Complex sentence: At higher temperatures, the electrons can easily cross the band gap in the crystal and reach the conduction band, where they are free to respond to the electric field, producing too much electrical noise to be useful as a spectrometer.
Split into simpler sentences: At higher temperatures, the electrons can easily cross the Band gap in the crystal and reach the conduction band, where they are free to respond to the electric field. The system therefore produces too much electrical noise to be useful as a spectrometer.