Red LED has 1/f noise

Interesting. Is it leaky in reverse bias? If so, it might have had a bunch of junction damage.

How are you driving it? RF power transistors work a lot better if you servo-control the bias current with some reasonable bandwidth--that prevents the device noise from intermodulating with the signal, and cuts

1/f noise in both amplitude and phase fairly dramatically.

Cheers

Phil Hobbs

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Now don't throw the mug at me for bringing this up, it's just meant as an idea: Did you cover the LED during the measurements? Those things also act as photodiodes. When the ambient illumination carries a lot of "gunk" on it and you are measuring miniscule signal levels this might cause issues. That would be a lot less severe in the IR range and light bulbs can't receive at all unless we are talking about an almost direct lightning hit.

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OK, I was just driving it too hard. I had about 50mA going through it, when I cut the current back to 20mA the '1/f' noise went away.

LED driven from a quite voltage source through a resistor.

George H.

Oh, I'd never throw a mug at you. I might slide you one full of beer.... it is Friday after all.

it's just meant as

Yup thanks... everything was in the dark inside a nice metal box.

But don't over drive your LED's they get cranky. :^)

George H.

Interesting. I wonder what the mechanism is?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

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I have no idea, 50mA was the number on the spec sheet for absolute maximum current. With warnings that if you ran it over 30mA for a long time bad things might start to happen... reduced light output. Is it internal heating that limits the maximum current through an LED?

George H.

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Mostly, but big 1/f noise sounds like you were propagating dislocations or maybe interstitial atoms. LEDs always have a bit of 1/f current noise when run from a resistor--since modern ones are pretty efficient, it would be interesting to see how the optical 1/f noise depends on the drive impedance.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

--- Since LEDs have a negative temperature coefficient of resistance, that's a "no". ;)

Seriously, though, since the life of a semiconductor is (AIUI) halved, exponentially, for each 10C rise in temp it experiences, that's a "yes".

--- JF

The aging mechanism that I know of in LEDs is aluminum migration -- at one point this was a barrier to progress in high intensity red LEDs, particularly at high temperatures.

I wonder if you were seeing the aluminum moving?

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This is a high intensity red led from Avago. I don't recall the part number. It may not be a problem, I think we can turn lemon leds into lemonade light sources that have excess noise. White noise is boring after a while. Did the other color leds not have an aluminum problem? I've got other colors from the same series in a drawer at work. Monday,

George H.

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I can try that, More 1/f noise at lower drive resistance? How about if I put a cap across the led?

George H.

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To leading order that should make it worse, especially if the cap has a lower impedance than the LED itself at the given drive current. A higher drive impedance might make it better.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

I'm curious: What kind of red LED is this? Mfr/brand and part no.? Any indication of chemistry type, what sort of efficiency, whatever?

I am aware of a couple interesting red LEDs:

1) Part number ER300 or ER-300, I don't know who made it. This is one of the first GaAlAsP red LEDs, made with GaP substrate. GaAlAsP was a breakthrough in high brightness, high efficiency red LEDs that became available in 1985 or so. Of course, now we have red LEDs ~20 times as efficient as the ER300 was, though the ER300 back then was considered laserlike blinding bright (300 mcd at 20 mA).

One interesting thing about the ER300 is that with reverse bias around 9 volts, in my experience they tend to have faint specks of white glow. I have not seen this in other LEDs yet, not even ones of the same chemistry such as Rohm SLA591LT3 (IIRC).

2) "Low Current Red" LEDs, chemistry GaP doped with ZnO. This dates back to the mid 1970's. One common characteristic is peak wavelength being especially long for a visible red LED - usually stated as 690, 697 or 700 nm. Their spectral bandwidth is more broad than usual for visible red LEDs. Examples are TLR147 (an older part number) and everythying I have ever seen by Panasonic listed in the Digi-Key catalog with a wavelength of 700 nm.

These are not always referred to as "low current red". Some post-1985 LEDs mentioned as "low current red" do not have this chemistry.

These have a nonlinearity where efficiency peaks when current is around or under 1 mA, and decreases quite noticeably as current increases past 3-4 mA. They are only a few times as bright at 20 mA as at 2 mA. Sometimes they look not much over twice as bright visually when current is increased by a factor of 10.

One interesting thing about these: Their spectrum has a secondary band peaking in the green (yellowish green) around 555-560 nm. This secondary spectral feature does not lose efficiency with increasing current the way the primary spectral feature does. As a result, many of these that have untinted packages glow orange at currents that are within their ratings.

--
 - Don Klipstein (don@misty.com)

In full overcurrent, the diode would develop destructive hot spots. Below that current, there would be thermal profile fluctuations (and you should be able with a video camera to see light output area getting patchy and fluctuating).

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Thanks for the reply Don, I'll look up the part number on Monday. I was searching on digikey, but after applying lots of filters I still had pages to look at....sigh..

I bought them about a year ago and was searching for the most intensity into a small area.

George H.

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Yeah, I did notice that when running 'over current' (50mA) that the DC light output was *slowly* decreasing. Maybe a part per thousand in 5 minutes?? I didn't write down any numbers. It may not be linear... It was running on my lab bench for several hours, while I did other sutff.

George H.

Yup, but the mechanism has to be something solid-statey like interstitial atom electromigration or dislocation propagation.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

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I didn't get a chance to fry any LED's today. I did grab the digikey package. Mfr. P/N HLMP-EG08-Y2000, digikey num. 516-1377 Mfr is Avago.

The days look busy ahead too. APS March meeting in a few weeks (Any SEDers live in Dallas?)

George H.