are my idea of surge protectors. Or SiC grinding
wheels between heavy plates.

 

[John Larkin]

On 20 Jun 2019 12:19:44 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Winfield Hill wrote...
John Larkin wrote...
Winfield Hill wrote:
Robert Baer wrote...
Winfield Hill wrote:
NPN signal transistors make pretty good
low-capacitance signal and MOSFET-gate
protection zeners ... ... Has anybody
taken pulsed high-current voltage-drop
measurements on these guys?

I would guess, that a reasonable limit
would be a ten microsecond pulse at 10mA.

Bah, humbug, I'll bet it can do amps.
Hope so anyway.

It's limited by Rbb. I have a book around here
somewhere that has a table of transistor Rbb's.

Yes, Rbb' Aha, good point, Table 8.1a, page 501.
We measured 110 ohms for the 2N3904, but that'd
be an extra 1.1V at 10mA, now I gotta go check.

OK, it appears Rbb' isn't an issue, at least the
parameter we measured with our noise measurements.
I measure ON-Semi and Fairchild parts, with both
C-B tied together, and with B alone.

Right, rbb' isn't a real resistance, it's a noise equivalent resistor.
There is a real series resistance, in series with the b-e junction,
that can be extracted from the DC e-i curve, but that's inherently
less than rbb', a different number. Does that have a name?


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Winfield Hill]

This is the paragraph I'm adding, as a
footnote to the graph in the x-Chapter
section on protection devices:

"The reverse avalanche breakdown of the 2N3904
is very sharp, with no current conducted at all
below 7 volts. Once it starts conducting, it
has a low dynamic resistance, with the breakdown
voltage increasing only 5mV / decade up to 1mA,
where its resistance is about 5 ohms. The
voltage increases by a volt at 100mA, and its
still about 5 ohms. We get only one more volt
by 0.5A, where the resistance is even lower.
Above 50mA the breakdown voltage will be a
little higher if you leave the collector pin
open, you need it connected to benefit from
its assistance with inverted-mode gain. The
2N3904 is fine carrying very high currents,
protecting your fragile devices, if the time
is short enough. It's worth noting that the
r_bb' base-resistance we measured and reported
in Table 8.1a, which was 110 ohms for the
2N3904, is not relevant to the breakdown."

I'm hoping that by the time the book goes to
print, I'll have replaced the graph with one
that goes to 10A. I'm confidant the 2N3904,
as with most other zener (actually avalanche)
devices, can handle full 10A peak currents in
the 8-20us surge tests, and pass. So it's a
totally viable low-capacitance protection device.

If one gets it in the SC-70 SOT-323 package
(at least four manufacturers*), it's not even
that large (I still prefer diode-package parts).
Fairchild offers a dual 2N3904 in an SC-70-6
package, FFB3904.** Huge stocking levels and
low prices, too, for all of these.
Lots of interest. Amazing, who knew?

* ON Semi MMBT3904WT1G, Diodes MMST3904-7-F,
Nexperia PMSS3904, MCC MMST3904-TP.

** and the FMB3946, get one each 2n3904 and
2n3906 in SC-70-6, and other manuf as well.


--
Thanks,
- Win

 

[John Larkin]

On 21 Jun 2019 03:05:47 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

This is the paragraph I'm adding, as a
footnote to the graph in the x-Chapter
section on protection devices:

"The reverse avalanche breakdown of the 2N3904
is very sharp, with no current conducted at all
below 7 volts. Once it starts conducting, it
has a low dynamic resistance, with the breakdown
voltage increasing only 5mV / decade up to 1mA,
where its resistance is about 5 ohms. The
voltage increases by a volt at 100mA, and its
still about 5 ohms. We get only one more volt
by 0.5A, where the resistance is even lower.
Above 50mA the breakdown voltage will be a
little higher if you leave the collector pin
open, you need it connected to benefit from
its assistance with inverted-mode gain. The
2N3904 is fine carrying very high currents,
protecting your fragile devices, if the time
is short enough. It's worth noting that the
r_bb' base-resistance we measured and reported
in Table 8.1a, which was 110 ohms for the
2N3904, is not relevant to the breakdown."

I'm hoping that by the time the book goes to
print, I'll have replaced the graph with one
that goes to 10A. I'm confidant the 2N3904,
as with most other zener (actually avalanche)
devices, can handle full 10A peak currents in
the 8-20us surge tests, and pass. So it's a
totally viable low-capacitance protection device.

If one gets it in the SC-70 SOT-323 package
(at least four manufacturers*), it's not even
that large (I still prefer diode-package parts).
Fairchild offers a dual 2N3904 in an SC-70-6
package, FFB3904.** Huge stocking levels and
low prices, too, for all of these.
Lots of interest. Amazing, who knew?

* ON Semi MMBT3904WT1G, Diodes MMST3904-7-F,
Nexperia PMSS3904, MCC MMST3904-TP.

** and the FMB3946, get one each 2n3904 and
2n3906 in SC-70-6, and other manuf as well.

BFT25 is a phenomenal diode:

https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1

The low end was limited by my measurement rig and may be even better.

Transistors in general make good diodes, better than the average
diode.

Using b-e as a zener, there seems to be a small drift of zener voltage
vs total charge conducted, but that's wouldn't effect its use as an
ESD diode. Blow some up!

What happens to collector current when you zener the b-e junction?
I've been meaning to investigate that for some decades now.







--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Winfield Hill]

John Larkin wrote...

BFT25 is a phenomenal diode:

https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1

600mV at 1uA. I'll keep it in mind next time I need a 100fA didoe.

Using b-e as a zener, there seems to be a small drift of
zener voltage vs total charge conducted, but that's
wouldn't effect its use as an ESD diode. Blow some up!

Yes.

What happens to collector current when you zener the
b-e junction? I've been meaning to investigate that
for some decades now.

Hmm, I coulda taken a look yesterday.


--
Thanks,
- Win

 

[George Herold]

On Friday, June 21, 2019 at 11:00:24 AM UTC-4, John Larkin wrote:

On 21 Jun 2019 03:05:47 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

This is the paragraph I'm adding, as a
footnote to the graph in the x-Chapter
section on protection devices:

"The reverse avalanche breakdown of the 2N3904
is very sharp, with no current conducted at all
below 7 volts. Once it starts conducting, it
has a low dynamic resistance, with the breakdown
voltage increasing only 5mV / decade up to 1mA,
where its resistance is about 5 ohms. The
voltage increases by a volt at 100mA, and its
still about 5 ohms. We get only one more volt
by 0.5A, where the resistance is even lower.
Above 50mA the breakdown voltage will be a
little higher if you leave the collector pin
open, you need it connected to benefit from
its assistance with inverted-mode gain. The
2N3904 is fine carrying very high currents,
protecting your fragile devices, if the time
is short enough. It's worth noting that the
r_bb' base-resistance we measured and reported
in Table 8.1a, which was 110 ohms for the
2N3904, is not relevant to the breakdown."

I'm hoping that by the time the book goes to
print, I'll have replaced the graph with one
that goes to 10A. I'm confidant the 2N3904,
as with most other zener (actually avalanche)
devices, can handle full 10A peak currents in
the 8-20us surge tests, and pass. So it's a
totally viable low-capacitance protection device.

If one gets it in the SC-70 SOT-323 package
(at least four manufacturers*), it's not even
that large (I still prefer diode-package parts).
Fairchild offers a dual 2N3904 in an SC-70-6
package, FFB3904.** Huge stocking levels and
low prices, too, for all of these.
Lots of interest. Amazing, who knew?

* ON Semi MMBT3904WT1G, Diodes MMST3904-7-F,
Nexperia PMSS3904, MCC MMST3904-TP.

** and the FMB3946, get one each 2n3904 and
2n3906 in SC-70-6, and other manuf as well.


BFT25 is a phenomenal diode:

https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1

The low end was limited by my measurement rig and may be even better.

Huh, can you say anything about the measurement rig.
(610 electrometer?)
I've measured current of about a pA with a ~10 pA bias current opamp
(difference measurement...

I think I was looking at c-b junction of a ~200V npn.
'HV' diode with low leakage.

George H.

Transistors in general make good diodes, better than the average
diode.

Using b-e as a zener, there seems to be a small drift of zener voltage
vs total charge conducted, but that's wouldn't effect its use as an
ESD diode. Blow some up!

What happens to collector current when you zener the b-e junction?
I've been meaning to investigate that for some decades now.







--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Fri, 21 Jun 2019 17:09:45 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Friday, June 21, 2019 at 11:00:24 AM UTC-4, John Larkin wrote:
On 21 Jun 2019 03:05:47 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

This is the paragraph I'm adding, as a
footnote to the graph in the x-Chapter
section on protection devices:

"The reverse avalanche breakdown of the 2N3904
is very sharp, with no current conducted at all
below 7 volts. Once it starts conducting, it
has a low dynamic resistance, with the breakdown
voltage increasing only 5mV / decade up to 1mA,
where its resistance is about 5 ohms. The
voltage increases by a volt at 100mA, and its
still about 5 ohms. We get only one more volt
by 0.5A, where the resistance is even lower.
Above 50mA the breakdown voltage will be a
little higher if you leave the collector pin
open, you need it connected to benefit from
its assistance with inverted-mode gain. The
2N3904 is fine carrying very high currents,
protecting your fragile devices, if the time
is short enough. It's worth noting that the
r_bb' base-resistance we measured and reported
in Table 8.1a, which was 110 ohms for the
2N3904, is not relevant to the breakdown."

I'm hoping that by the time the book goes to
print, I'll have replaced the graph with one
that goes to 10A. I'm confidant the 2N3904,
as with most other zener (actually avalanche)
devices, can handle full 10A peak currents in
the 8-20us surge tests, and pass. So it's a
totally viable low-capacitance protection device.

If one gets it in the SC-70 SOT-323 package
(at least four manufacturers*), it's not even
that large (I still prefer diode-package parts).
Fairchild offers a dual 2N3904 in an SC-70-6
package, FFB3904.** Huge stocking levels and
low prices, too, for all of these.
Lots of interest. Amazing, who knew?

* ON Semi MMBT3904WT1G, Diodes MMST3904-7-F,
Nexperia PMSS3904, MCC MMST3904-TP.

** and the FMB3946, get one each 2n3904 and
2n3906 in SC-70-6, and other manuf as well.


BFT25 is a phenomenal diode:

https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1

The low end was limited by my measurement rig and may be even better.
Huh, can you say anything about the measurement rig.
(610 electrometer?)

I built a thing:

https://www.dropbox.com/s/zmilmhn1tg4ts4q/99A260A1.JPG?raw=1

https://www.dropbox.com/s/j57mqvywvytrtwh/99A260A3.JPG?raw=1

https://www.dropbox.com/s/6ia0wgo88jnbwbq/99S260A.JPG?raw=1

After it was all built, I discovered that the crummy nylon banana
terminals were leaky as heck, so I had to mill out the holes and add
the polycarb insulator. That was a major nuisance.

I have a set of Pomona dual bannana plugs with various resistors from
1M to 1T ohms to use with that.

I'm thinking that instead I could get a really good film cap and
charge it with a device's leakage current. Let that sit for some hours
or days and then measure the cap voltage. That would easily get below
1 fA. Some good film caps have self-discharge time constants of
decades.

1 nF would charge 1 uV per second at 1 fA. That gives 86 mV/day per
fA.



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Winfield Hill]

George Herold wrote...

John Larkin wrote:

BFT25 is a phenomenal diode:
https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1

Huh, can you say anything about the measurement rig.
(610 electrometer?)
I've measured current of about a pA with a ~10 pA
bias current opamp (difference measurement...

Lots of JFET gates can make it well below 1pA.
Our Table 5.3, Nine Low-input-current Op-amps,
on AoE III, page 303, lists three CMOS op-amps.
Try the LMP7721, rated 3fA typ, 20fA max. The
ADA4530-1 has guard pins, and claims < 1fA.


--
Thanks,
- Win