Why is the necessary energy for a photon to lift an electron higher than the band gap energy? The band gap energy of silicon is around 1 eV and though the required energy for a photon to lift an electron up into the conduction band is around 3.6 eV.
Why is this?
Is the Energy of an absorbed photon exactly the energy of the band gap? is quite similar but - whyever - they do not answer it respectively do not use any material.
 A: Silicon has an indirect band gap. This means that although there is a conduction-band state which is only 1eV above the top of the conduction band it occurs at a different Bloch momentum ${\bf k}$. The nearest state with the samae ${\bf k}$ value is 3.6eV above the top of the valance band. Photons have a wavelength $\approx 600\mu$ that it is much larger than the inter-atom spacing and so their crystal momentum ${\bf k}$ is much smaller than the size of the Brillouin zone. Their momentum  is therefore effectively zero as far as band theory is concerned. Therefore, for single-photon absorbtion with none of the energy going into phonons (to make up the momentum change) you need 3.6 eV photons.  
LED's and other devices that play well  with light are made of III-V or (or even II-VI) materials such as Gallium Arsenide or Indium Arsenide that have direct band gaps, meansing that the lowest energy conduction-band state has the same ${\bf k}$ as topmost valence band state.
There is some discussion of this in he Wikipedia page on Direct and indirect band gaps      
