Semiconductivity
Materials with band gap of less than
Holes
When electrons jump into conduction band, it leaves a hole in the valence band. The hole can be treated as a positive charge, having an equal and opposite charge value as an electron.
Under the influence of an electric filed, an electron in valence band can jump into a hole, creating new hole in the electron’s original position.
Both electrons and holes contribute to the current flow of a semiconductor.
Types of Semiconductors
2 types based on the crystal structure.
- Intrinsic: Made of a pure compound (no dopants)
- Extrinsic: Made of a pure compound and a dopant
Intrinsic Semiconductors
Fermi level lies in between conduction and valence bands. At higher temperatures some electrons can be thermally excited and jump to conduction band.
Conductivity of Intrinsic Semiconductors
Because holes and electrons are equal in count:
Here:
- conductivity - carrier concentration - electron’s charge - carrier mobility of electrons - carrier mobility of holes
In intrinsic semiconductors, conductivity is low due to small number of charge carriers.
Extrinsic Semiconductors
A doped semiconductor. Has more conductivity compared to intrinsic semiconductors.
Doping
Introduction of a foreign atom (impurities) into a intrinsic semiconductor. The foreign atoms are usually either pentavalent (i.e. Sb, P, As) or trivalent (i.e. B, Ga, In).
2 types based on the dopant.
- n-type: When dopant is a pentavalent atom.
- p-type: When dopant is a trivalent atom.
n-type
Increased number of electrons. Fermi level is shifted upwards, due to extra electron energy states (donor state).
At room temperature, thermal energy is sufficient to excite the electrons from donor states.
p-type
Increased number of holes. Extra hole energy levels are introduced above valence band (acceptor state).
At room temperature, electrons in valence band can jump into acceptor state, facilitating movement of holes