Semiconductor -band structure

The band structure of semiconductors is not dissimilar to that of insulators. The valence band is full but the band gap, E_g, is much smaller, of the order of 1 eV, similar to the energies achievable by thermal excitation.

Under normal conditions the conduction band will be populated to some extent and the material will exhibit limited conductivity. 

Cooling the material will reduce the number of electrons in the conduction band and make it much easier to detect the extra excitations due to the gamma-ray interactions.

See insulator and conductor for comparison.