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Radiation effect on silicon transistors in mixed neutrons–gamma environment
Author's Accepted Manuscript
Radiation effect on Silicon transistors in mixed neutrons-gamma environment J. Assaf, R. Shweikani, N. Ghazi
www.elsevier.com/locate/rad-
PII: DOI: Reference:
S0969-806X(14)00233-3 http://dx.doi.org/10.1016/j.radphyschem.2014.05.050 RPC6496
To appear in:
Radiation Physics and Chemistry
physchem
Received date: 23 December 2013 Revised date: 8 May 2014 Accepted date: 25 May 2014 Cite this article as: J. Assaf, R. Shweikani, N. Ghazi, Radiation effect on Silicon transistors in mixed neutrons-gamma environment, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2014.05.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Radiation effect on Silicon transistors in mixed neutrons‐gamma environment J. Assaf1, R. Shweikani and N. Ghazi Atomic Energy Commission of Syria, Damascus, P. O. Box 6091, Syria
Abstract
The effects of gamma and neutron irradiation on two different types of transistors, Junction Field Effect Transistor (JFET) and Bipolar Junction Transistor (BJT), were investigated. Irradiation was performed using a Syrian research reactor (RR) (Miniature Neutron Source Reactor (MNSR)) and a gamma source (Co‐60 cell). For RR irradiation, MCNP code was used to calculate the absorbed dose received by the transistors. The experimental results showed an overall decrease in the gain factors of the transistors after irradiation, and the JFETs were more resistant to the effects of radiation than BJTs. The effect of RR irradiation was also greater than that of gamma source for the same dose, which could be because neutrons can cause more damage than gamma irradiation. Keywords: Absorbed Dose, Radiation Effect, BJT, JFET. 1. Introduction Electronic components are widely used in control and measurement of military, aerospace, nuclear and high‐energy physics systems and could be exposed to nuclear radiation (Candelori, 2001; Colder et al., 2002; Schrimpf, 2004; Dalla Betta et al., 2007). Studies of radiation effects on semiconductor devices are useful for industrial companies to produce devices that are more resistant to radiation. In general, the structure of electronic components consists of different junctions and contacts of Si, SiO2 and metal. The most radiation‐sensitive areas of these components are the Si, SiO2 and the Si/SiO2 interface. Radiation creates defects in these areas that cause damage to the component functions, such as decreasing gain and increasing noise, etc. 1
Fax.: +963 11 6112289
E‐mail address: [email protected]
1
The amount of radiation delivered to the components and the effects of that radiation depend on the radiation energy, types of radiation and the structure and composition of the devices (Messenger, 1992)
In general, gamma rays and neutrons react with semiconductors via ionization and atomic displacement to different degrees (Akkerman et al., 2001; Pien, 2010). The contribution of ionization is high for gamma rays, while the contribution of displacements is high for neutrons. Moreover, the radiation effects depend on the dose that is absorbed by the component materials. For neutrons, the flux is measured directly, and the dose is then calculated, whereas, for a gamma source, the dose is directly measured. To compare the effects of different radiation types on component behavior, similar doses must be used. In this work, the radiation effect of neutron irradiation in RR is compared with that resulting from a gamma source. 2. Irradiation facilities and irradiated component materiel The tested components were irradiated by gamma radiation from a gamma irradiation cell and by neutrons from Syrian RR. The average radiation energy of the gamma source was 1.25 MeV. The MNSR reactor is classified as a thermal neutron reactor. A thermal neutron flux achieved, at the nominal power of 30 kW, a maximum of about 1×1012 neutrons/sec∙cm2. In addition, neutrons with higher energy and associated fission gamma rays were also present. The neutron beams produced by the reactor were thermal, resonance absorption, epithermal and fast. The studied components were JFET and BJT discrete commercial transistors (N‐JFET (2N5434) and NPN‐BJT (2N2222), respectively) with metallic and Teflon cover cases (packages), respectively. The transistors were irradiated to different gamma source dose levels, as measured by a chemical dosimeter, or were irradiated inside one of the RR internal channels for different irradiation times. The ranges of gamma doses and irradiation times are shown in Table 1
2
Table 1 Measurement irradiation ranges and an example of transistors gain factors reduction ratio after irradiation.
Transistor type
Measurement irradiation ranges
Gamma dose and irradiation time producing a gain factors reduction of 65%
Gamma source
Reactor
Gamma source
Reactor
JFET
0.1‐10000 kGy
5‐2000 sec
10000 kGy
1000 sec
BJT
0.1‐1 kGy
5‐50 sec
0.35 kGy
10 sec
3. Experimental results The resistance to radiation of the irradiated components was characterized by the behavior of their gain factors, such as the transconductance (gm) for JFETs and current gain (hFE) for BJTs. The variations of gm and hFE were characterized and reported. The results showed that, in general, the gain factors decrease with increasing dose/irradiation time for both types of transistors. This decrease is much lower in JFETs than in BJTs, i.e., hFE reduction is much higher than that of gm for the same dose/irradiation time. For example, the dose/irradiation time required to produce a reduction of 65% for both gain factors was measured and is reported in Table 1. 4. Dose calculation using MCNP code The total absorbed dose, contribution of fission gammas and neutrons from irradiation inside the RR is proportional to the exposure time. It is difficult to experimentally measure the radiation dose inside the RR. Therefore, the dose must be calculated using adequate code. In this study, the dose was calculated using the MCNP through the following steps:
3
•
Modeling of MNSR reactor.
•
Testing the accuracy of the model and verifying the validity of the modeling.
•
Modeling of the studied electronic components.
•
Calculating the absorbed doses by the electronic items according to the established models.
4.1 Modelling of the reactor The details of modeling the reactor were similar to those described by other work (Hainoun and Alissa, 2005). The goal of this modeling is to better understand the full energy spectrum (energy vs. flux) of different neutron beams and fission gamma rays to calculate the doses. The calculated fission gamma energy ranges from 0.05 MeV up to 10 MeV. 4.2 Modelling the structure of the transistors The studied electronic components were modeled according to the axis of the core. The geometric design was set to resemble the real shape and dimensions of the transistors as closely as possible. The functional parts of transistors are mainly composed of Si, SiO2 and doping elements. Because the gamma and neutron interaction coefficients are similar for Si and SiO2, the model can be simplified by considering only Si in the calculated model. The materials that cover the bodies of the transistors have also been considered. Therefore, from a modelling point of view, the calculated dose is then the dose absorbed by the Si shielded by stainless steel or by Teflon. The two configurations are designated in the text as M‐Si and I‐Si corresponding to metal and isolator packages, respectively. 4.3 The Basic of dose calculation The dose rate was calculated using the following equation (Li et al., 2009): ⎛ 1 dE ⎞ • =⎜ DCal ⎟ ×φ ⎝ ρ dx ⎠
(1)
4
where 1/ ρ (dE / dx) is the ratio of the change of particles energy dE to path length dx normalized to the density of matter ρ, which is known as the stopping power. Finally, ϕ is the radiation flux. If the following units are used for the terms of equation (1) as 1/ ρ (dE / dx) in MeV∙cm2∙g‐1 and • ϕ in cm‐2∙sec‐1, then the unit of DCal will be MeV∙g‐1∙sec‐1 and the calculated dose DCal in
MeV∙g‐1. To calculate the absorbed radiation dose through MCNP, the amount of deposited energy in the unit weight per second has been calculated within the cell representing the silicon core of the electronic item estimated by MeV∙g‐1∙sec‐1. This value was multiplied in the next step by three other values: the flux of specific neutron beam ϕ , the irradiation time, and conversion factor to Gy which is equal to 1.602×10‐10 Gy.g.MeV‐1 (Li et al., 2009). 4.4 Results of the calculated doses According to the spectra of neutrons and gamma fission, the doses received by the transistors models were calculated using MCNP. These calculations include the different contributions of neutron doses corresponding to different neutron beams and the total dose of fission gamma rays. The calculation was performed for several irradiation times. Silicon film thicknesses of 10, 100 and 1000 μm in both structures were also considered. The results showed that the absorbed dose is independent of the Si volume at this scale. Because the Si thickness of the real structure is close to 10 μm, the doses corresponding to this thickness value were chosen for all • subsequent analyses. Table 2 illustrates the obtained values of DCal . for both modeled
structures. The two structures receive similar gamma doses, while the dose of neutrons 25% lower with the metallic shielding. 5
Table 2 • The values of the total calculated dose rate DCal . of fission gamma and neutrons resulted from
reactor irradiation for both models considered. ‐1 • DCal . [Gy.sec ]
Structure model
Total fission gamma
Total neutrons
Total fission gamma plus neutrons
M‐Si
19.30
0.325
19.625
I‐Si
20.40
0.424
20.824
4.4.1 Details of the dose contributions As Table 2 shows, the fission gamma contribution was much higher than that of the neutrons in both structures. The ratios of these contributions are shown in Table 3, where the neutrons and gamma rays constitute approximately 2% and 98% of the total, respectively. Table 3: Neutrons and fission gamma contributions for the model structures of M‐Si and I‐Si. Contributions ratio%
Structure model
neutron/Gamma
neutrons/total
M‐Si
1.69%
1.66%
I‐Si
2.08%
2.04%
The total neutrons and fission gamma doses versus irradiation time are shown in Fig.1 A similar • result was obtained for the Si‐M model. The details of DCal . values and its percentage of total
neutrons for different beams are shown in Table 4.
6
Table 4 • The details of the DCal . values and the percentage of total neutrons for different beams. • DCal . [Gy/sec]
Structure model
Thermal
Epithermal
Resonance
Fast
Total
3.29 ×10−4
2.58 ×10−4
8.46 × 10−2
2.4 ×10−1
3.25 ×10−1
(0.101%)
(0.079%.)
(26.01%)
(73.80%)
(100%)
3.44 ×10−4
1.79 ×10−4
1.03 ×10−1
3.21×10−1
4.24 ×10−1
(0.081%)
(0.0424%)
(24.26%)
(75.61%)
(100%)
M‐Si
I‐Si
The results presented in Tables 2, 3 and 4 indicate that the package material has a relatively small effect on the dose absorbed by Si, and the dose can be considered to be similar in the two models. Therefore, the effect of radiation on transistors depends mainly on their real structures and functions. 5. Gamma and neutrons effects in dose scale After calculating the dose rate of the neutron beams of RR in Gy/sec, the experimental values of gm and hFE reported in section 3 were calculated as a function of the dose in Gy instead of the irradiation time. These results are shown in Fig.2 and Fig. 3 for JFET and BJT, respectively. The effects of irradiation by the gamma source are also illustrated in these figures. The curves show that gm and hFE begin to decrease for the doses where the effect of the "Gamma source" (only gamma) is an almost negligible. Therefore, the decrease in gm and hFE in the "Reactor" curve (neutrons plus fission gamma) were attributed to neutrons; even they have small dose compared with that of fission gamma. Fig.2 and Fig. 3 also confirm the higher radiation effect on BJT relative to JFET; additionally, the neutron effect was higher than the gamma effect, even though the neutron dose contributions were small (approximately 2%). It is useful to examine the ratio between the dose resulting from irradiation by reactor DR and that resulting from gamma source DS, which gives the same degradation. Table 5 presents
7
details about the experimental example presented in the Table 1 for calculation this ratio (DR / DS) for JFET and BJT. Table 5 Comparison between gamma source dose and calculated doses resulting from reactor irradiation, which give the same effect on the gain factors. Doses producing a gain factor reduction of 65% Transistor
gamma
type
source
Reactor DR/DS
(DS)
Irradiation time
neutron dose
fission gamma dose
JFET
10000 kGy
1000 sec
325.19 Gy
19300 Gy
BJT
0.35 kGy
10 sec
4.25 Gy
204 Gy
Total dose (DR) 19625.19 Gy 1.965 ×10−3 208.25 Gy
5.95 ×10−1
In the above discussion, the effect of gamma source and fission gamma on silicon is considered to be the same because there is an insignificant difference in the mass energy absorption coefficient μ for both types of gamma rays in silicon. In fact, the average value of μ of fission gamma equals to 3.15 ×10−2 cm2/g, while for gamma source it equals to 2.652 ×10−2 cm2/g (Hubbell, and Seltzer, 2004). 6. Conclusion The irradiation of JFET and BJT transistors by gamma and neutrons shows a degradation in the gain factors. This degradation was higher in BJT than in JFET. In cases of mixed gamma –neutrons fields, such as the irradiation inside a fission reactor, the dose was estimated using MCNP code but was measured experimentally only for the gamma source. The dose calculation was achieved by modeling the reactor and silicon structures and two shielding materials (metallic and isolator), which represent simplified transistor models.
8
The output of MCNP provided the values of dose rate. Therefore, the doses for different irradiation times for neutron beams and associated fission gamma have been obtained. Calculated result shows that the two structures receive an almost similar fission gamma dose, while the neutrons dose was less by 25% in the metallic shield. In both structure, the neutrons and gamma contribution were approximately 2% and 98%, respectively. The highest neutron contributions come mainly from the fast and resonance absorption neutrons. Comparison between radiation effect by only gamma source and that by reactor shows that the decrease in gain factors is attributed mainly to neutrons even though neutrons deliver a smaller dose than the gamma source does. The ratios between the neutron dose and the gamma dose that give the same decrease in gain factors were found to be 1.963 ×10−3 and 5.950 ×10−1 for JFET and BJT structure, respectively. Acknowledgements The authors would like to express their thanks to the Director General of AECS Prof. I. Othman for his continuous encouragement and support.
References Akkerman A. et al.,2001, Update NIEL calculation for estimating the damage induced by particles and g‐rays in Si and GaAs, Radiation Physics and Chemistry 62, 301‐310 . Candelori A.,2001, Low and high energy proton irradiations of standard and Oxygenated Silicon
diodes, IEEE Transactions on Nuclear Science Vol. 48, No. 6, 2270 ‐2277 . Colder A. et al.2002, Effects of ionizing radiation on BiCMOS components for space applications, Proceedings of the European Space Components Conference, ESCCON 2002, Toulouse, France 377‐382. Dalla Betta G. F. et al.2077, Radiation damage studies of detector‐compatible Si‐JFETs, Nucl. Instr. Meth. A 572, 287-289. Li Xingji et al.,2009, Radiation effects on BJTs and ICs produced by different energy Br ions, Nucl. Instr. And Meth in Nucl.Physc Resea. A612,171‐175.
9
Hainoun A., and Alissa S., 2005, Full‐scale modeling of the MNSR reactor to simulate normal operation, transients and reactivity insertion accidents under natural circulation conditions using the thermal hydraulic code ATHLET, Nuclear Engineering and Design 235, 33‐52. Hubbell J. H., and Seltzer S. M., 2004, Tables of X‐Ray Mass Attenuation Coefficients and Mass Energy‐ Absorption Coefficients (version 1.4), National Institute of Standards and Technology, Gaithersburg, MD, 2004. Messenger G. C.,1992, A summery review of displacement damage from high energy radiation
in Silicon Semiconductors and Semiconductors devices, IEEE Transactions on Nuclear Science Vol. 39, No.3, 468‐473 . Pien F., et al.,2010, Effect of total ionizing dose on Bipolar junction transistor, American Journal of Applied Sciences Vol. 7, No. 6, 807 ‐810 . Schrimpf R. 2004, Gain degradation and enhanced low dose rate sensitivity in BJT transistors, International Journal of High Speed Electronics and Systems Vol. 14, 503‐517. Item
Comment' reviewer
Authors reply
1
A change in the last line of the Abstract . A correction in the introduction. A correction in the page 5, lines 4445.
It is now changed as the reviewer suggested
2 3
It is now corrected as the reviewer suggested It is now corrected. Instead of " a little " , it becomes now " much higher", as it is shown on the table 2.
10
Figure Captions Figure 1: The total calculated doses of gamma and neutrons e as function of irradiation time inside reactor for the structure. Figure 2: Variation of the gm versus irradiation dose. The dose is experimentally measured for
gamma source in kGy, and calculated as equivalent dose for irradiation inside the reactor during the range from 0 to 2000 sec. Figure 3: Variation of the hFE versus irradiation dose. The dose is experimentally measured directly in kGy for the gamma source, and calculated as equivalent dose for irradiation inside the reactor during the range from 0 to 50 sec.
11
1.E+05
fission gamma neutrons
Calculated dose (Gy)
1.E+04
1.E+03
1.E+02
1.E+01
1.E+00 1
10
100
1000
Irradiation time (sec) ‐b‐ Figure 1
JFET
gm (mA/V)
100
10 Gamma source Reactor
1 0.001
0.01
0.1
1
10
Dose [kGy]
12
100
1000
10000
Figure 2
BJT
1000
100
hFE
10 Gamma source 1
0.1 0.01
Reactor
0.1
1
10
Dose [kGy]
Figure 3
13
Highlights • • • • •
We measure the gain degradation after irradiation of BJT and JFET transistors. We model the reactor and transistors in order to calculate irradiation dose. The gain degradation was higher in BJT than in JFET.
Neutrons and gamma dose contributions of reactor were 2% and 98%, respectively. The highest neutron contribution comes from the fast and resonance absorption neutrons
14
Radiation effect on Silicon transistors in mixed neutrons-gamma environment J. Assaf, R. Shweikani, N. Ghazi
www.elsevier.com/locate/rad-
PII: DOI: Reference:
S0969-806X(14)00233-3 http://dx.doi.org/10.1016/j.radphyschem.2014.05.050 RPC6496
To appear in:
Radiation Physics and Chemistry
physchem
Received date: 23 December 2013 Revised date: 8 May 2014 Accepted date: 25 May 2014 Cite this article as: J. Assaf, R. Shweikani, N. Ghazi, Radiation effect on Silicon transistors in mixed neutrons-gamma environment, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2014.05.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Radiation effect on Silicon transistors in mixed neutrons‐gamma environment J. Assaf1, R. Shweikani and N. Ghazi Atomic Energy Commission of Syria, Damascus, P. O. Box 6091, Syria
Abstract
The effects of gamma and neutron irradiation on two different types of transistors, Junction Field Effect Transistor (JFET) and Bipolar Junction Transistor (BJT), were investigated. Irradiation was performed using a Syrian research reactor (RR) (Miniature Neutron Source Reactor (MNSR)) and a gamma source (Co‐60 cell). For RR irradiation, MCNP code was used to calculate the absorbed dose received by the transistors. The experimental results showed an overall decrease in the gain factors of the transistors after irradiation, and the JFETs were more resistant to the effects of radiation than BJTs. The effect of RR irradiation was also greater than that of gamma source for the same dose, which could be because neutrons can cause more damage than gamma irradiation. Keywords: Absorbed Dose, Radiation Effect, BJT, JFET. 1. Introduction Electronic components are widely used in control and measurement of military, aerospace, nuclear and high‐energy physics systems and could be exposed to nuclear radiation (Candelori, 2001; Colder et al., 2002; Schrimpf, 2004; Dalla Betta et al., 2007). Studies of radiation effects on semiconductor devices are useful for industrial companies to produce devices that are more resistant to radiation. In general, the structure of electronic components consists of different junctions and contacts of Si, SiO2 and metal. The most radiation‐sensitive areas of these components are the Si, SiO2 and the Si/SiO2 interface. Radiation creates defects in these areas that cause damage to the component functions, such as decreasing gain and increasing noise, etc. 1
Fax.: +963 11 6112289
E‐mail address: [email protected]
1
The amount of radiation delivered to the components and the effects of that radiation depend on the radiation energy, types of radiation and the structure and composition of the devices (Messenger, 1992)
In general, gamma rays and neutrons react with semiconductors via ionization and atomic displacement to different degrees (Akkerman et al., 2001; Pien, 2010). The contribution of ionization is high for gamma rays, while the contribution of displacements is high for neutrons. Moreover, the radiation effects depend on the dose that is absorbed by the component materials. For neutrons, the flux is measured directly, and the dose is then calculated, whereas, for a gamma source, the dose is directly measured. To compare the effects of different radiation types on component behavior, similar doses must be used. In this work, the radiation effect of neutron irradiation in RR is compared with that resulting from a gamma source. 2. Irradiation facilities and irradiated component materiel The tested components were irradiated by gamma radiation from a gamma irradiation cell and by neutrons from Syrian RR. The average radiation energy of the gamma source was 1.25 MeV. The MNSR reactor is classified as a thermal neutron reactor. A thermal neutron flux achieved, at the nominal power of 30 kW, a maximum of about 1×1012 neutrons/sec∙cm2. In addition, neutrons with higher energy and associated fission gamma rays were also present. The neutron beams produced by the reactor were thermal, resonance absorption, epithermal and fast. The studied components were JFET and BJT discrete commercial transistors (N‐JFET (2N5434) and NPN‐BJT (2N2222), respectively) with metallic and Teflon cover cases (packages), respectively. The transistors were irradiated to different gamma source dose levels, as measured by a chemical dosimeter, or were irradiated inside one of the RR internal channels for different irradiation times. The ranges of gamma doses and irradiation times are shown in Table 1
2
Table 1 Measurement irradiation ranges and an example of transistors gain factors reduction ratio after irradiation.
Transistor type
Measurement irradiation ranges
Gamma dose and irradiation time producing a gain factors reduction of 65%
Gamma source
Reactor
Gamma source
Reactor
JFET
0.1‐10000 kGy
5‐2000 sec
10000 kGy
1000 sec
BJT
0.1‐1 kGy
5‐50 sec
0.35 kGy
10 sec
3. Experimental results The resistance to radiation of the irradiated components was characterized by the behavior of their gain factors, such as the transconductance (gm) for JFETs and current gain (hFE) for BJTs. The variations of gm and hFE were characterized and reported. The results showed that, in general, the gain factors decrease with increasing dose/irradiation time for both types of transistors. This decrease is much lower in JFETs than in BJTs, i.e., hFE reduction is much higher than that of gm for the same dose/irradiation time. For example, the dose/irradiation time required to produce a reduction of 65% for both gain factors was measured and is reported in Table 1. 4. Dose calculation using MCNP code The total absorbed dose, contribution of fission gammas and neutrons from irradiation inside the RR is proportional to the exposure time. It is difficult to experimentally measure the radiation dose inside the RR. Therefore, the dose must be calculated using adequate code. In this study, the dose was calculated using the MCNP through the following steps:
3
•
Modeling of MNSR reactor.
•
Testing the accuracy of the model and verifying the validity of the modeling.
•
Modeling of the studied electronic components.
•
Calculating the absorbed doses by the electronic items according to the established models.
4.1 Modelling of the reactor The details of modeling the reactor were similar to those described by other work (Hainoun and Alissa, 2005). The goal of this modeling is to better understand the full energy spectrum (energy vs. flux) of different neutron beams and fission gamma rays to calculate the doses. The calculated fission gamma energy ranges from 0.05 MeV up to 10 MeV. 4.2 Modelling the structure of the transistors The studied electronic components were modeled according to the axis of the core. The geometric design was set to resemble the real shape and dimensions of the transistors as closely as possible. The functional parts of transistors are mainly composed of Si, SiO2 and doping elements. Because the gamma and neutron interaction coefficients are similar for Si and SiO2, the model can be simplified by considering only Si in the calculated model. The materials that cover the bodies of the transistors have also been considered. Therefore, from a modelling point of view, the calculated dose is then the dose absorbed by the Si shielded by stainless steel or by Teflon. The two configurations are designated in the text as M‐Si and I‐Si corresponding to metal and isolator packages, respectively. 4.3 The Basic of dose calculation The dose rate was calculated using the following equation (Li et al., 2009): ⎛ 1 dE ⎞ • =⎜ DCal ⎟ ×φ ⎝ ρ dx ⎠
(1)
4
where 1/ ρ (dE / dx) is the ratio of the change of particles energy dE to path length dx normalized to the density of matter ρ, which is known as the stopping power. Finally, ϕ is the radiation flux. If the following units are used for the terms of equation (1) as 1/ ρ (dE / dx) in MeV∙cm2∙g‐1 and • ϕ in cm‐2∙sec‐1, then the unit of DCal will be MeV∙g‐1∙sec‐1 and the calculated dose DCal in
MeV∙g‐1. To calculate the absorbed radiation dose through MCNP, the amount of deposited energy in the unit weight per second has been calculated within the cell representing the silicon core of the electronic item estimated by MeV∙g‐1∙sec‐1. This value was multiplied in the next step by three other values: the flux of specific neutron beam ϕ , the irradiation time, and conversion factor to Gy which is equal to 1.602×10‐10 Gy.g.MeV‐1 (Li et al., 2009). 4.4 Results of the calculated doses According to the spectra of neutrons and gamma fission, the doses received by the transistors models were calculated using MCNP. These calculations include the different contributions of neutron doses corresponding to different neutron beams and the total dose of fission gamma rays. The calculation was performed for several irradiation times. Silicon film thicknesses of 10, 100 and 1000 μm in both structures were also considered. The results showed that the absorbed dose is independent of the Si volume at this scale. Because the Si thickness of the real structure is close to 10 μm, the doses corresponding to this thickness value were chosen for all • subsequent analyses. Table 2 illustrates the obtained values of DCal . for both modeled
structures. The two structures receive similar gamma doses, while the dose of neutrons 25% lower with the metallic shielding. 5
Table 2 • The values of the total calculated dose rate DCal . of fission gamma and neutrons resulted from
reactor irradiation for both models considered. ‐1 • DCal . [Gy.sec ]
Structure model
Total fission gamma
Total neutrons
Total fission gamma plus neutrons
M‐Si
19.30
0.325
19.625
I‐Si
20.40
0.424
20.824
4.4.1 Details of the dose contributions As Table 2 shows, the fission gamma contribution was much higher than that of the neutrons in both structures. The ratios of these contributions are shown in Table 3, where the neutrons and gamma rays constitute approximately 2% and 98% of the total, respectively. Table 3: Neutrons and fission gamma contributions for the model structures of M‐Si and I‐Si. Contributions ratio%
Structure model
neutron/Gamma
neutrons/total
M‐Si
1.69%
1.66%
I‐Si
2.08%
2.04%
The total neutrons and fission gamma doses versus irradiation time are shown in Fig.1 A similar • result was obtained for the Si‐M model. The details of DCal . values and its percentage of total
neutrons for different beams are shown in Table 4.
6
Table 4 • The details of the DCal . values and the percentage of total neutrons for different beams. • DCal . [Gy/sec]
Structure model
Thermal
Epithermal
Resonance
Fast
Total
3.29 ×10−4
2.58 ×10−4
8.46 × 10−2
2.4 ×10−1
3.25 ×10−1
(0.101%)
(0.079%.)
(26.01%)
(73.80%)
(100%)
3.44 ×10−4
1.79 ×10−4
1.03 ×10−1
3.21×10−1
4.24 ×10−1
(0.081%)
(0.0424%)
(24.26%)
(75.61%)
(100%)
M‐Si
I‐Si
The results presented in Tables 2, 3 and 4 indicate that the package material has a relatively small effect on the dose absorbed by Si, and the dose can be considered to be similar in the two models. Therefore, the effect of radiation on transistors depends mainly on their real structures and functions. 5. Gamma and neutrons effects in dose scale After calculating the dose rate of the neutron beams of RR in Gy/sec, the experimental values of gm and hFE reported in section 3 were calculated as a function of the dose in Gy instead of the irradiation time. These results are shown in Fig.2 and Fig. 3 for JFET and BJT, respectively. The effects of irradiation by the gamma source are also illustrated in these figures. The curves show that gm and hFE begin to decrease for the doses where the effect of the "Gamma source" (only gamma) is an almost negligible. Therefore, the decrease in gm and hFE in the "Reactor" curve (neutrons plus fission gamma) were attributed to neutrons; even they have small dose compared with that of fission gamma. Fig.2 and Fig. 3 also confirm the higher radiation effect on BJT relative to JFET; additionally, the neutron effect was higher than the gamma effect, even though the neutron dose contributions were small (approximately 2%). It is useful to examine the ratio between the dose resulting from irradiation by reactor DR and that resulting from gamma source DS, which gives the same degradation. Table 5 presents
7
details about the experimental example presented in the Table 1 for calculation this ratio (DR / DS) for JFET and BJT. Table 5 Comparison between gamma source dose and calculated doses resulting from reactor irradiation, which give the same effect on the gain factors. Doses producing a gain factor reduction of 65% Transistor
gamma
type
source
Reactor DR/DS
(DS)
Irradiation time
neutron dose
fission gamma dose
JFET
10000 kGy
1000 sec
325.19 Gy
19300 Gy
BJT
0.35 kGy
10 sec
4.25 Gy
204 Gy
Total dose (DR) 19625.19 Gy 1.965 ×10−3 208.25 Gy
5.95 ×10−1
In the above discussion, the effect of gamma source and fission gamma on silicon is considered to be the same because there is an insignificant difference in the mass energy absorption coefficient μ for both types of gamma rays in silicon. In fact, the average value of μ of fission gamma equals to 3.15 ×10−2 cm2/g, while for gamma source it equals to 2.652 ×10−2 cm2/g (Hubbell, and Seltzer, 2004). 6. Conclusion The irradiation of JFET and BJT transistors by gamma and neutrons shows a degradation in the gain factors. This degradation was higher in BJT than in JFET. In cases of mixed gamma –neutrons fields, such as the irradiation inside a fission reactor, the dose was estimated using MCNP code but was measured experimentally only for the gamma source. The dose calculation was achieved by modeling the reactor and silicon structures and two shielding materials (metallic and isolator), which represent simplified transistor models.
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The output of MCNP provided the values of dose rate. Therefore, the doses for different irradiation times for neutron beams and associated fission gamma have been obtained. Calculated result shows that the two structures receive an almost similar fission gamma dose, while the neutrons dose was less by 25% in the metallic shield. In both structure, the neutrons and gamma contribution were approximately 2% and 98%, respectively. The highest neutron contributions come mainly from the fast and resonance absorption neutrons. Comparison between radiation effect by only gamma source and that by reactor shows that the decrease in gain factors is attributed mainly to neutrons even though neutrons deliver a smaller dose than the gamma source does. The ratios between the neutron dose and the gamma dose that give the same decrease in gain factors were found to be 1.963 ×10−3 and 5.950 ×10−1 for JFET and BJT structure, respectively. Acknowledgements The authors would like to express their thanks to the Director General of AECS Prof. I. Othman for his continuous encouragement and support.
References Akkerman A. et al.,2001, Update NIEL calculation for estimating the damage induced by particles and g‐rays in Si and GaAs, Radiation Physics and Chemistry 62, 301‐310 . Candelori A.,2001, Low and high energy proton irradiations of standard and Oxygenated Silicon
diodes, IEEE Transactions on Nuclear Science Vol. 48, No. 6, 2270 ‐2277 . Colder A. et al.2002, Effects of ionizing radiation on BiCMOS components for space applications, Proceedings of the European Space Components Conference, ESCCON 2002, Toulouse, France 377‐382. Dalla Betta G. F. et al.2077, Radiation damage studies of detector‐compatible Si‐JFETs, Nucl. Instr. Meth. A 572, 287-289. Li Xingji et al.,2009, Radiation effects on BJTs and ICs produced by different energy Br ions, Nucl. Instr. And Meth in Nucl.Physc Resea. A612,171‐175.
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Hainoun A., and Alissa S., 2005, Full‐scale modeling of the MNSR reactor to simulate normal operation, transients and reactivity insertion accidents under natural circulation conditions using the thermal hydraulic code ATHLET, Nuclear Engineering and Design 235, 33‐52. Hubbell J. H., and Seltzer S. M., 2004, Tables of X‐Ray Mass Attenuation Coefficients and Mass Energy‐ Absorption Coefficients (version 1.4), National Institute of Standards and Technology, Gaithersburg, MD, 2004. Messenger G. C.,1992, A summery review of displacement damage from high energy radiation
in Silicon Semiconductors and Semiconductors devices, IEEE Transactions on Nuclear Science Vol. 39, No.3, 468‐473 . Pien F., et al.,2010, Effect of total ionizing dose on Bipolar junction transistor, American Journal of Applied Sciences Vol. 7, No. 6, 807 ‐810 . Schrimpf R. 2004, Gain degradation and enhanced low dose rate sensitivity in BJT transistors, International Journal of High Speed Electronics and Systems Vol. 14, 503‐517. Item
Comment' reviewer
Authors reply
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A change in the last line of the Abstract . A correction in the introduction. A correction in the page 5, lines 4445.
It is now changed as the reviewer suggested
2 3
It is now corrected as the reviewer suggested It is now corrected. Instead of " a little " , it becomes now " much higher", as it is shown on the table 2.
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Figure Captions Figure 1: The total calculated doses of gamma and neutrons e as function of irradiation time inside reactor for the structure. Figure 2: Variation of the gm versus irradiation dose. The dose is experimentally measured for
gamma source in kGy, and calculated as equivalent dose for irradiation inside the reactor during the range from 0 to 2000 sec. Figure 3: Variation of the hFE versus irradiation dose. The dose is experimentally measured directly in kGy for the gamma source, and calculated as equivalent dose for irradiation inside the reactor during the range from 0 to 50 sec.
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1.E+05
fission gamma neutrons
Calculated dose (Gy)
1.E+04
1.E+03
1.E+02
1.E+01
1.E+00 1
10
100
1000
Irradiation time (sec) ‐b‐ Figure 1
JFET
gm (mA/V)
100
10 Gamma source Reactor
1 0.001
0.01
0.1
1
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Dose [kGy]
12
100
1000
10000
Figure 2
BJT
1000
100
hFE
10 Gamma source 1
0.1 0.01
Reactor
0.1
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Dose [kGy]
Figure 3
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Highlights • • • • •
We measure the gain degradation after irradiation of BJT and JFET transistors. We model the reactor and transistors in order to calculate irradiation dose. The gain degradation was higher in BJT than in JFET.
Neutrons and gamma dose contributions of reactor were 2% and 98%, respectively. The highest neutron contribution comes from the fast and resonance absorption neutrons
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