The resistivity of a semiconductor depends on $........$.

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

$A$ solid reflects incident light and its electrical conductivity decreases with temperature. The binding in this solid is

When the temperature of a semiconductor is increased,its resistance and electric conductivity respectively.

This question has Statement $1$ and Statement $2$. Of the four choices given after the Statements,choose the one that best describes the two Statements.
Statement $1$ : $A$ pure semiconductor has a negative temperature coefficient of resistance.
Statement $2$ : On raising the temperature,more charge carriers are released into the conduction band.

Give an explanation of energy band,valence band,and conduction band in a crystal.