which component has exponential i-v characteristics down to

A

alan

Guest
Looking for a circuit element that is pretty exponential in i-v down to
about 1pA. Maybe even 10pA. Starting from maybe 10nA. Trying to
simulate a vacuum tunnel junction for STM. I know a lot of stuff has
this, e.g. diode, jfet, bipolar, but I don't have a good idea if they
pitter out at such low currents. For example, I think a particular
diode has exponential iv with one slope from 10mA-.1uA, then a different
slope from .1uA to 100pA, then pitters out after that.

Even though I'm looking for "only" 4 decades of exponential-ness, I
don't think it is a good idea to get these 4 decades at a higher
current, and then divide down the current, coz that adds a lot of noise.
Oh yeah, I'm looking for low noise too.

Thanks
 
alan wrote...
Looking for a circuit element that is pretty exponential in i-v down
to about 1pA. Maybe even 10pA. Starting from maybe 10nA. Trying to
simulate a vacuum tunnel junction for STM. I know a lot of stuff has
this, e.g. diode, jfet, bipolar, but I don't have a good idea if they
pitter out at such low currents. For example, I think a particular
diode has exponential iv with one slope from 10mA-.1uA, then a
different slope from .1uA to 100pA, then pitters out after that.
Click to expand...
OK, don't use that diode!

I'm sure there are many small high-beta BJTs that continue to work
well in the pA region, but I haven't taken measurements lately and
don't recall right now where to look for some old ones. But I did
post on one of my web pages some diode measurements that show good
results to 1pA for PAD-1 diodes and 2n4117A gate-source junctions,
see http://www.picoamp.com/win/elec/comp/diode/diode-curves.html

The curves show logarithmic linearity over seven to eight decades.

I should revisit these parts and extend the measurement below 1pA,
using my lab's Keithley electrometers, recently-acquired on eBay.
One of my instruments, a 642 with remote head, goes down to 100aA.


--
Thanks,
- Win
 
Phil Hobbs wrote:
You might try using an ordinary display LED. I've measured them to have
leakages of less than 50 fA over -5V to +0.5 V, so they don't go ohmic
near zero bias the way silicon diodes do. Make sure you keep it in the
dark!
Click to expand...
Just as you would your Engineers, if you were a manager. Make sure to
tell them that the company is doing "just great" from time to time.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
Tim Wescott wrote:

Phil Hobbs wrote:


You might try using an ordinary display LED. I've measured them to
have leakages of less than 50 fA over -5V to +0.5 V, so they don't go
ohmic near zero bias the way silicon diodes do. Make sure you keep it
in the dark!


Just as you would your Engineers, if you were a manager. Make sure to
tell them that the company is doing "just great" from time to time.
Click to expand...
<snort> ROTFLMAO

Cheers
Terry
 
On 25 Nov 2004 05:51:49 -0800, in sci.electronics.design Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

alan wrote...

snip
I should revisit these parts and extend the measurement below 1pA,
using my lab's Keithley electrometers, recently-acquired on eBay.
One of my instruments, a 642 with remote head, goes down to 100aA.
Click to expand...
Hi Win,
I was just wondering how much tech stuff do you/Rowland.org buy off
Ebay?





martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.
 
alan wrote:
Looking for a circuit element that is pretty exponential in i-v down to
about 1pA. Maybe even 10pA. Starting from maybe 10nA. Trying to
simulate a vacuum tunnel junction for STM. I know a lot of stuff has
this, e.g. diode, jfet, bipolar, but I don't have a good idea if they
pitter out at such low currents. For example, I think a particular
diode has exponential iv with one slope from 10mA-.1uA, then a different
slope from .1uA to 100pA, then pitters out after that.

Even though I'm looking for "only" 4 decades of exponential-ness, I
don't think it is a good idea to get these 4 decades at a higher
current, and then divide down the current, coz that adds a lot of noise.
Oh yeah, I'm looking for low noise too.

Thanks
Click to expand...
Thanks for the quick replies
 
"alan" <no-longer-valid@yahoo.com> wrote

Looking for a circuit element that is pretty exponential in i-v down to
about 1pA. Maybe even 10pA.
Click to expand...
Hey, we suggest them for everything else: Why not use a PIC?

TIM -> A/D -> PIC -> D/A

Any simulation curve you like ...

* * *

Zeke Zeke@zyzzly.net wrote

My carburetor linkage broke . . .
Click to expand...
Use a PIC! Drill a hole in both ends of the PIC and insert into the
broken linkage, crimp broken linkage ends to secure.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/
 
martin griffith wrote...
Hi Win,
I was just wondering how much tech stuff do you/Rowland.org
buy off Ebay?
Click to expand...
Probably about $50k per year.


--
Thanks,
- Win
 
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