$A$ solid which is not transparent to visible light and whose conductivity increases with temperature is formed by

Choose the correct statement. In conductors,

In a semiconducting material,the mobilities of electrons and holes are $\mu_e$ and $\mu_h$ respectively. Which of the following is true?

Choose the correct statement. In semiconductors,the valence band and conduction band

In an extrinsic $n$-type semiconductor,the free electrons donated by the impurity atoms occupy energy levels in

Assertion $(A)$: When a wire of aluminium and another wire of silicon are heated from room temperature to $80^{\circ} C$,the conductivity of aluminium decreases and that of silicon increases.
Reason $(R)$: Aluminium has a positive temperature coefficient of resistivity and silicon has a negative temperature coefficient of resistivity.