The operating principle of Semiconductor detectors is similar to gas ionization detectors: except that instead of ionization of gas atoms, free electrons and holes are produced which create a signal at the electrodes.  The advantage of solid state detectors is the greater resolution of the resultant energy spectrum.  Usually NaI(Tl) detectors are used; for more precise applications Ge(Li) and Si(Li) detectors have been developed.  For extra sensitive measurements high-pure germanium detectors are used under a liquid nitrogen environment.