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2.18 Electronic Disorder

Unlike intrinsic point defeats, intrinsic electronic defects are optically or thermally created.
This occurs in materials having a forbidden energy gap between conduction and the valence band and are categorized as semiconductors and insulators (see footnote**).
Defect density is in number per unit volume of the crystal.
Disorder implies elevation of electrons into higher energy levels creating vacant states in lover energy bands which are called as holes.

Excitation of electrons across the bandgap into conduction band
Bandgap (Eg) for semiconductors is typically below 2.5 eV e.g. Si has bandgap of ~1.1 eV whereas for insulators it is typically above 2.5-3 eV.
The band diagram for a semiconductor or insulator can be seen below.

Figure 2.8 Band diagram for an insulator

Band gap energy values for a few selected materials are shown in the table below:

Si	1.1 eV	NaCl	7.3 eV
Ge	0.7 eV	MgO	7.8 eV
Diamond	5.4 eV	NiO	4.2 eV
GaAs	1.43 eV	FeO	2.0 eV
GaP	2.25 eV	BaTiO₃	2.8 eV
BN	4.8 eV	TiO₂	3.0 eV
CdTe	1.44 eV	UO₂	5.2 eV
ZnO	3.2 eV	SiO₂	8.5 eV
	3.1 eV	MgAl₂O₄	7.8 eV

** Basically, materials having a well defined band gap show conduction band, band of higher energy and valence band, bands of lower energies with maximum of valence band and minimum of conduction band separated by the forbidden energy gap i.e. E_g. The position of Fermi energy, E_F, lies in this forbidden gap. At 0 K, all the states in the valence band are filled while the states in the conduction band are empty. Another way to express this is that all the energy states below E_F are filled while those above E_F, are empty at 0 K. It is just that for basic physics reasons, carriers cannot reside in the forbidden energy gap.
Elementary physics of bands in materials can read from any book related to solid state physics or electronic properties of materials as listed in the bibliography.