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Radiation effect on a fast bipolar amplifier
596
Nuclear Instruments and Methods m Physics Research A308 (1991) 596-598 North-Holland
Radiation effect on a fast bipolar amplifier Hirokazu Ikeda and Norihiko Ujiie
National Laboratory for High Energy Physics, 1-1 Oho, Tsukuba, Ibaraki-ken, 305 Japan
Koichi Kawaguchi
Institute of Applied Physics, University of Tsukuba, Ten-noudai, Tsukuba, 305 Japan
Received 13 May 1991
In order to verify the radiation hardness and to evaluate the increase in noise level, a bipolar amplifier fabricated with a fast bipolar transistor process was irradiated with a gamma ray source of '() Co .
1. Introduction We have investigated the radiation hardness of a bipolar amplifier. When using a bipolar amplifier for a front end analog circuit of a silicon microstrip detector in a high radiation environment of a high luminosity hadron collider experiment, the circuit must withstand a radiation dose of 1 Mrad/yr of gamma rays and charged particles and also 10 13 neutrons /cm2/yr for more than a few years of operation. Our study began with an investigation of the dependence of the transistor's degradation on the radiation dose . At first we studied the effect of neutrons [1]. In the second step we studied the effect of gamma rays from 'Co [2]. The degradation of the transistors was evaluated in terms of common emitter current gain (h fe ) decrease . We certified that the bipolar transistors from NTT's bipolar SST process was reasonably radiation hard even with a low collector current operation . In this article the survivability of an amplifier circuit as a whole is investigated and the degradation of its signal to noise ratio is evaluated . We expected a degradation of the noise level due to a decrease of h fe . Another source of noise increase might come from an increase in the 1/f noise. The amplifier chips used in this study were prototype [3] amplifiers fabricated with a bipolar SST process, details of which were described in ref. [3].
2. Procedure of radiation exposure The irradiation was done at the cobalt source facility of UC Santa Cruz . The total dose covered in this
experiment was from 0.1 to 5 .0 Mrad . We chose five irradiation levels : 0.1, 0.5, 1.0, 2.0, and 5 Mrad . Table 1 shows the actual dose and the duration of the exposure . The dose estimated here is accurate to about 5% . Completing the whole procedure of irradiation took two weeks. During the exposure the preamplifier chips were biased by -1 .5 V at the most negative voltage rail, while the other voltage rails were grounded . The preamplifiers used in this experiment are listed in table 2. We had three types of preamplifiers whose head transistors were different in size, i.e . their maximum collector currents Ic(n,,X), while they were operated with an identical collector current of 100 ltA. Because the larger transistor has a smaller base spreading resistance (r i, b ,), it is advantageous to achieve a lower noise with a capacitive signal source . One the other hand, it is easier to be degraded in h fe by radiation because it is operated with lower current density. We used 18 preamplifier chips; three types of amplifier chips for each radiation level and three additional chips for backup at an irradiation dose of 1 Mrad .
Table 1 Irradiation dose Dose [Mrad] 0.1
0 .5 1 .0 2 .0 5 .0
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
Irradiation time [min] 232.8 1157.3 2435 .4 5446 .4 11024 .3
Dose in practice [Mrad] 0 .097 0.482 1 .01 2.27 4.59
597
H. Ikeda et al. / Radiation effect on a fast bipolar ampliflier
3. Evaluation of electronic noise
2500
Since in the actual detector, damage and annealing occur simultaneously and a long persistent damage was to be investigated, the amplifier chips were left for a month after the exposure at room temperature without any electrical biases . Even after a month, we found some samples which showed significant noise during the first 20 or 30 min when nominal power supply voltages were applied. This noise showed characteristics of the burst noise, which faded out completely in the subsequent noise measurement procedure. The setup for the noise measurement was identical with that described in ref. [3]. The output of the preamplifier was fed into a shaping amplifier. The peak pulse heights of the shaping amplifier were measured and processed statistically to evaluate the electronic noise. The peaking time of the output of the shaping amplifier was 20 to 30 ns, while an intrinsic shaping time constant was 15 ns . Fig. 1 shows the electronic noise for the amplifiers with an irradiation dose of 1 and 0.1 Mrad . In order to decompose the noise characteristics and identify the possible source of the major noise contribution, we fitted the noise data with a quadratic polynomial as a function of CD :
1500
2000
ene2 =X f +XZ CD,
(1)
where CD is the capacitance of the input signal source . XI can be decomposed as :
z 0
1000
a Û w
500 0 2500
w a
2000 1500
U W
1000 500
ó z
0
F
a z
2500
1500 1000 500 0
0 10 20 30 CAPACITANCE OF INPUT SIGNAL SOURCE IN pF
Fig. 1. Electronic noise for (A) type A amplifier, (B) type B amplifier, and (C) type C amplifier. The open circles designate data points for 1 .0 Mrad, and the cross marks ( x ) designate data points for 0.1 Mrad . The solid lines in the figure come from a polynomial fit to the data .
800000
where q is the electronic charge, Ie is the collector current of the input transistor, k is Boltzmann's constant, T is the absolute temperature in K, Rf is the feedback resistance of the preamplifier, and TM is the measurement time of the shaping amplifier. XZ is expressed as : re
1
2 ~ jM
+
Type A Type B Type C
2.0 4.0 8.0
600000 500000 400000 300000 200000
C\2
z
x
800000
0 .5
1
- O
(D
m
5
700000 600000 500000 400000
EZ w U
300000
p
700000
U
0 .1 1,1
200000 800000
uL 0 1
0 .5
,I
1
5
600000 500000
Table 2 Preamplifier chips irradiated ',(max) [mAl
700000
U q
Wfmaise),
where rbb' is the base spreading resistance of the input transistor and re is the dynamic emitter resistance of the input transistor . re was about 250 fl for our operation condition. A typical value of r bb , was 120, 60, and 30 d for type A, type B, and type C amplifier,
Preamplifier
30
0
2000
Ie I, 4kT Xt a 2q + TM , ( hfe Rf )
X2 Q 4kT (rbb' +
0
400000 300000
rbb'
-120 -60 -30
200000
0 1
0 .5 1 RADIATION DOSE IN MRAD
5
Fig 2. Coefficient Xf for (A) type A amplifier, (B) type B amplifier, and (C) type C amplifier. The solid lines in the figure come from f 15% tile due to li fe and R f variations .
598
H. Ikeda et al. / Radiation effect on a fast bipolar ampliflier 4000 3000
0
showed no contradiction with the degradation of hfe of the input transistor. Xz is shown in fig . 3 . The type A amplifier shows a higher value of XZ than the other amplifiers due to its larger r bb , . The solid lines in the figure are simply a ± 15% tile for the data point at 0 .1 Mrad . This tile could come from a possible parameter variation of rbb , . Data points were well contained in this tile area up to 5 .0 Mrad. We observed no significant increase of noise which could be attributed to the 1/f noise .
(D
2000 1000 L70 4000 3000 2000
0.1
. . . . . 11 0.5 1
0.1
0.5
H
1000 0 4000
1
4. Summary
5
The preamplifier fabricated with the bipolar SST technology survived for a radiation dose of 5 Mrad . The increase in electronic noise, if it existed anyway, was a few hundred electrons . We observed no significant indication of increase in the l/f noise . The behavior of the shot noise was consistent with the degradation of h fe of the input transistor .
3000
m
2000 1000 0
0.1
0.5 1 RADIATION DOSE IN MRAD
5
Fig . 3 . Coefficient XZ for (A) type A amplifier, (B) type B amplifier, and (C) type C amplifier. The solid lines in the figure come from ± 15% tile associated with the data point at 0 .1 Mrad. respectively. X1 is shown in fig . 2 . In order to incorporate a radiation effect in eq . (2), we derived an additional term according to the experimental data in ref. [2l : 1
29Ied1-~ =2qIc h,
1 .0 x 10 -6 X D0 y 45 ( IC/IC(--»
,
(4)
where Dy is the radiation dose in rad, and I4max) was 2 .0, 4 .0, and 8 .0 mA for type A, type B, and type C amplifier, respectively. In order to estimate the effect of process variation on hfe and Rf we calculated ± 15% tile for these parameters . The central values assumed for h fe and Rf were 200 and 20 kf, respectively . The, measurement time assumed here was 15 ns . Data points were consistently contained in this tile area . The increase of the noise, if it existed anyway,
Acknowledgements The directors of Physics Department of KEK, S . Iwata, F . Takasaki, and M . Kobayashi, are acknowledged for their encouragement during the work. Dr . H . Sadrozinski, UC Santa Cruz, kindly provided a cobalt source facility for our research . The work was partially supported by the Grant-in-aid for Developmental Scientific Research of the Japanes ministry of edcucation, science and culture and by the US-Japan scientific collaboration program .
References [11 H . Ikeda and N . Ujiie, Nucl. Instr. and Meth . A281 (1989) 508 . [2] H . Ikeda and N . Ujiie, Nucl. Instr. and Meth . A290 (1990) 462 . [31 H . Ikeda, N . Ujiie, K. Kawaguchi and Y . Akazawa, Nucl . Instr . and Meth. A300 (1991) 335 .
Nuclear Instruments and Methods m Physics Research A308 (1991) 596-598 North-Holland
Radiation effect on a fast bipolar amplifier Hirokazu Ikeda and Norihiko Ujiie
National Laboratory for High Energy Physics, 1-1 Oho, Tsukuba, Ibaraki-ken, 305 Japan
Koichi Kawaguchi
Institute of Applied Physics, University of Tsukuba, Ten-noudai, Tsukuba, 305 Japan
Received 13 May 1991
In order to verify the radiation hardness and to evaluate the increase in noise level, a bipolar amplifier fabricated with a fast bipolar transistor process was irradiated with a gamma ray source of '() Co .
1. Introduction We have investigated the radiation hardness of a bipolar amplifier. When using a bipolar amplifier for a front end analog circuit of a silicon microstrip detector in a high radiation environment of a high luminosity hadron collider experiment, the circuit must withstand a radiation dose of 1 Mrad/yr of gamma rays and charged particles and also 10 13 neutrons /cm2/yr for more than a few years of operation. Our study began with an investigation of the dependence of the transistor's degradation on the radiation dose . At first we studied the effect of neutrons [1]. In the second step we studied the effect of gamma rays from 'Co [2]. The degradation of the transistors was evaluated in terms of common emitter current gain (h fe ) decrease . We certified that the bipolar transistors from NTT's bipolar SST process was reasonably radiation hard even with a low collector current operation . In this article the survivability of an amplifier circuit as a whole is investigated and the degradation of its signal to noise ratio is evaluated . We expected a degradation of the noise level due to a decrease of h fe . Another source of noise increase might come from an increase in the 1/f noise. The amplifier chips used in this study were prototype [3] amplifiers fabricated with a bipolar SST process, details of which were described in ref. [3].
2. Procedure of radiation exposure The irradiation was done at the cobalt source facility of UC Santa Cruz . The total dose covered in this
experiment was from 0.1 to 5 .0 Mrad . We chose five irradiation levels : 0.1, 0.5, 1.0, 2.0, and 5 Mrad . Table 1 shows the actual dose and the duration of the exposure . The dose estimated here is accurate to about 5% . Completing the whole procedure of irradiation took two weeks. During the exposure the preamplifier chips were biased by -1 .5 V at the most negative voltage rail, while the other voltage rails were grounded . The preamplifiers used in this experiment are listed in table 2. We had three types of preamplifiers whose head transistors were different in size, i.e . their maximum collector currents Ic(n,,X), while they were operated with an identical collector current of 100 ltA. Because the larger transistor has a smaller base spreading resistance (r i, b ,), it is advantageous to achieve a lower noise with a capacitive signal source . One the other hand, it is easier to be degraded in h fe by radiation because it is operated with lower current density. We used 18 preamplifier chips; three types of amplifier chips for each radiation level and three additional chips for backup at an irradiation dose of 1 Mrad .
Table 1 Irradiation dose Dose [Mrad] 0.1
0 .5 1 .0 2 .0 5 .0
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
Irradiation time [min] 232.8 1157.3 2435 .4 5446 .4 11024 .3
Dose in practice [Mrad] 0 .097 0.482 1 .01 2.27 4.59
597
H. Ikeda et al. / Radiation effect on a fast bipolar ampliflier
3. Evaluation of electronic noise
2500
Since in the actual detector, damage and annealing occur simultaneously and a long persistent damage was to be investigated, the amplifier chips were left for a month after the exposure at room temperature without any electrical biases . Even after a month, we found some samples which showed significant noise during the first 20 or 30 min when nominal power supply voltages were applied. This noise showed characteristics of the burst noise, which faded out completely in the subsequent noise measurement procedure. The setup for the noise measurement was identical with that described in ref. [3]. The output of the preamplifier was fed into a shaping amplifier. The peak pulse heights of the shaping amplifier were measured and processed statistically to evaluate the electronic noise. The peaking time of the output of the shaping amplifier was 20 to 30 ns, while an intrinsic shaping time constant was 15 ns . Fig. 1 shows the electronic noise for the amplifiers with an irradiation dose of 1 and 0.1 Mrad . In order to decompose the noise characteristics and identify the possible source of the major noise contribution, we fitted the noise data with a quadratic polynomial as a function of CD :
1500
2000
ene2 =X f +XZ CD,
(1)
where CD is the capacitance of the input signal source . XI can be decomposed as :
z 0
1000
a Û w
500 0 2500
w a
2000 1500
U W
1000 500
ó z
0
F
a z
2500
1500 1000 500 0
0 10 20 30 CAPACITANCE OF INPUT SIGNAL SOURCE IN pF
Fig. 1. Electronic noise for (A) type A amplifier, (B) type B amplifier, and (C) type C amplifier. The open circles designate data points for 1 .0 Mrad, and the cross marks ( x ) designate data points for 0.1 Mrad . The solid lines in the figure come from a polynomial fit to the data .
800000
where q is the electronic charge, Ie is the collector current of the input transistor, k is Boltzmann's constant, T is the absolute temperature in K, Rf is the feedback resistance of the preamplifier, and TM is the measurement time of the shaping amplifier. XZ is expressed as : re
1
2 ~ jM
+
Type A Type B Type C
2.0 4.0 8.0
600000 500000 400000 300000 200000
C\2
z
x
800000
0 .5
1
- O
(D
m
5
700000 600000 500000 400000
EZ w U
300000
p
700000
U
0 .1 1,1
200000 800000
uL 0 1
0 .5
,I
1
5
600000 500000
Table 2 Preamplifier chips irradiated ',(max) [mAl
700000
U q
Wfmaise),
where rbb' is the base spreading resistance of the input transistor and re is the dynamic emitter resistance of the input transistor . re was about 250 fl for our operation condition. A typical value of r bb , was 120, 60, and 30 d for type A, type B, and type C amplifier,
Preamplifier
30
0
2000
Ie I, 4kT Xt a 2q + TM , ( hfe Rf )
X2 Q 4kT (rbb' +
0
400000 300000
rbb'
-120 -60 -30
200000
0 1
0 .5 1 RADIATION DOSE IN MRAD
5
Fig 2. Coefficient Xf for (A) type A amplifier, (B) type B amplifier, and (C) type C amplifier. The solid lines in the figure come from f 15% tile due to li fe and R f variations .
598
H. Ikeda et al. / Radiation effect on a fast bipolar ampliflier 4000 3000
0
showed no contradiction with the degradation of hfe of the input transistor. Xz is shown in fig . 3 . The type A amplifier shows a higher value of XZ than the other amplifiers due to its larger r bb , . The solid lines in the figure are simply a ± 15% tile for the data point at 0 .1 Mrad . This tile could come from a possible parameter variation of rbb , . Data points were well contained in this tile area up to 5 .0 Mrad. We observed no significant increase of noise which could be attributed to the 1/f noise .
(D
2000 1000 L70 4000 3000 2000
0.1
. . . . . 11 0.5 1
0.1
0.5
H
1000 0 4000
1
4. Summary
5
The preamplifier fabricated with the bipolar SST technology survived for a radiation dose of 5 Mrad . The increase in electronic noise, if it existed anyway, was a few hundred electrons . We observed no significant indication of increase in the l/f noise . The behavior of the shot noise was consistent with the degradation of h fe of the input transistor .
3000
m
2000 1000 0
0.1
0.5 1 RADIATION DOSE IN MRAD
5
Fig . 3 . Coefficient XZ for (A) type A amplifier, (B) type B amplifier, and (C) type C amplifier. The solid lines in the figure come from ± 15% tile associated with the data point at 0 .1 Mrad. respectively. X1 is shown in fig . 2 . In order to incorporate a radiation effect in eq . (2), we derived an additional term according to the experimental data in ref. [2l : 1
29Ied1-~ =2qIc h,
1 .0 x 10 -6 X D0 y 45 ( IC/IC(--»
,
(4)
where Dy is the radiation dose in rad, and I4max) was 2 .0, 4 .0, and 8 .0 mA for type A, type B, and type C amplifier, respectively. In order to estimate the effect of process variation on hfe and Rf we calculated ± 15% tile for these parameters . The central values assumed for h fe and Rf were 200 and 20 kf, respectively . The, measurement time assumed here was 15 ns . Data points were consistently contained in this tile area . The increase of the noise, if it existed anyway,
Acknowledgements The directors of Physics Department of KEK, S . Iwata, F . Takasaki, and M . Kobayashi, are acknowledged for their encouragement during the work. Dr . H . Sadrozinski, UC Santa Cruz, kindly provided a cobalt source facility for our research . The work was partially supported by the Grant-in-aid for Developmental Scientific Research of the Japanes ministry of edcucation, science and culture and by the US-Japan scientific collaboration program .
References [11 H . Ikeda and N . Ujiie, Nucl. Instr. and Meth . A281 (1989) 508 . [2] H . Ikeda and N . Ujiie, Nucl. Instr. and Meth . A290 (1990) 462 . [31 H . Ikeda, N . Ujiie, K. Kawaguchi and Y . Akazawa, Nucl . Instr . and Meth. A300 (1991) 335 .