A semiconductor is a material with a high resistance at low temperatures, but a much lower resistance at higher temperatures, or when a voltage is applied, or when light is shone on it. There is no flow of electrons initially because of the presence of an empty energy band. Energy must be given to the electrons so they can be excited into the 'conduction band' and be free to move. Most electrons in a material occupy the lowest available energy level.If there is a big gap between the highest occupied energy band in a material, and the next highest energy level, an electron will need to acquire a lot of energy level to make the jump.
In transparent materials like many glasses, this energy gap occurs for photons with energies between about 175 and 310 eV, corresponding to electromagnetic radiation with wavelengths between 400 and 900 nm (nanometres).