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Ionization vs. photo-generation - what is the difference?

  1. Nov 25, 2009 #1
    Hello, I am new to the forum.. I hope someone can help me with this problem cause this has been killing me for a while now..

    photo-generation is excitation of electrons into the conduction band when a photon interacts with an electron.. energy required for photo-generation is equal (or greater than) the band-gap energy of a material --> 1.1eV Silicon.. it results in creation of an electron-hole pair.. thats how solar cells work..

    ionization is a removal (or addition) of an electron from an atom.. energy required for ionization is 2-3 times greater --> 3.7eV Silicon.. it results in creation of an electron-hole pair in the crystal lattice as well..

    now.. donor atoms in doped semiconductors are considered ionized in room temperature; electrons from these atoms are in the conduction band, allowed to move freely through the crystal lattice; just as photo-generated electrons that are excited to the conduction band by light of energy above the band gap energy.. but we don't consider photo-generation - ionization, it is not the same..

    so whats the difference? is it that ionization radiation elevates an electron very high into the conduction band?

    thanks for any comments on this topic.. id really appreciate any help..

    cheers, woj..
     
  2. jcsd
  3. Nov 26, 2009 #2

    ZapperZ

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    You have some things here mixed up in very different situations.

    Your definition of ionization is valid, but these are ONLY applicable to ISOLATED ATOMS. While you can certainly ionize solids, the mechanism here is slightly different because of the presence of BANDS. For example, I need only to overcome the work function of a solid to cause the emission of an electron. But if you look at the ionization energy of the atom that makes up that solid, you'll need a different energy than the work function.

    The point here is that in solids, there are COLLECTIVE EFFECT that are more dominant than the behavior of individual atoms. So your mixing of "ionization of atoms" and then "electron hole pairs in lattice" doesn't quite makes sense, since "lattice", by definition, is a congregation of many, many atoms, i.e. a collective state.

    Going back to your question, the donor atoms providing electrons in the conduction band is usually due to thermal excitation, i.e. the ambient temperature (room temperature) of the solid is sufficient to give those electrons enough energy to be in the conduction band. But this doesn't cause the donor atoms to be ionized since, technically, on average, it is still neutral and the electrons it donated have not left the solid. After all, do you consider copper atoms as being ionized even though the valence electrons are not localized at any particular copper atom? Same thing here.

    Zz.
     
  4. Nov 26, 2009 #3
    thanks for the clarification..

    ionization of a solid refers to the emission of charge carriers from the material.. and the energy needed for them to escape is the work function that is higher than the band gap and lower than the ionization energy for a single atom (because of collective effects between multiple atoms in close proximity)..

    photo-generation just increases the amount of charge carriers in the conduction band..

    the reason why I mixed it up, was that a book that I was reading referred to ionization energy as the energy needed for electron emission from a solid.. while technically one should use work function..

    moreover, donor atoms are often referred to as ionized, but thats probably because of the build-in potential they create as a result of carriers diffusion during PN-junction creation..
    I kinda see why one could refer to them as ionized, but thats again incorrect..

    Thanks a lot for putting few things in order for me..

    Cheers.. woj
     
  5. Nov 27, 2009 #4
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