What are elementary semiconductors?

The explanation of the band structure in solids is due to:

The energy band diagrams for three semiconductor samples of silicon are as shown. We can then assert that :-

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

Sodium has a body-centered cubic $(BCC)$ packing. If the distance between two nearest atoms is $3.7 \mathring{A}$,then its lattice parameter is.....$\mathring{A}$

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 $c = 3 \times 10^8 \ m/s$).