Energy Band Theory
In every atom, electrons occupy discrete energy levels in atomic orbitals, arranged into shells and subshells. In an isolated atom, electrons occupy well-defined energy states.
When
Energy bands in a solid
- Valence band: The highest energy band that can be occupied by electrons at
. - Conduction band: The empty band just above the valence band.
At higher temperatures, electrons which have sufficient energy can jump to conduction band. At this energy level, electrons can move freely through the material.
Fermi Energy
The energy level which is occupied by the highest electron orbital at
In absolute zero temperature, electrons settle into lowest available energy states and build a Fermi Sea of electron energy states. Fermi energy is the surface of this sea and no electron has energy to rise above this surface.
Fermi Level
The energy level where Fermi-Dirac distribution equals to 0.5.
The closer the Fermi level is to the conduction band energy, the easier it will be for electrons in the valence band to transition into the conduction band.
For an electron to become free, it must be excited or promoted into one of the
empty and available energy states above
Fermi-Dirac Distribution
In a system which is in thermodynamic equilibrium, the probability of finding an
electron in a single energy state
Here:
- Fermi level - Energy of the th energy state - Boltzmann constant - Absolute temperature
At T=0
: No electrons above fermi level : All electrons are below fermi level
At T>0
There is more chance that the electrons can be available in conduction band. The chance increases with increasing temperature.