Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Conductors and valence bands

  1. Oct 8, 2009 #1
    Hi all

    I am reading about conductors and energy bands, and I undertand the math. I just need for it to be more intuitive. This is how I have understood it:

    We look at e.g. a semiconductor at zero temperature made of silicon . Since silicon has 4 valence electrons, these 4 electrons of one Si-atom are in (covalent?) bonds with the electrons of the other Si-atoms.

    Now if we turn on the heat, then the electrons are excited to the conduction band, but what happens to the electron and the bonds? Do the bonds stay the same? Do the electrons get excited to a higher shell or what?
    Last edited: Oct 8, 2009
  2. jcsd
  3. Oct 8, 2009 #2
    The bonds stay the same - the distribution of the electrons with respect to these bonds changes (distribution is temperature dependent).
  4. Oct 8, 2009 #3
    Hey there.

    Please understand the band theory is only an analogy that is used to explain the energy requirements to promote conductivity. In other words, electrons do not PHYSICALLY jump into a conduction band upon conduction. It simply proposes the relative amounts of energy needed to promote conductivity.

    You are right in that an electron is released due to heat, i.e. ONE covalent bond of silicon is broken. This electron then moves depending on the polarity of the superconductor. The other bonds DO NOT break. Electrons simply move from one atom to another. In other words, the JUMP positions and bonds.

    I suggest you have a look at this video. Although, please note there is not physical CONDUCTION BAND. It is simply used to describe electrons that are not bonded to a specific atom.

    Hope this helps. Good luck!
    Last edited by a moderator: Sep 25, 2014
  5. Oct 9, 2009 #4
    Actually, the electron does move.
    The valence band is actually a collection of orbitals which contribute to bonding, meaning that the force of attraction between the silicon nucleii and the electrons in these orbitals is greater than the repulsion between silicon nucleii. These bonding orbitals are lower in energy than that of electrons in free silicon atoms (like in space or something) and can be thought of as a 'constructive interference' of wavefunctions of electrons in free silicon atoms.

    The converse case of 'destructive interference' causes anti-bonding orbitals which are higher in energy. These orbitals make up the conduction band. When silicon is heated, some electrons gain enough energy to be promoted to the conduction band.

    Conduction can only occur if there are free states for electrons to move into. Once promotion has occurred there are many free spaces in the conduction band for the newly promoted electron to move to. Similarly, the hole left behind in the valence band can be moved into by an electron in the valence band.

    Its important to remember that the anti-bonding orbitals are still orbitals of the silicon crystal. They are solutions to the wave equation, meaning that they are possible states which electrons can occupy. All that changes with heat is where the electrons are.
  6. Oct 9, 2009 #5
    I don't think the chemical bonds will stay exactly the same. These bonds will actually be weakened. As the electrons are excited to higher energy states, the system's free energy increases, and the Hellman-Feymann force decreases, namely, the bonds are diminished. But if the band gap is quite large while the temperature is not much high, the bonds will only be very mildly affected.
  7. Oct 10, 2009 #6
    Thank you for this video. It really helped. And thanks to everyone for your comments: They were also helpful.

    I have one final question. Please take a look at this website: http://britneyspears.ac/physics/basics/basics.htm. Please look at the part about doping and donor/acceptor levels (the picture I am refering to is just above the sentence "These shallow level impurities are known as hydrogenic impurities").

    Ok, so we have calculated that the binding energy of the donor-electrons of a donor-impurity is very low. When the donor-electrons are thermally excited, they can contribute to the conduction. Have I understood it correctly that when they are bound to the donor-nucleus (and thus cannot conduct), then they are in the "E_d donor level"? And from this level, they can "switch places" with holes in the conduction band?

    Likewise, the holes in the "E_a acceptor level" can "switch places" with electrons in the valence band? If this is correct, how many electrons can there be in the "E_a acceptor level"?
    Last edited by a moderator: Sep 25, 2014
  8. Nov 6, 2009 #7
    if this Molecular orbital theory of urs works then can u please explain me about p-type and n-type semiconductors(in accordance with bonding and antibonding molecular orbitol)
    if possible can u plz show me the molecular orbital energy level diagram of silicon the bonding and antibonding molecular orbitol

    i cant understand how hole has positive charge totally confused about concept of hole forward and reverse bias plz plz plz waiting for urs answers
  9. Nov 6, 2009 #8
    you may want to check out the first 2-3 pages of "Impurities in type-II staggered InAs/AlSb superlattices" by John D.Dow et al in Superlattices and Microstructures Vol 13 No 4 1993 which gives a pretty detailed explanation of the real meaning of acceptor and donor levels in semiconductors. I found it amazing, but it is quite complicated.
    Remember that the conduction band is really a combination of each antibonding orbital and the valence band is from the bonding orbitals.

    The hole is like a bubble in a sea of electrons. If a nearby electron moves into the bubble, it leaves behind a bubble where it was before. Kind of like, if you fill a jar all the way to the top, but there are a few bubbles. When you tilt the jar, the water moves according to gravity which makes the bubble appear to move the other way.

    The absence of the electron moves in the direction opposite to the direction the electrons are moving. Therefore it appears to have a positive charge equal to the electron's.
  10. Nov 6, 2009 #9
    thanks pseudophonist for ur reply

    i have this doubt and its making me mad
    how does battery work in
    discharge tube, thomsons exp, millikans oil drop exp, gm counter, van de graff gen, vacuum tube etc etc
    in all these devices they say the plates are maintained at positive potential what is the meaning of it .But actually if u see the diagram the circuit is open then how can the battery work or supply charges in open circuit.

    what my hypothesis is that the battery supplies charges which stick on one plate and induce the other plate so that the charges reach back the battery but in the case of van de graff generator my hypothesis flops
    plz plz plz guide me
    also in almost all electronic circuit diagrams the negative terminal of battery is connected to ground why is that

    where can i find this book Impurities in type-II staggered InAs/AlSb superlattices

    check out this site
    http://www.owlnet.rice.edu/~chem152/lecture/ [Broken]
    its cool and explains band theory superbly go to this site and click Supplementary Reading Material
    Last edited by a moderator: May 4, 2017
  11. Nov 6, 2009 #10
    Re: Ground
    Either they mean that the negative is physically connected to the ground, or they are declaring that the negative terminal is the datum of the circuit (this is the point to which voltages are referenced to; voltage or potential difference is always between two nodes)

    Applying a positive potential to a plate with respect to another plate means that positively charged particles between the two will be repelled from the positive plate. There is an electric field between the plates. Whether this potential is generated using a battery or a van de graff generator doesn't matter, it still operates the same way.

    When the battery is connected, a very tiny electron current flows but gets jammed up at the negative plate. As this plate charges up it repells the current trying to flow until the current drops to zero. (basic principle of a capacitor) In the steady state there is a net charge, a non-zero voltage/potential, and no current
  12. Nov 6, 2009 #11
    Re: Battery

    see the attachment
    plz explain the function of battery and ground in the picture
    my doubt was that in discharge tubes the 2 plates are connected to some voltage source
    let it be battery for instance see the fig in this case between the two plates there is empty space then what does the battery here do
    does it send electrons to cathode which deposits on it and the cathde induces +charge on the anode and electrons return back
    that is my doubt am i right or wrong
    but when i consulted my physics teacher he told me that it cant induce to such long distances , he told that what happens is that electrons in anode are attracted by +terminal of the battery and when they reach the +terminal they repel the electrons in the - terminal which goes to the cathode as a result of repulsion
    WHO IS RIGHT HERE PLZ PLZ PLZ explain pseudophonist

    Attached Files:

  13. Nov 6, 2009 #12
    In the case of the cathode ray tube, the battery establishes a potential difference, and electric field, between the cathode and anode plates. electrons travel from the negative terminal of the battery to the cathode plate. They are ejected from the cathode plate and accellerate toward the anode plate through the gas (or possibly vacuum) of the tube. They are then absorbed by the anode plate and return to the battery via the positive terminal.

    The cathode doesn't induce a charge; the battery sets up all the potentials.
  14. Nov 7, 2009 #13
    Re: establishing a potential difference

    thats what my question is how does it establish a potential difference what do u mean by establishing a potential difference how does it generate electric field between the plates
    what is the meaning of potential difference here
    also could u also explain my doubt in that van de graff that i mention before
  15. Nov 7, 2009 #14
    Last edited by a moderator: May 4, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook