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Characteristics of ELDRS at high and low-level injection in double
polysilicon self-aligned NPN bipolar transistors
S Chen,
2 Y. H. Yang,
1 K. F. Zhu 1, Y. Zh ong 1
Q . N. Yl�,P. J. Zh ang 2* ,X. Wu2 ,W..
2
1 Chongqing Semi-chip Electronics Co .. Ltd, Chongqing 401332, China
Science and Technology on Analog Integrated Circuit Laboratory, Chongqing 400060, China
* Email: pjzhang@whu.edu.cn
Abstract
A comparative investigation of y-ray total dose
ionization damage at high and low-level injection
(HLl/LLI) for different dose rate irradiation in double
poly silicon self-aligned bipolar NPN transistors is
presented. The transistors reveal anomalous dose rate
radiation responses for Emitter-Base (E-B) electrical
field strength in forward active mode. This effect is
probably associated with the different types of radiation
induced defects, which depends on the E-B junction
electric field strength, play the key role to the increase of
excess base current.
1. Introduction
The degradation of the current gain caused by total
ionizing dose radiation in bipolar transistors has been a
serious problem for microelectronics application in
extreme environments[I-21. During the past several
decades, the radiation damage including total dose
effects and enhanced low dose rate sensitivity (ELORS)
in bipolar transistors have been intensively investigated
in various transistors[3- 81. Lots of physical models have
been presented to explain ELORS mechanisms, which
can be mainly sorted into 3 categories: (1) space charge
models, (2) bimolecular process models, and (3) a binary
reaction rate model[81. The excess base current, which is
the result of increased recombination in the Emitter-Base
depletion region, is based on two interacting effects: (I)
surface recombination velocity increase and (2) E-B
depletion region spread[4.5 1. It is believed that the
physical mechanisms that induce the degradation of
electrical parameters occur at the interface of the silicon
with the transistor base oxide and within the base oxide.
However, most of the studies focused on the degradation
performances in low E-B bias region[6.71, very little
reports regarding current gain degradation characteristics
in the HLI.
It has been proved that the significant degradation of
NPN bipolar transistors is caused by the low electric
field in the screen oxide over the E-B junction edge. In
this case, the great amount of net positive charge is
captured in screen oxides[61. The E-B bias stress altered
978-1-4673-9719-3/16/$31.00 �161EEE
the magnitude of the fringing electric field, which
determines the electric field in the base oxide and plays a
key role in the buildup of radiation-induced
oxide-trapped charges and interface states[6.71. This is just
suit for the low and medium E-B bias condition, while
the situation in high E-B field is still lacked. This paper
will reveal the different radiation response characteristics
of polysilicon emitter (PE) NPN bipolar transistors,
which under low dose rate (LOR) and high dose rate
(HOR) with the same total dose irradiation, in LLI and
HLI.
2. Experimental and Results
The details for the transistors which used in this paper
have been illustrated elsewhere[91. The electrical
parameters were measured by a Keithly 4200
Semiconductor Parameter Analyzer. Three to five
identical transistors were performed under each test
condition to mInImIze uncertainties caused by
manufacturing process fluctuations and to ensure the
accuracy of the experiments. Typical Gummel curves
and current gain characteristics for PE NPN transistors in
forward bias condition for HOR and LOR are depicted in
Fig I. It has been widely reported that the collector
current kept approximately constant during the gamma
ray irradiation in both of the two dose rate situations,
except at HLI where small changes to the parasitic base
and emitter resistance happen. It indicated that the most
severe degradation occurred in LLI region as the same as
that discussed in many earlier scientific works on
conventional silicon bipolar transistors[4.l01. The primary
result of ionizing radiation in low E-B junction fringing
electric field region on bipolar junction transistors is an
increase in the base current resulting from enhanced
recombination in the E-B depletion region[21. The
recombination-rate increase occurs mainly located in the
depletion region which intersects the Si/Si02 interface,
due to formation of interface traps that serve as
recombination centers[2.51.
As can be seen in Fig l(b), obvious radiation induced
current gain degradation in HLI region only occurred in
the case of LOR irradiation and this effect in the case of
It indicates in Fig 2(a) that room temperature annealing
HDR is so small that it can be ignored
. (;;
"
120
90
-=
60
o
~
~
(a~
30
(b)
ok
HDR
11,[:17 7
8
0.4
0.6
0 .8
1.0
1 .2 0.4
0.6
0 .8
1.0
1 .2
Base-Emitter Voltage (A)
Figure 1. Typical electrical parameters degradation
behaviors versus base-emitter voltage. (a) HOR and (b)
LDR.
2.1 ELDRS effects in LLI & HLI for PE-BJTs
Fig.2 exhibits the normalized current gain degradation in
the forward active mode for LOR, LOR and HOR plus
post annealing. It can be seen that the degradation at the
end of a LOR irradiation is obviously greater than the
degradation measured after irradiation to the same total
dose at HDR followed by a room temperature anneal for
a time at least as long as the irradiation time at LDR to
various total dose levels. It indicates that the PE-NPN
transistors show the true dose rate effect both at LLI and
HLI in our case. The ELDRS effect in HLI region,
which has rarely been reported to the authors'
knowledge.
RTA ( H)
150
300
5OJ
0
450
1.0
\[
E
~
u
~
�
ideality factor, depends upon oxide charge and forward
voltage[4l . [n Fig.3, excess current for lOOk total ionizing
dose under LOR, HOR and post-annealing following
HDR are comparatively illustrated and simulated basing
on the above recombination current relation. It indicates
that in the LLI region the ideality factor in the case of
LDR is the same as that in the case of HDR, which
indicates that the major physical origin induced current
gain degradation for HOR is the same as that for LOR in
the LLI. The post-annealing process following HOR
irradiation caused excess base current further increase
but not slightly recover.
3:
--<>- HDR loak
1�-9
--0- LDR l OOk
E
~
::l
~~t
I-+--<>- LDRI
I
MB = MBa exp(f3V In) in p-njunction where n, the
U
~-~----
"EN 0.9
600
Ca)
@0 .7V
following HDR irradiation causes a significant increase
in the current gain degradation in LLI. The magnitude of
the degradation comes from post-annealing following
HOR irradiation is even much worse than the
degradation at the end of a LOR irradiation. [n contrast,
post-annealing effect on the current gain degradation in
HLI can be neglected compared with that in LLI as
shown in Fig 2(b).
2.2 Characteristics of excess base current
[t is shown that the value of oxide charge can be
detennined by plotting the excess base current versus
base emitter voltage, as in Fig. 3. The excess base
current is a recombination current in the E-B depletion
region. The recombination current can be expressed as
~
OJ
o:l
en
en
. - HDR 100k+RTA 72H
? HDR IOOk+RTA 4 32H
? HDR IOOk+RTA 930H
0,)
HDR
100k+ RD\I
U
~
I E-II
0
Z
0.8
20
40
60
80
100
300
450
045
0. 50
0.55
0.60
0.65
Base-Emitter V oltage (V)
Total Dose (krad/Si)
RTA (H )
150
040
600
20
Tota l Dose (krad/S i)
Figure 2. Nonnalized current gain versus accumulated
dose and annealing time @ (a) VBE=0.7V, (b) VBE =1.0V
in the case of LOR, HOR and HOR plus annealing
process. Error bars represent one standard deviation on
the tested population.
Figure 3. Log of the excess base current versus E-B
voltage of lOOk total ionizing dose for LOR, HOR and
HOR plus room temperature post-annealing.
2.3 Effect of post-annealing
FigA exhibits the normalized excess base current change
of the PE-NPN transistor as a function of room
temperature post-annealing time. Based on the result in
FigA(a), the normalized excess base current of the
transistors for HOR further increase with annealing time
to the maximum value, which is as much as or even
bigger than that of the transistors for LOR in LLI. When
the annealing time exceeds 432 hours, the excess base
current reaches the maximum value and starts to
decrease with annealing time further increase. When the
annealing time exceeds 620 hours, the excess base
current saturated. In contrast, the normalized excess base
current of the transistors for HDR in the HLI shows
week dependence on annealing time. When the
annealing time exceeds 620 hours the excess base
current reaches the maximum value but the maximum
value is much smaller than that of the transistors for
LDR in HLI region as indicated in Fig. 4(b). Based on
the result in Fig. 4, room temperature post-annealing
process
only
modulated
the
radiation-induced
oxide-trapped charge and interface defect states which
mainly located in E-B interface region[2.9 l.
8
-0- HDR
?
LDR
"
HDR
? HDR
(a)
l OOk
lOOk
IOOk+RTA 72H
IOOk+RTA 432H
neutral base. As a result, the excess base current shows
strong dependence on post-annealing time in LLI but
week dependence on post-annealing time in HLI.
4. Summary
It was shown experimentally that the different radiation
response of bipolar transistors on the E-B bias strength.
The PE-NPN transistors show obvious ELDRS effect
both in LLI and HLI regions. A physical reason for the
effect is probably associated with the different types of
radiation induced defects that play the key role to the
increase of excess base current, which depends on the
E-B junction electric field strength.
Acknowledgments
0.5
0.4
0.6
0.7
Base-Emitter Voltage (V)
(b)
0.2
~
~-
'"
玸
0 1
???
.
....
? ....,
-0- HDR l OOk
??
LDR l OOk
???
HDR I OOk+ 72H
???
~.- HDR IOOk+432H
???
7
HDR IOOk+620 H
~f- HDR IOOk+930 H
I--+--
0.0 ' - - _........_ _ _---"_ _ _ _..............
10
1.1
1.2
Base-Emi tter Voltage (V)
Figure 4. Normalized excess base current versus E-B
voltage (a) LLI, (b) HLI.
3. Discussion
In the case of LLI, the electric field in the base oxide
over the E-B region is determined by the fringing field of
the E-B junction space charge, which plays a key role in
the buildup of radiation-induced oxide-trapped charge
and interface states in the base oxide no matter for LOR
or HOR. As a result, the excess base current shows the
same E-B bias dependence in both LDR and HDR[4.l0l.
In contrast to the case of LLI, the recombination in the
intrinsic base becomes the dominating response effect of
ionizing radiation in HLI. For a large E-B forward bias
voltage, the injected excess minority carrier
concentration can become comparable to or even exceed
the dopant concentration in the base. The excess base
current due to radiation induced defects related
recombination at the interface has been suppressed[9l.
Current gain degradation not only primarily comes from
increased recombination in the depletion region of the
E-B junction but it also comes from recombination in the
We thank J. C. Jia, and professor W. Lu for the help and
assist in the total dose experiments. This project is
supported in part by Science and Technology on Analog
Integrated Circuit Laboratory Foundation under No.
9140All050315DZ34052 and OC09YJTJl503 and the
National Natural Science Foundation of China under No.
61404013.
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