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How do silicon and germanium semiconductor detectors detect X-rays?

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detector output connected to a measuring circuit — radiation measurement Figure 1: (A) A simple equivalent circuit for the development of a voltage pulse at the output of a detector. R represents the resistance and C the capacitance of the circuit; V(t) is the time (t)-dependent voltage produced. (B) A representative current pulse due to the interaction of a single quantum in the detector. The total charge Q is obtained by integrating the area of the current, i(t), over the collection time, t_c. (C) The resulting voltage pulse that is developed across the circuit of (A) for the case of a long circuit time constant. The amplitude (V_max) of the pulse is equal to the charge Q divided by the capacitance C.

Silicon and germanium semiconductor detectors detect X-rays by converting the energy of incident photons into measurable electrical signals.1

When an X-ray photon is absorbed by the semiconductor material, it transfers energy to electrons, exciting them from the valence band to the conduction band. This process creates electron-hole pairs—free electrons in the conduction band and corresponding vacancies, or "holes," in the valence band.123

Because these detectors maintain an electric field throughout their active volume, the freed electrons and holes are forced to drift toward electrodes on the surface of the material. This movement generates a current pulse, and the total charge collected is proportional to the energy of the original X-ray photon, allowing the device to function as a spectrometer.21

Germanium is often preferred for higher-energy applications due to its higher atomic number, which increases the probability of complete photon absorption. Because both materials have relatively small band gaps, germanium detectors are typically cooled to liquid-nitrogen temperatures to minimize thermally generated charge carriers that could interfere with the signal.42