**3.1. Photodiodes and phototransistors response to neutron radiation**

Free neutrons, in small amounts, are everywhere in nature. The main source of neutrons is cosmic radiation. They also occur in nuclear reactions of natural α radiation and spontaneous fission of heavy nuclei. Neutron is a unique particle, it is uncharged, has a relatively large mass, and leads to radioactive disintegrations. From the point of nuclear reaction, neutrons are much more important than any other particles. As neutral particles, neutrons do not have the ability for direct ionization of materials. The basic mechanism of neutron interaction with the matter is via elastic collisions with atomic nuclei of environment. The interaction with electrons, although it exist, is negligible. Thereby, neutron loses some of its energy and slows down, while the environment can suffer different types of transformation.

Neutrons interact with the material in two different ways:


In the case of high-energy neutrons (fast one), the dominant process is elastic scattering, while with low-energy neutrons, the absorption process is more likely [3].

Displacement of atoms can be compared with a collision between two solid spheres. If the transferred energy higher than the energy required for displacement (*displacement energy Ed*) atom will be shifted from their original positions in the lattice, and there will be defect (*PKA —primary knock-on atom*). Assuming that there is enough energy, displaced atom could be able to move other atoms or to produce electron-hole pairs. In the case of very high energy particles, a cascade distortion can be formed.

Different types of displacement defects could occur due to neutron irradiation (**Figure 1**):


Figures 2–4 show the results of measurements of PIN photodiodes and phototransistors spectral response before and after neutron irradiation and after a period of 30 days recovery are presented [4]. As can be seen from Figures 2 to 4, neutron radiation caused the deterioration of photodiodes and phototransistors characteristics.

High-energy particles like neutrons create much more displacement damages than gamma radiation. When an atom is ejected from its position, it creates a vacancy in the lattice. The ejected atom may recombine with a vacancy or stay in an interstitial position in the lattice.The

**Figure 1.** Displacement defects [3].

**3. Results and discussion**

72 Radiation Effects in Materials

**3.1. Photodiodes and phototransistors response to neutron radiation**

while the environment can suffer different types of transformation.

with low-energy neutrons, the absorption process is more likely [3].

Neutrons interact with the material in two different ways:

**•** through collisions with other particles,

**•** through the process of absorption.

a cascade distortion can be formed.

of photodiodes and phototransistors characteristics.

**•** vacancies,

**•** divacancies,

**•** interstitials,

**•** Schottky defects,

**•** Frenkel defects.

Free neutrons, in small amounts, are everywhere in nature. The main source of neutrons is cosmic radiation. They also occur in nuclear reactions of natural α radiation and spontaneous fission of heavy nuclei. Neutron is a unique particle, it is uncharged, has a relatively large mass, and leads to radioactive disintegrations. From the point of nuclear reaction, neutrons are much more important than any other particles. As neutral particles, neutrons do not have the ability for direct ionization of materials. The basic mechanism of neutron interaction with the matter is via elastic collisions with atomic nuclei of environment. The interaction with electrons, although it exist, is negligible. Thereby, neutron loses some of its energy and slows down,

In the case of high-energy neutrons (fast one), the dominant process is elastic scattering, while

Displacement of atoms can be compared with a collision between two solid spheres. If the transferred energy higher than the energy required for displacement (*displacement energy Ed*) atom will be shifted from their original positions in the lattice, and there will be defect (*PKA —primary knock-on atom*). Assuming that there is enough energy, displaced atom could be able to move other atoms or to produce electron-hole pairs. In the case of very high energy particles,

Different types of displacement defects could occur due to neutron irradiation (**Figure 1**):

Figures 2–4 show the results of measurements of PIN photodiodes and phototransistors spectral response before and after neutron irradiation and after a period of 30 days recovery are presented [4]. As can be seen from Figures 2 to 4, neutron radiation caused the deterioration

High-energy particles like neutrons create much more displacement damages than gamma radiation. When an atom is ejected from its position, it creates a vacancy in the lattice. The ejected atom may recombine with a vacancy or stay in an interstitial position in the lattice.The

**Figure 2.** Spectral response of photodiodes before and after neutron irradiation.

vacancies are mobile and combine with other vacancies or with impurities of the semiconduc‐ tor [5, 6], thus creating recombination centers that cause the reduction of charge carrier lifetime. Axness *et al*. [7] showed that the damage to the crystal lattice and reduction of the charge carriers lifetime are spatially dependent. Sporea *et al*. [8] have calculated that the major degradation of the photodiode responsivity, for the total gamma dose of 1.23 MGy and to the neutron fluence of 1.2 × 1013 n/cm2 , occurs in the case of neutron irradiation (37.5%) as compared to the gamma irradiation (7.2%).

Steady defects act as recombination centers and traps for charge carriers and because of that the resistance of the material could be increased [6]. Mobile vacancies represent a strong recombination instrument for capturing of minority charge carriers and thus reduce their lifetime. Defects responsible for the capture of electrons called E-defects while the H-defects actually traps holes [3]. Displacement defects mainly affect the electrical characteristics of the semiconductor substrate and thus the electrical characteristics of the whole electronic compo‐ nents. As a result, there have been the reduction of the spectral response and lower photocur‐ rent photodiode (**Figure 2**).

**Figure 3.** Spectral response of phototransistor BPW40 before and after neutron irradiation.

Phototransistors are very susceptible to neutron radiation. Neutron radiation affects the characteristics of phototransistors primarily by creating defects in the crystal lattice which can dramatically increase the level of charge carriers recombination. On the other hand, the increment of the recombination rate reduces the current gain. Many studies of the damage relocation mechanism in bipolar transistors have shown that the current gain of the transistor with a common emitter decreases with increasing of recombination centers number. The measurement data of phototransistors before and after irradiation showed that the adverse effects of neutron radiation are the most pronounced on transistors base current. Phototran‐ sistor is light controlled device where the output current is controled by the base current and brightness. Cluster defects caused by fast neutrons are the dominant mechanism for damaging of phototransistors exposed to neutron radiation. Number of displaced atoms caused by neutrons is very large. The result is forming of recombination-generation centers. Electronhole recombination causes a decrease of current gain. Generation of electron-hole pairs cause an increase in leakage current. Removing the majority charge carriers and the reduction of carrier mobility causing an increase in voltage between the collector and emitter. Current gain is determined by the number of majority carriers emitted from the emitter which are passing through the base as minority carriers and are collected by collectors as the major carriers. Increasing of density of recombination-generation centers due to defects created by radiation causes a reduction of minority carrier lifetime, and because of that, the rate of electron-hole recombination in the base increases. Accordingly, the current gain decreases as a result of reduced injection of charge carriers from the emitter to the collector and, as a result, the photocurrent and spectral response decreases (Figures 3 and 4) [9].

**Figure 4.** Spectral response of phototransistor LTR4206 before and after neutron irradiation.

nents. As a result, there have been the reduction of the spectral response and lower photocur‐

**Figure 3.** Spectral response of phototransistor BPW40 before and after neutron irradiation.

photocurrent and spectral response decreases (Figures 3 and 4) [9].

Phototransistors are very susceptible to neutron radiation. Neutron radiation affects the characteristics of phototransistors primarily by creating defects in the crystal lattice which can dramatically increase the level of charge carriers recombination. On the other hand, the increment of the recombination rate reduces the current gain. Many studies of the damage relocation mechanism in bipolar transistors have shown that the current gain of the transistor with a common emitter decreases with increasing of recombination centers number. The measurement data of phototransistors before and after irradiation showed that the adverse effects of neutron radiation are the most pronounced on transistors base current. Phototran‐ sistor is light controlled device where the output current is controled by the base current and brightness. Cluster defects caused by fast neutrons are the dominant mechanism for damaging of phototransistors exposed to neutron radiation. Number of displaced atoms caused by neutrons is very large. The result is forming of recombination-generation centers. Electronhole recombination causes a decrease of current gain. Generation of electron-hole pairs cause an increase in leakage current. Removing the majority charge carriers and the reduction of carrier mobility causing an increase in voltage between the collector and emitter. Current gain is determined by the number of majority carriers emitted from the emitter which are passing through the base as minority carriers and are collected by collectors as the major carriers. Increasing of density of recombination-generation centers due to defects created by radiation causes a reduction of minority carrier lifetime, and because of that, the rate of electron-hole recombination in the base increases. Accordingly, the current gain decreases as a result of reduced injection of charge carriers from the emitter to the collector and, as a result, the

rent photodiode (**Figure 2**).

74 Radiation Effects in Materials

In this experiment, a long-term isothermal annealing at room temperature was applied. The recovery period, labelled as a short-term annealing, begins immediately after the occurrence of damage and fully complete within a few minutes to 1 hour after irradiation. Damage, remaining after, that are often referred as a permanent damage. However, relatively slow process of recovery or long-term annealing, continues even after the short-term annealing is completed [10]. Recovery causes partially increasing of spectral response and the photocur‐ rent.

Figures 2–4 show that the response of new, unused photodiode and phototransistor to neutron irradiation is in accordance with the theoretical principles described in the literature.
