2.12. LOCALIZED STATES IN SOLIDS 85

Ev Ec

Valence band

Perfect structure

ELECTRON ENERGY

D

ENSITY OF STATES

(a)

Ev Ec

Structure with Defects

ELECTRON ENERGY

D

ENSITY OF STATES

(b)

Defect states

}

Conduction band

Valence band Conduction band

No bandgap

states

Figure 2.39: Schematic of density of states (a) in a perfectly periodic solid and (b) in a material
with defects. The presence of bangap states influences semiconductor devices.