In which of the following is an ionic bond present?

$A$ solid which is transparent to visible light and whose electrical conductivity increases with temperature is formed by:

In a semiconductor,which of the following statements are correct?
$(i)$ There are no free electrons at $0\, K$.
$(ii)$ There are no free electrons at any temperature.
$(iii)$ The number of free electrons increases with temperature.
$(iv)$ The number of free electrons is less than that in a conductor.

The resistance of a semiconductor at $0 \ K$ is:

If pieces of copper and germanium are cooled from room temperature to $77 \ K$,their resistance will:

At absolute zero temperature,an intrinsic semiconductor behaves as