i have a question about behavior of IR LED

[UyAb]

why does IR LED acts like a receiver when it is reversed bias?

 

[FvM]

Any semiconductor junction does.

 

[UyAb]

Any semiconductor junction does.

I think your answer is right, but in the different question..
What I'm asking is why does it behaves like that?is it kind of a phenomenon,theory or something?what is it?

 

[godfreyl]

There's an explanation here: https://en.wikipedia.org/wiki/Photodiode

 

[albert22]

The PN junction reaches an equilibrium that is distorted when energy is introduced. Light energy releases electrons producing electron/holes pairs so current increases.
Of course this explanation is very simplistic for more detail you can go into the Physics of the PN junction for example at wikipedia.
There are other devices that exhibit light dependence even when not designed to do so:
In the very old times we used to cut the top of a OC71 (very old Germanium transistor) to use it as a light sensor.
A DRAM (dynamic RAM) with its lid off, has been used as a low resolution camera.

 

[godfreyl]

In the very old times we used to cut the top of a OC71

Or just scrape the black paint off.
I did break one open once to see what was inside. It was filled with something resembling Vaseline. Any idea what that was? At the time I just filed it mentally under "do not eat" and left it at that.

 

[albert22]

godfreyl:
I see that you are from the old times as me. Of course I was talking about the metallic encapsulation. As I recall those types had a white stuff resembling the silicon grease used for heatsinks.
look history here:
http://www.wylie.org.uk/technology/semics/Mullard/Mullard.htm

These transistors were filled with Alundum paste: an aluminium oxide and silicon oil mix that made them lightproof and also conducted the heat away from the transistor junctions. The devices were dipped in a light blue silicon "Bouncing" putty before putting them into the Alundum filled envelope, and scraping off the black 'dope' often reveals a blue tinged whitish interior. However, the OCP71 capsules were filled with a clear silicon grease to allow light through

Regards

 

[godfreyl]

Thanks for the history. I didn't know they were available in a metal can. Back in my schooldays, my father brought an OC71 and an OC45 home from work for me to play with. It's amazing how much fun you can have with just one transistor - radio transmitters etc. By the time I started buying my own parts, they were all silicon.

Back on topic:
What about modern glass-encapsulated diodes like 1n4148? Can they inject hum into a circuit if they're exposed to flickering light, e.g. from a fluorescent lamp?

 

[albert22]

I didnt experience (or realized) the hum effect but this is from the Robert A. Pease book:

Once I built a precision test box that worked right away and gave exactly the right
readings until I picked up the box to look at some waveforms. Then the leakage test
shifted way off zero. Every time I lifted up the box, the meter gave an indication; I
thought I had designed an altimeter. After some study, I localized the problem to an
FD300 diode, whose body is a clear glass DO-35 package covered with black paint.
This particular diode’s paint had been scratched a little bit, so when I picked up the
test box, the light shone under the fixture and onto the diode. Most of these diodes
didn’t exhibit this behavior; the paint wasn’t scratched on most of them.

Extraneous, unwanted light impinging on the pn junction of a semiconductor is only
one of many tricky problems you can encounter when you try to design and operate
precision amplifiers-specially high-impedance amplifiers. Just like a diode’s pn
junction, a transistor’s collector-base junction makes a good photodiode, but a transistor’s
plastic or epoxy or metal package normally does a very good job of blocking
out the light.
When light falls onto the pn junction of any diode, the light’s energy is converted
to electricity and the diode forward biasses itself.

UyAb
Another form of energy that disturbes the equilibrium of a PN junction is thermal. It also releases electrons. Causing increasing current.

- - - Updated - - -

this is from a forum:
http://forum.allaboutcircuits.com/archive/index.php/t-61061.html

It is possible that there might be an issue with light sensitivity. 1N914 is usually found in a glass encapsulation, although the geometry of the package is hardly optimised for use as a photodiode. Other than obtaining a device with a light-proof package, screening e.g. with a black insulating sleeve could be advisable. I have personally had experience of mil-spec 1N3595 diodes apparently leaking a little more than the tiny expected (<1nA) amount. This was traced to photocurrent, despite their having visually blackened glass bodies.

That said, any photocurrents are likely to be in the microamp region or lower under normal ambient lighting. There may thus be significant issues with light sensitivity if operation is required to very low currents, and it is also possible that excessive noise or ripple may be seen on the output, especially under bright electric lighting.

 

[FvM]

The photocurrent is well detectable with Si-diodes in usual SOD27 glass package (1N4148 and similar). For normal light intensities, it's in the same order of magnitude as specified leakage currents. In other words, where leakage currents matter, you should better go for light absorbing packages.