How to tell LEDs apart from photodiodes in a bin

[Cyberdragon]

My organizatonal skills are GARBAGE. I throw everything with "diode" in it into the same bin.

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?

[stj]

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)

[T3sl4co1l]

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.

Tim

[Cyberdragon]

I found one part with .6V breakdown but no response to infrared or visable light.

[Cyberdragon]

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.

[jonovid]

I use a video camera with no infrared filter to check for infrared

[mikeselectricstuff]

Don't forget phototransistors....
On DMM diode test these will show a low forward voltage (<100mV) when exposed to bright light

[Twoflower]

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.

[Cyberdragon]

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).

[T3sl4co1l]

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.

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. )

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!

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?)

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