Definitions [7]
The material with electrical conductivity between that of a conductor and an insulator, whose number of charge carriers can be controlled as per requirement, is called a semiconductor. (e.g. Silicon, Germanium)
The different energy levels with continuous energy variation are called energy bands.
The range of energies possessed by valence electrons is called valence band.
The range of energies possessed by conduction electrons is called conduction band.
The energy difference between the valence band and the conduction band is called forbidden energy gap.
The solids which have a large number of free electrons are called conductors. (e.g. Iron, Aluminium)
The solids which have very small number of free electrons are called insulators. (e.g. Glass, Wood)
Key Points
- Conductors → Eg = 0 - bands overlap, electrons flow freely.
- Semiconductors → Eg < 3 eV — small gap, conducts at room temperature.
- Insulators → Eg > 5 eV — large gap, no conduction.
- Ge = 0.72 eV, Si = 1.1 eV — both semiconductors.
- Metal conductivity decreases with temp. Semiconductor conductivity increases with temp.
