Radiation-hardness of silicon p–i–n photodiodes operated under illumination by light of different wavelengths

The photo response of silicon p–i–n photodiodes has been investigated for light of different wavelengths using I–V and C–V techniques. The measurements were carried out prior to and after radiation damage by 1 MeV neutrons. A main indication is that effects due to incident photons are more pronounced at low radiation fluence but they become negligible as the fluence increases. This is due to defect levels induced by the irradiation in the energy gap of the silicon. The number of these levels increases with radiation fluence and they act mainly to recombine photo-generated carriers. This recombination reduces the number of mobile charges and hence the measured current and capacitance of the photodiode. The results show that silicon gets quickly damaged by radiation in the initial stages of the irradiation process but as the fluence increases the material becomes resistant to further damage. We contend that silicon becomes radiation-hard after initial heavy damage by 1 MeV neutrons.