Silicon-based semiconductor detectors are mainly used for charged particle detectors (especially for tracking charged particles) and soft X-ray detectors while germanium is widely used for gamma ray spectroscopy. A large, clean and almost perfect semiconductor is ideal as a counter for radioactivity. However, it is difficult to make large crystals with sufficient purity. The semiconductor detectors have, therefore, low efficiency, but they do give a very precise measure of the energy. Detectors based on silicon have sufficiently low noise even by room temperature. This is caused by the large band gap of silicon (Egap= 1.12 eV), which allows us to operate the detector at room temperature, but cooling is prefered to reduce noise. The drawback is that silicon detectors are much more expensive than cloud chambers or wire chambers and require sophisticated cooling to reduce leakage currents (noise). They also suffer degradation over time from radiation, however this can be greatly reduced thanks to the Lazarus effect.
Principle of Operation of Silicon Detectors
The operation of semiconductor detectors is summarized in the following points:
- Ionizing radiation enters the sensitive volume of the detector and interacts with the semiconductor material.
- Particle passing through the detector ionizes the atoms of semiconductor, producing the electron-hole pairs. The number of electron-hole pairs is proportional to the energy of the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band.
- Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit,
- This pulse carries information about the energy of the original incident radiation. The number of such pulses per unit time also gives information about the intensity of the radiation.
The energy required to produce electron-hole-pairs is very low compared to the energy required to produce paired ions in a gaseous ionization detector. In semiconductor detectors the statistical variation of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good. Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.