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which component has exponential i-v characteristics down to 1 pA?

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

Reply to
alan
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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

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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
Reply to
Winfield Hill

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
Reply to
Tim Wescott

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!

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Thanks for the quick replies

Reply to
alan

Probably about $50k per year.

--
 Thanks,
    - Win
Reply to
Winfield Hill

snip

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.

Reply to
martin griffith

"alan" wrote

Hey, we suggest them for everything else: Why not use a PIC?

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

Any simulation curve you like ...

  • * *

Zeke snipped-for-privacy@zyzzly.net wrote

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/
Reply to
Nicholas O. Lindan

ROTFLMAO

Cheers Terry

Reply to
Terry Given

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