Various textbooks mention, but not go into detail, how semiconductor devices are optimized for their particular function. E-k space is trascendental to understand this, given that it depends on the direct and indirect nature of the semiconductor. Yet, I am confused regarding this part. I am specially interested in optoelectronic devices (photodiode, LED, solar cell, and semiconductor laser). LEDs are made of direct semiconductors, because electron hole recombination can occur without phonon participation. Solar cells can be made of both. In solar cells you dont want any type of recombination. How does the directness or indirectness of the material play a role here?

  • $\begingroup$ Would Electrical Engineering be a better home for this question? $\endgroup$
    – Qmechanic
    Jan 26, 2015 at 20:46
  • $\begingroup$ I'm unclear what exactly is being asked for. Since E-k space was mentioned I'm not sure this is a device question, but what physics enlightenment is wanted is not clear. $\endgroup$
    – Jon Custer
    Jan 26, 2015 at 21:08
  • $\begingroup$ @JonCuster: e.g. on which grounds is a certain material chosen for a certain device. It basically has to do with recombination. A LED should have a lot of radiative recombination and not the other types. In a solar you don't want any type of recombination. How do you manage to do that? I know that silicon cells are very thick because the material is indirect (reduced absorption coefficient due to phonon participation in eh-pair generation). But wouldn't that also mean that thin film cells, which are direct, would also be bad, because recombination is also easy? $\endgroup$
    – nomadStack
    Jan 26, 2015 at 21:54
  • $\begingroup$ @nomadStack you should add these details to your question to make it specific. E.g. "why do direct band gap materials make good LEDs" or "why can solar cells be made from both direct and indirect materials" are both excellent questions. The way it's written at the moment make it hard to answer without a very general rambling post. $\endgroup$
    – boyfarrell
    Jan 27, 2015 at 14:16

1 Answer 1


The main design parameters (at least on a conceptual level) for solar cells are the band gap energy and the minority carrier diffusion length. The former determines at which point in the solar spectrum the semiconductor starts absorbing light, the latter determine how far minority carriers diffuse before recombining. The goal of a solar cell is to have the photogenerated minority carriers cross the junction before they recombine.

Direct band gap materials have strong optical transitions between the valence and conduction band. However indirect materials have fairly weak optical transitions. This is because absorption and emission of a photon must occur with the simultaneous absorption or emission of a phonon (thus conversing momentum).

If you compare the design of a GaAs (direct material) solar cell to a Si (indirect material) then you will find that Silicon cells are much thicker: on the order of hundreds of microns. This is done to compensate for much weaker absorption. Moreover, because Silicon is a poor absorber of light, simply having a greater thickness means that you can absorb nearly all of incoming photons.

On the surface this answers your questions. However there is another level of detail.

Considering only optical properties, it is clearly advantageous to have a thick active layer. However, if you made a GaAs or Silicon solar cell much thicker the efficiency, counterintuitively, would decrease! This is because of the minority carrier diffusion length.

The minority diffusion length of carriers in Silicon is very long, mean carriers can move hundreds of microns before spontaneously recombining. This it is possible to get a good balance of optical generation and carrier collection with a thick active layer.

However, the minority carrier diffusion length in GaAs is very short, on the order of tens of microns. By good fortune, GaAs has a large absorption coefficient and so cells only have to be several microns thick to achieve a good balance between absorption and carrier collection.

In summary, it's all about balancing optical absorption, by changing thickness, and carrier collection, by making sure the thickness is smaller than the minority carrier diffusion length. Provided you can achieve this balance you can make solar cells from direct or indirect materials.

  • $\begingroup$ Would this imply that if a direct semiconductor that is thicker than carrier diffusion length would work as a solar cell that emits light? In a LED you want radiative recombination. In a solar cell you don't want any type of recombination. How do you control which recombination mechanism occurs? SRH is pretty straightforward (make a crystal as perfect as possible). But how about inducing radiative over Auger? $\endgroup$
    – nomadStack
    Feb 1, 2015 at 0:57
  • $\begingroup$ These are all good questions but will take time to discuss and don't lead themselves well to the comment box. Why not synthesis this into a new question about recombination? $\endgroup$
    – boyfarrell
    Feb 1, 2015 at 9:24
  • $\begingroup$ @boyfarell: Done: physics.stackexchange.com/questions/162931/… $\endgroup$
    – nomadStack
    Feb 2, 2015 at 10:47
  • $\begingroup$ @nomadStack OK, nice question! Typed you something over my lunch break. Upvotes and accepted answers are appreciated. $\endgroup$
    – boyfarrell
    Feb 2, 2015 at 12:41

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