i have a question about behavior of IR LED

FvM said:

Any semiconductor junction does.

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albert22 said:

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

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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

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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.

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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.

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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.

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