What is the crystalline structure of an intrinsic semiconductor?

The valence band and conduction band of a solid overlap at low temperature. The solid may be:

What is the maximum wavelength of electromagnetic radiation that can create an electron-hole pair in a material with a band gap of $0.7 \ eV$? (Planck's constant $h = 4.136 \times 10^{-15} \ eV \cdot s$, velocity of light $c = 3 \times 10^8 \ m/s$).

Choose the correct statement.

$A$ piece of semiconductor is connected in series in an electric circuit. On increasing the temperature,the current in the circuit will

Pure sodium $(Na)$ is a good conductor of electricity because the $3s$ and $3p$ atomic bands overlap to form a partially filled conduction band. By contrast,the ionic sodium chloride $(NaCl)$ crystal is: