Why LEDs Have a Broadened Spectra?

[Lee]

Why is it that LEDs do not sharper spectra? I've been considering this recently, and was wondering if it was due to the uncertainty principle (like homogeneous broadening in a laser) or is it due to a fluctuating band gap? I've searched high and low and can't find any explanation for the broadening in LED spectra.

 

[cesiumfrog]

As I understand it, the main difference between a diode laser and an LED is that one is additionally an optical resonator. But ignoring that, I don't know what mechanism most contributes [to the LED] line width (if I had to guess, I'd say the band gap transition isn't just between discreet energy levels, unlike in an elementary gas where broadening might have to be attributed to Doppler).

 

[Lee]

Yeah, with a LED laser you sheer each side so they have some sort of reflectivity, creating your cavity, and rather conveniently the depletion region has a different refractive index from the rest of the material creating a waveguide (for small angles) which helps to confine the light to the desired region.

I know that you can get 2 types of broadening in lasers, homogeneous and inhomogeneous, one is attributed to Doppler shifts and the other to the uncertainty principle. I think, it might be through the variable nature of the band gap within the PN junction, that with each photon created the depletion range will change and causing the energy of the next photon to be released to be different, and this causes the broadening. Is this the primary cause?

 

[Cthugha]

I know that you can get 2 types of broadening in lasers, homogeneous and inhomogeneous, one is attributed to Doppler shifts and the other to the uncertainty principle.

Doppler shifts are usually taken into account, if you have a look at gas lasers or something similar. In solid state lasers the case is a bit different.

The theory of the linewidth of semiconductor lasers depends strongly on coupling between intensity and phase. Spontaneous emission occurring in a laser operating at steady state will introduce some change in the gain and the refractive index, which leads to relaxation oscillations and to linewidth broadening.

Maybe this classical paper might help you:

C. H. Henry, "Theory of the linewidth of semiconductor lasers", IEEE J. Quantum. Electron. 18 (2), 259 (1982)

 

[Lee]

That's very interesting. Will the broadening within the spectrum of the everyday LED also be caused by the coupling between the intensity and phase?

 

[Claude Bile]

LEDs have a broad output spectrum because the gain spectrum is broad, the gain spectrum being determined (I think) by the band structure of the semiconductor.

I think you are going down the wrong path by looking at laser broadening mechanisms if you are looking at LEDs.

Claude.

 

[Dr Transport]

As per Chuang, Physics of Optoelectronic Devices, the difference between an LED and a Laser Diode is the role of spontaneous emission and amplified spontaneous emission. He details everything nicely starting on page 405. I believe that Sze's The Physics of Semiconductor Devices also does a good job of explaining the differences, but that text is on my desk at work so I can't get into it until later.