In the band structure of $n$-type semiconductor, the free electrons donated by 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.

If $Cu$ (Copper) and $Ge$ (Germanium) are cooled to $70 \ K$,then:

Read the following statements carefully:
$Y:$ The resistivity of a semiconductor decreases with an increase in temperature.
$Z:$ In a conducting solid,the rate of collision between free electrons and ions increases with an increase in temperature.
Select the correct statement from the following:

Although carbon,silicon,and germanium have the same lattice structure and four valence electrons each,their band structure leads to the energy gaps as:

The resistivity of a semiconductor at room temperature is in between