SCEN103 Class 15
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Introduction to Electronic Band Structure
To continue our path of trying to understand the physics of semiconductors,
we must begin to bring many atoms together to form a solid.
- Pauli exclusion principle still holds!
- Electrons on neighboring atoms "overlap" and must shift energies
slightly to that each stays out of the others' states.
---- more detailed quantum physics
- Energy levels "blur" into energy bands.
Conductor or Insulator?
- Depends on separation of bands and
whether band is full or not.
- With N atoms making up the solid,
each band may hold 2N electrons.
---- related to filling of electronic orbitals
- Consider example of following two elements,
referring to our abridged Periodic Table
|| 1s2, 2s2, 2p6
|| 1s2, 2s2, 2p6 3s1
|| or [neon] 3s1
- Since the number of electrons in sodium is odd,
the last band up is only half-filled.
The even number of electrons in neon fills the final band.
So how does this help explain why sodium is metallic and
solid neon is an insulator?
- To have an electrical current in a solid, conduction electrons
must be able to move through the solid by addition of some energy
from an applied voltage.
- The energy level of conduction electrons can be approximated
by the thermal energy, 0.025 eV at room temperature,
smaller for lower temperature.
- The relative separation of bands must be considered;
How far away is the next unfilled band?
- Remember that electrons are not permitted in the forbidden bands
- In neon,
- The energy separation between the nearest empty band
and the closest filled band is around 20 eV.
- For conduction to take place in neon, the electrons in the upper most band,
completely filled, would have to
"jump" across the band gap of 20 eV,
very unlikely with only 25 meV of energy kT!
- In sodium,
- The band is partially filled, thus a "little bit" of energy can be added to
the electrons -- no quantum jump is involved.
- The final electrons "added" to make sodium, the 3s1 electrons,
are very easy to strip from the "neon" ion core.
They become the conduction electrons, often known as the free electrons
in the metal.
- Note the orbital configuration of the following elements that
happen to be excellent conductors as solids:
|| [neon], 3s2, 3p1
|| [argon], 3d10, 4s1
|| [krypton], 4d10, 5s1
|| [xenon], 4f14, 5d10, 6s1
- This is not to say that all conductors have one electron in an
unfilled band -- but the best conductors do!
My article on
Insulators written for encyclopedia
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Last updated Jan. 28, 1998.
Copyright George Watson, Univ. of Delaware, 1996.