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Semi-conductors and how they work?

  1. Jul 18, 2011 #1
    Hey guys!

    I'm researching and writing notes on semi-conductors and how they work. I've learnt about the electron energy profile for conductors, insulators and semi-conductors and have a basic understanding on energy bands of each and how they're put together.

    I'm having difficulties learning how semi-conductors actually conduct electricity. I've read that they have a small energy gap and therefore, thermal energy, at room temperature, is enough to excite electrons in the valence band and jump them into the conduction band, ready for electricity. However, as they jump over due to enough quantised energy provided, they leave positive 'holes' behind. These 'holes' and the electrons provide current flow.

    I don't quite understand how the positive holes can provide the current (even though current is the flow of CHARGES), if the band theory is about the energy levels of electrons and what they can exist in when their atoms form a solid, how does valence band conduct current? isn't it just a band of energy levels or does the the band of energy levels refer to the structure of the solid and where the electrons of the solid is placed (i.e in chemical bonds)?

    Thanks in advance for anyone who helps!
     
  2. jcsd
  3. Jul 18, 2011 #2
    The whole key to understanding this is to move on from atoms.

    I don't know if you have studied molecular orbitals in bonding ?

    When atoms combine to form molecules their individual energy levels combine to form a molecular energy level which spans the whole molecule. It is no longer attributable to an individual atom.

    Take this process one stage further. When atoms or molecules combine in the bulk to form solid masses, their individual energy levels combine to form energy levels spanning the whole solid mass from end to end.

    We call these super levels bands.

    An electron in one of these bands can therefore roam the entire solid mass, according to this band theory.
    That is how conduction takes place.

    This applies to all three conductors, semiconductors and insulators. The bands exist for all substances. It then becomes a question of are there any electrons and how many in a given band.

    If this helps let us know and someone can continue with part II about holes.
     
  4. Jul 18, 2011 #3
    We haven't done electron orbitals as such but we've learnt on a basic level that atoms contain energy shells which correspond to their potential energy levels. The higher the electron shell from the nucleus, the higher the energy the electron contains.

    However, I've also read that when atoms combine and form bonds, their individual electron energy and energies available combine to form a continuous levels of energy states in the molecular form or solid form. Yes but I don't understand how this happens or what exactly happens, we've just been told.

    So, these energy "bands" are an attribute to the whole solid mass?
     
  5. Jul 18, 2011 #4
    Exactly so.

    I should add that it is only the outer electrons in an atom's shells that take part in bonding or form bands. The inner ones remain with their respective atoms. This will usually restrict the contribution per atom to 3,4 or 5 electrons at most.
     
  6. Jul 18, 2011 #5
    Okay, I understand that but could you explain how atoms combine to form these molecular energy bands?

    I'm having troubles understands how solids or moleculars can combine energy levels to form energy bands
     
  7. Jul 18, 2011 #6
    It is a good question so it would be worth taking a step back and asking why rather than how.

    Take a single atom.
    You can draw a ladder diagram for the energy levels.

    You populate the levels with whatever electrons there are, starting at the lowest.
    Starting at the lowest is a basic principle of physics systems - try to achieve minimum energy.

    Now take a second atom with a similar ladder diagram and place the two side by side.

    You cannot do this with any two random atoms and expect them to bond. They will only bond if the resultant molecular energy levels are lower than those in the original atoms. If this condition is met the atoms will likely bond (there may be other factors) forming a molecule with lower energy levels and releasing the difference in energy as heat and perhaps light.

    Now there are rules which say that each level can only accomodate one electron so if you add the electrons from each atom you need twice as many levels as with a single atom. there are thus twice as many levels in a molecule. But the levels come closer together.
    In a single molecule that is still not many levels, but when we move to an array of billions of atoms you can see there are many mnay levels required. However the levels are so close they form bands.

    I have tried to illustrate this in the sketches.
     

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  8. Aug 1, 2011 #7
    I'm very sorry for the late reply, I was held up by revising maths and totally forgot about physics. Anyways, Thanks for the explanation and the visual sketches, it really helped me a lot.

    My first thoughts about this weren't wrong but I needed additional information to piece all the loose info together.

    Thanks Studiot for the help, I appreciate it!
     
  9. Aug 1, 2011 #8
    I'm very sorry for the late reply, I was held up by revising maths and totally forgot about physics. Anyways, Thanks for the explanation and the visual sketches, it really helped me a lot.

    My first thoughts about this weren't wrong but I needed additional information to piece all the loose info together.

    Thanks Studiot for the help, I appreciate it!
     
  10. Aug 1, 2011 #9
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