**4. Conclusion**

**Figure 15.** Cross-section of a phototransistor [41].

86 Radiation Effects in Materials

**Probability of creating new photons Interaction that creates photons**

1.1333E-06 Positron-electron annihilation

7.0135E-03 Electrons knocked out in a collision with impact electrons

According to the simulation results in any semiconductor field within the phototransistor, there was no interaction in which are created or disappeared photons. **Table 8** show the

1.3333E-05 Bremsstrahlung

5.1667E-06 Electron x-rays

**Probability of creating new electrons Interaction that creates electrons**

**Table 7.** Probability of creating new electrons per incident particle (photon).

**Table 6.** Probability of creating new photons per incident particle (photon).

1.4667E-06 Pair production 2.1764E-03 Compton effect 1.1633E-05 Photoelectric effect 2.3333E-07 Auger photons 1.7910E-04 Auger electrons

> Gamma and neutron radiation, applied individually, affect the semiconductor material creating defects and changing the existing structure, which results in a change in the output characteristics of the device and reducing their functionality.

> Gamma irradiation of silicon semiconductor causing numerous defects of the crystal lattice. Monte Carlo simulations showed that in this experiment were represented almost all of the

effects described in the literature: displacement of atoms (PKA), Auger electrons, Compton scattering, photoelectric effect, pair production. The impact of all these effects are manifested in the generation of energy levels in the energy gap of crystal lattice which decreases the the minority charge carriers lifetime resulting in a decrease in the photocurrent and spectral response. The big change of phototransistors output characteristics can be explained by the influence of radiation on the current gain. The current gain is proportional to the minority charge carriers lifetime so the degradation of their lifetime directly affects the degradation of current gain. This degradation is caused by a displacement of atoms in the semiconductor bulk which affects the increase in the number of recombination centers and also oxidation of the oxide pasivisation layer especially over the emitter-base junction.

Neutron irradiation causes damage in the photovoltaic detector which is primarily related to the displacement of silicon atoms from their positions in a grid and creating vacancies. Together with the vacancies, other effects appeared. Monte Carlo simulations showed that after the vacancies, the most frequent are Auger electrons, Compton scattering, pair produc‐ tion, and the photoelectric effect. Because of the combination of complex defects, defects that act as recombination centers are created and reduce the minority charge carriers' lifetime which can lead to the degradation of electrical parameters of photovoltaic detectors.

When the semiconductor photovoltaic detectors are first exposed to gamma radiation and after a month to neutron, one can see that neutron radiation, applied after gamma radiation, partially corrects the characteristics of semiconductor devices which are exacerbated by gamma radiation, and that is manifested through increased spectral response and output photocurrent. This behavior of photodiodes and phototransistors can be explained by the increased generation of charge carriers as a result of direct transfer (tunneling) of the charge through the traps (recombination centers). Direct (intercenter) charge transfer is a process where charge carriers spend some time trapped in the defect of material (traps) before tunneling through the barrier. To become free (transferred from the valence to the conductive band), an electron must have enough energy to overcome the energy gap. However, if the traps, that represent energy levels, are located near the edge of the conduction and valence band and near the Fermi level on both sides (according to Dharival-Rajvanshi model), then moving electrons from the valence band into the trap require notably less energy than for direct transit to the conductive band, which means that the traps actually facilitate the process of generation of free carriers. Also, according to the Shockley–Read–Hall model, there is one quasi-stationary energy level within the gap where the electron or hole could come. The probability that an electron will fall into the trap and spend some time in it depends, among other causes, on the density of defects in the energy gap. One of the ways to increase the density of defects in the energy gap is creating a large number of vacancies located physically close to each other in semiconductor material. Monte Carlo simulation of γ-photons transfer through the photovoltaic detectors showed that gamma radiation leaves behind itself a number of displaced atoms (vacancies). Since the radiation damage caused by neutrons primarily related to the displacement of atoms from their positions in the lattice of silicon semiconductor, i.e. forming of vacancies, so neutron irradiation of photovoltaic detectors applied after gamma irradiation gives a possibility for the creation of a sufficient number of divacancies which can cause intercenter transfer and increased generation of charges and thereby increasing the photocurrent and other parameters. The requirement for creation of divacancies by neutron irradiation is the existence of vacancies in a semiconductor caused by previous gamma radiation.
