Yellow colour in NaCl is due to metal excess defect due to which unpaired electrons occupy anionic sites, known as F-centres. These electrons absorb energy from the visible region for the excitation which makes crystal appear yellow.

In the crystals of FeO , some of the $\mathrm{Fe}^{2+}$ cations are replaced by $\mathrm{Fe}^{3+}$ ions. Three $\mathrm{Fe}^{2+}$ ions are replaced by two $\mathrm{Fe}^{3+}$ ions to make up the loss of positive charge. Eventually there would be less amount of metal as compared to stoichiometric composition.

On heating ZnO loses oxygen as follows

$$\mathrm{ZnO} \xrightarrow{\text { Heat }} \mathrm{Zn}^{2+}+\frac{1}{2} \mathrm{O}_2+2 e^{-}$$

$\mathrm{Zn}^{2+}$ ions and electrons move to interstitial sites and F -centres are formed which impart yellow colour to $\mathrm{ZnO}(\mathrm{s})$.

The gap between conduction band and valence band is small in semiconductors. Therefore, electrons from the valence band can jump to the conduction band on increasing temperature. Thus, they become more conducting as the temperature increases.

On doping germanium with galium some of the positions of lattice of germanium are occupied be galium. Galium atom has only three valence electrons. Therefore, fourth valency of nearby germanium atom is not satisfied and the place remains vacant. This place is deficient of electrons and is therefore called electron hole or electron vacancy. Electron from neighbouring atom comes and fills the hole, thereby creating a hole in its original position. Under the influence of electric field electrons move towards positively charged plates through these and conduct electricity. The holes appear to move towards negatively charged plates.