idea to make an LED with variable light output frequency

A semiconductor junction makes a pretty good capacitor. You can try forcing a lot of voltage across it, unfortunately those darn electrons keep slipping past, and you need exponentially greater currents to increase the bias (plus linear voltage to overcome resistive losses in the bulk semiconductor and interconnects).

That said, green LEDs (GaN) turn yellow and orange nicely when pushed well beyond rated current.

Tim

-- Deep Friar: a very philosophical monk. Website:

For a diode bandgap of 2V over nanometers, it would require a kV electric field from the capacitor to have much effect on the bandgap voltage, but if the bandgap voltage was only be modulated 1% (+-0.02V), that would still allow a multi-GHz RF modulation to be put directly into the light which could be good for fiber optic data transfer.

cheers, Jamie

Can the bandgap of a semiconductor be changed with an applied electric field?

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If you're doing it for fiber, why not use a laser diode and modulate the cavity length. It may take less magic than modulating the material bandgap, and the line width would be tighter to start with.

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I've seen red LED's turn Org when the ambient temperate around it gets hot!!!!!!!!!!! Maybe its a design of the LED, who knows.

JAmie

Try dipping them in LN2.

Best regards, Spehro Pefhany

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"The Journey is the reward"

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Yeah the band gap is pretty much set by the atomic spacing. So, I think, you might be able to squeeze on it and change the wavelength. Or change the temperature as Spehro suggests. I've heard that most LED's move to higher wavelengths as you cool (shrink) them, but some go the other way. Yellow's become orange, or something like that.

George H.

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Hi,

Instead of using a capacitor to try to change the bandgap properties, it would be better to use a miniature high-gain antenna that has its beam focused onto the bandgap directly, and then modulate the antenna RF for the desired light modulation.

cheers, Jamie

On a sunny day (Tue, 08 Nov 2011 20:31:10 -0500) it happened Jamie wrote in :

I have some Nichia blue LEDs here (you have seen them in my radiation counter AVI movie perhaps), and the datasheet gives a forward current versus wavelength graph on page 10 (of

15). It changes from about 474 nm at 1 mA, to 486 nm at 100 mA. It also changes from 469.5 nm at -40 °C to 472 nm at 80 °C for 20 mA constant current.

So it seems the color change is more for a delta I. google (is on conrad.nl site): 180763-da-01-en-LED_3MM_BLAU_15_8_200_MCD_NICHIA.pdf or look up the LED type directly: NSPB300B

AVI movie perhaps),

(of 15).

current.

LEDs are not particularly monochromatic with an output bandwidth of typically 50nm either side of the nominal peak frequency. Some of them have very wide wings - the first (pale) green ones originally contained a lot of yellow as any basic spectroscope would quickly show.

I'd be quite interested to find a 20nm or narrower high intensity amber/yellow LED centred on 590nm or as close as possible to it. There is a Cree XP-C part that isn't too far off the mark still a bit too much of a tail into the green for what I want to do though.

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Regards,
Martin Brown

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^^^^^^^^^ oops frequencies, (shorter wavlengths..sorry)

ounter AVI movie perhaps),

ge 10 (of 15).

A constant current.

Maybe add an interference filter on the output? You can get ~80% transmission in a 10nm window. Or does that 'blow the budget'?

George H.

counter AVI movie perhaps),

(of 15).

constant current.

Basically yes. I use a Schott colloidal glass OG590 low pass filter at present which sees off the unwanted green very well, but it would be nice to avoid any extra optics (and associated cost).

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Regards,
Martin Brown

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Hi Jamie, Do you have any solid state physics books? It's not easy to change the band gap of a material. Just reading quickly in Sze's "physics of semiconductor devices" That the bandgaps of Ge and GaAs increase with pressure and for Si it decreases (go figure). We temperature tune laser diodes, but it's only about 0.25 nm/deg C

George H.

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Hi George,

Here is another way to change the bandgap properties:

cheers, Jamie

The nomenclature is a bit confusing; that is a description of modulating the field of the depletion region, and the BANDGAP voltage is a pressure-modified variant of the electronic orbital energies. You can change the bandgap with pressure (very high pressure), but not with an electrode.

When the bandgap is zero, the material becomes a conductor rather than a semiconductor (this is called the Mott transition).

Hi,

The stark/zeeman effect show that the bandgap electron orbital energies can be changed with an electromagnetic field:

cheers, Jamie

Hi,

Is it correct to think of the bandgap as an "electron accelerator" that doesn't actually have any electron orbital transitions during operation?

cheers, Jamie