Author Topic: How to tell LEDs apart from photodiodes in a bin  (Read 3640 times)

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Offline Cyberdragon

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How to tell LEDs apart from photodiodes in a bin
« on: September 01, 2016, 11:46:10 pm »
My organizatonal skills are GARBAGE. I throw everything with "diode" in it into the same bin. :palm:

The problem is I was checking the color of clear LEDs to find a specific color, and some of them wouldn't light. Now I don't know whether they are dead LEDs or photodiodes/phototransistors (non-infrared, the infrared receivers have a huge dye).

Is there any way to tell, either by looking at them (with a magnifier) or with a cicuit that could test for both?
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Offline stj

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #1 on: September 01, 2016, 11:54:12 pm »
put them on a meter set to diode-check and point an ir remote at them.
if the readings change - it's a photodiode.

or: power them up and if you cant see them - look with a cellfone camera. (they see ir)
 

Offline T3sl4co1l

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #2 on: September 01, 2016, 11:57:07 pm »
Actually, use diode-check and measure Vf.

Si photodiode: < 1V
IR LED: ~1.1V
Red: ~1.3V
etc.

Blue/white may not light up because meters don't put out much voltage on diode-check.

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Offline Cyberdragon

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #3 on: September 02, 2016, 02:07:35 am »
I found one part with .6V breakdown but no response to infrared or visable light.
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Offline blueskull

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #4 on: September 02, 2016, 02:22:52 am »
I found one part with .6V breakdown but no response to infrared or visable light.

Schottky?
 

Offline Cyberdragon

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #5 on: September 02, 2016, 03:01:04 am »
I mention it because it looks like an LED. I hope I haven't blown any of them by putting power to them. I used a 1k\$\Omega\$ resistor an 3-5V to test the LED colors and nothing seemed to draw excessive current.
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Offline jonovid

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #6 on: September 03, 2016, 09:28:22 am »
I use a video camera with no infrared filter to check for infrared
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Offline mikeselectricstuff

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #7 on: September 03, 2016, 10:34:09 am »
Don't forget phototransistors....
On DMM diode test these will show a low forward voltage (<100mV) when exposed to bright light
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Offline Twoflower

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #8 on: September 03, 2016, 10:43:59 am »
If you try a camera you should try it first on a known IR-LED. Some cameras have a very good IR filter. The iPhone 4s main camera did not notice even the slightest output here. Obviously it has a very effective IR filter. I think I even blown a LED as I reduced the series resistor because of the zero output... The selfie cam worked much better.
 

Offline Cyberdragon

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #9 on: September 03, 2016, 04:22:14 pm »
I have some very old LEDs that are fading, that's why I was wondering if some are are just dead. The infrared trick does work,  it shows up bright green in the camera.

EDIT: odd, it's green in the rear cam but purple in the selfie cam (some don't even show up in the rear cam). Must have different filters. So far the mystery devices don't show up as infrared LEDs (if they do, I'll mark them with a red marker).
« Last Edit: September 03, 2016, 04:41:24 pm by Cyberdragon »
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Offline evb149

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #10 on: September 03, 2016, 08:26:27 pm »
I also wonder how to tell the difference.
If you had enough characterization equipment to tell if it is a *good* LED or *good* photodiode then that would probably answer the question -- it is what is seems best fit to be.

But there are lots of not very good LEDs and not very good photodiodes out there.

Almost all old LEDs will be photosensitive and will operate as photodiodes even at near IR and visible wavelenghs.
After all an IR LED and IR photodiode both have to have around the same bandgap and some kind of transparent area over the junction.  Not much has to be different in the gross sense.

Some of the more exotic phosphor based LEDs may well be photosensitive as well, but the sensitive wavelength range and the efficiency will be harder to estimate and generalize about.

Many old photodiodes will, in fact, emit light as well if you drive a current through them.  Sometimes by a LED like mechanism and sometimes as an incandescent thermal source, depending on how you drive them. :)

So if you have something that is in a very generic package that could be either a LED or a photodiode and it acts as a weak near IR LED at least a little and it acts as a photodiode to near IR or visible at least a little, I'm not sure how you'd tell the difference between them.

Many LEDs have a bonding wire that terminates in the middle of the die on the top face.  Some photodiodes do as well, though there are variations in the ways that PDs and LEDs may be packaged.  Usually the die's rear  is mouted right to the case or lead frame but the way the other terminal is made can more often vary.

I guess photodiodes were slightly more likely to be in metal packages than LEDs for a while, decades ago, and probably even moreso today.

Photodiodes might have top surfaces that are more optically homogenous and specular than LEDs some of which have pretty structured top faces with the wire bond and all.

I'm still not sure why there are / were so very many "not so good" LED and photodiode models out there that might only have 2% to 10% or less of the performance of "good" devices with similar packages and die sizes.  I guess to a point there has to be a cost benefit to making them with lower performance but for a given die size and given package type I'd think there would be a point of diminishing returns as to how much you can cut corners on performance to save packaged device cost.

Of course some of the more weakly performing devices could just be exotic ones that are simply intended for some other use like a much longer wavelength or high speed operation or low noise or high temperature or low capacitance or something.

I wonder what the best "LEDs as photodiodes" are today for common-ish devices.

 

Offline T3sl4co1l

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #11 on: September 03, 2016, 09:37:19 pm »
Almost all old LEDs will be photosensitive and will operate as photodiodes even at near IR and visible wavelenghs.
After all an IR LED and IR photodiode both have to have around the same bandgap and some kind of transparent area over the junction.  Not much has to be different in the gross sense.

Direct vs. indirect bandgap.

Silicon has an indirect bandgap, and as a diode, emits very little light indeed.  And that light would be at or below the bandgap (>1400nm?), where other Si detectors have little or no sensitivity.

GaAs is a direct bandgap material, so it makes a good LED.  The bandgap is somewhat higher than Si, so the emitted light (~1000nm) is well received by Si diodes.

Quote
Many old photodiodes will, in fact, emit light as well if you drive a current through them.  Sometimes by a LED like mechanism and sometimes as an incandescent thermal source, depending on how you drive them. :)

You can light up a 2N3055 by reverse-biasing B-E.  At 1A (and about 8V), it's just barely visible if you cup your hands over it (ah, and cut off the metal can first... ;) ).  It gets hot pretty fast at that power level, so don't leave it too long.  (And probably toasts the junction fairly well... but it's a 2N3055, so good riddance anyway. ;D )

I don't know that the mechanism of avalanche light emission is well known or understood... I haven't heard anything about it.
...Nevermind, it's been known for a long time:
http://cdn.intechopen.com/pdfs-wm/47465.pdf
...which explains the otherwise ridiculous sounding proposals for putting phosphors in/on silicon.  Because, what's going to excite them?  Well, hot carriers, of course, from an avalanche (or near-avalanche) condition.

BTW, hot carriers are why JFET gates leak more at high Vds, and why EPROM and Flash are possible (the floating gate is charged by dumping high Vds through the transistor).  Physics!


Quote
So if you have something that is in a very generic package that could be either a LED or a photodiode and it acts as a weak near IR LED at least a little and it acts as a photodiode to near IR or visible at least a little, I'm not sure how you'd tell the difference between them.

I would think GaAs photodiodes are very rare indeed, even throughout history.  (On a quick search, it looks like they're used for Gbit fiber.  But probably more as modules rather than components, except for specialized applications?)

Quote
I wonder what the best "LEDs as photodiodes" are today for common-ish devices.

Probably, for most of them, the gain is very poor...

Red or IR should be best, as the sensitivity will include the visible spectrum, assuming that's what you want to look at.  (Whereas green LEDs shouldn't respond to red light, and so on.  Minding that GaP is a weird semiconductor, and probably does respond to red light anyway...)

An important difference between photodiodes and LEDs: current density.  Photodiodes (and solar cells) can afford very thin connections, maximizing absorption.  LEDs can't, because they have to operate at high currents.

Tim
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Offline evb149

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Re: How to tell LEDs apart from photodiodes in a bin
« Reply #12 on: September 04, 2016, 10:18:48 pm »
Thank you very much, Tim, for the information.

I stand corrected about the bandgap / junction emission similarity aspects of common PDs and LEDs.
It was folklore that I've heard since the 1980s or so but never tested myself with known PD devices.  It may be that some of the "surplus mystery devices" that were said to be PDs that worked (a bit) as LEDs were actually intended LEDs that happened to work as PDs instead of the other way around, or it may be that the avalanche mode feeble light emission which you described was what was sometimes being referred to. 
I remember seeing a wider variety of longer wave LEDs in the past decades (e.g. more common 900nm and longer stuff), though I don't know if such manufacturing variange would have related to variations of surplus PDs on the market at the time.  I'm not sure what composition those were / are even when they were more commonly "seen" .

I remember a thread on SED from a few years ago where someone was suggesting that they might have even been able to achieve a kind of very noisy and low overall QE but occasionally "single photon sensitive" detection using LEDs as PDs.  The report of that seems to have generated at least one follow up research publication:
http://www.ccsenet.org/journal/index.php/mas/article/view/52573


Direct vs. indirect bandgap.

Silicon has an indirect bandgap, and as a diode, emits very little light indeed.  And that light would be at or below the bandgap (>1400nm?), where other Si detectors have little or no sensitivity.

GaAs is a direct bandgap material, so it makes a good LED.  The bandgap is somewhat higher than Si, so the emitted light (~1000nm) is well received by Si diodes.

I would think GaAs photodiodes are very rare indeed, even throughout history.  (On a quick search, it looks like they're used for Gbit fiber.  But probably more as modules rather than components, except for specialized applications?)

Probably, for most of them, the gain is very poor...

Red or IR should be best, as the sensitivity will include the visible spectrum, assuming that's what you want to look at.  (Whereas green LEDs shouldn't respond to red light, and so on.  Minding that GaP is a weird semiconductor, and probably does respond to red light anyway...)

An important difference between photodiodes and LEDs: current density.  Photodiodes (and solar cells) can afford very thin connections, maximizing absorption.  LEDs can't, because they have to operate at high currents.

Tim
 


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