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Metals, semiconductors and insulators: MO theory and band theory

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... bands result - a band of bonding MO's filled with electrons (valence band) and ... The separation in energy of the two bands is the band gap ... – PowerPoint PPT presentation

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Title: Metals, semiconductors and insulators: MO theory and band theory


1
Chemical Bonding Theory
  • Metals, semiconductors and insulators MO theory
    and band theory
  • Solid bulk samples of metals have a large number
    of MOs because many AOs from each metal atom
    are available to form LCAOs
  • The resulting MOs are closely spaced in energy
    and are said to produce a band of delocalized
    MOs spread out throughout the bulk solid
  • Each MO in a band can accommodate 2 electrons

2
Chemical Bonding Theory
  • In a metal, there are not enough electrons to
    fill all the orbitals of the band
  • The highest occupied band is called the valence
    band
  • The state with lowest energy - electrons filling
    the lowest energy orbitals of a band - occurs
    only at 0 K
  • The highest energy level occupied at 0 K is the
    Fermi Level
  • A slight input of energy will promote an electron
    to a higher orbital resulting in two
    half-filled MOs, one above the Fermi level and
    one below the Fermi level
  • Movement of electrons in these half-filled levels
    close the the energy of the Fermi level is
    responsible for electrical conduction in metals

3
Chemical Bonding Theory
  • Metals, semiconductors and insulators MO theory
    and band theory
  • The energy difference between the MOs in a band
    is very small and a continuum of energy states
    is produced
  • There are electrons that can absorb light of
    almost any wavelength
  • Absorption of light excites the electron, but it
    can immediately reemit a photon equal in
    wavelength to the incident, exciting photon
  • This produces the metallic luster characteristic
    of metals

4
Chemical Bonding Theory
  • Nonmetals behave as insulators because the
    valence band is completely filled with electrons
  • The next highest MOs are a band of anitbonding
    MOs at much higher energy
  • Promoting an electron in such a solid is not
    likely and the solid is not electrically
    conductive
  • Example the bonding in diamond can be considered
    to consist of a band of bonding MOs made up of
    C atom AOs spread out over the entire crystal
  • Two bands result - a band of bonding MOs filled
    with electrons (valence band) and a band of
    empty anitibonding MOs (conduction band)
  • The separation in energy of the two bands is the
    band gap

Empty levels Conduction band
Band gap
Filled levels Valence band
5
Chemical Bonding Theory
  • Metals, semiconductors and insulators MO theory
    and band theory
  • Semiconductors have electrical conductivities
    between metals and insulators
  • The band gap is smaller (50-300 kJ/mol) than
    insulators (500 kJ/mol)
  • Some electrons can be promoted to the conduction
    band
  • Si and Ge are intrinsic semiconductors
  • When an electron is promoted, a positive hole is
    created in the valence band
  • Both the electrons and holes carry charge - in
    opposite directions
  • A hole moves as a nearby electron fills the hole
    creating a new hole some distance away
  • As the temperature increases, more electrons
    occupy the conduction band

6
Chemical Bonding Theory
  • Metals, semiconductors and insulators MO theory
    and band theory
  • Semiconductors
  • Extrinsic semiconductors have dopants added to
    control conductivity
  • Substituting some of the Si atoms with Al or B
    atoms result in some bonds in the crystal to be
    electron deficient
  • Group 3A elements have 3 valence electrons
    instead of the 4 valence electrons in Si or Ge
  • A new band - the acceptor level - is created at
    slightly higher energy than the valence band
  • Electrons are easily promoted to the acceptor
    level and positive holes are created in the
    valence band
  • Such semiconductors are called p-type
    semiconductors

7
Chemical Bonding Theory
  • Metals, semiconductors and insulators MO theory
    and band theory
  • Semiconductors
  • Extrinsic semiconductors have dopants added to
    control conductivity
  • Substituting some of the Si atoms with P atoms
    results in some bond in the crystal to have
    extra electrons
  • P has one more valence electron than Si or Ge
  • A new, filled band - the donor level - is
    established just below the conduction band
  • Electrons are easily promoted to the conduction
    band and carry electric current
  • Such semiconductors are called n-type
    semiconductors because they make use of
    negative charge carriers

8
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