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Y^K 621.382
The Effect of Gamma Rays on the Main Static Characteristics of SiGe Transistors
Dvornikov 0. V.1, Dziatlau V. L.1, Prokopenko N. N.2'3, Tchekhovski V. A.4
1Public Joint Stock Company "MNIPI", Minsk, Belarus 2Don State Technical University, Rostov-on-Don, Russia institute for Design Problems in Microelectronics of RAS, Zelenograd, Russia institute for Nuclear Problems BSU, Minsk, Belarus
E-mail: prokopniklOJO&gniaiLcom
The article considers the effect of 60Co gamma rays on the characteristics (the major ones for the analog ICs) of SiGe n-p-n transistors of SGB25V technology: the voltage across the forward-biased base-emitter junction, the dependence of the static base current gain {¡3) in the common-emitter configuration on emitter current., the output characteristic in the common-emitter configuration.
Key words: radiation hardness, SiGe-transistors, analog microcircuits, gamma rays, main static characteristics of transistor
Introduction
It is known that the sensors of spacecraft are often located outside the heated blocks, protected against the penetrating radiation (PR). Therefore, they are subjected to simultaneous exposure to low temperatures and PR. To improve the signal-to-noise ratio of these sensors it is reasonable to place an interface device closely to them, which realizes the sensor signal preprocessing and conveys information to the impenetrable block across the wire for final processing.
This interface is most often an analog IC. which should remain operative while simultaneously exposing to low temperatures and PR.
In some cases it is reasonable to apply the precision analog interfaces, implemented on bipolar transistors (BTs) for sensor signal processing.
Our studies [1]. supplementing the works [2 4]. have shown that SiGe BTs manufactured using IHP's SGB25V technology are appropriate for designing high-quality low-temperature analog ICs.
In the literature, the high radiation hardness of SiGe BTs [5 8] to the effect of gamma quanta [9 12]. protons [13 15]. including at low temperatures [16]. as well as to the simultaneous influence of different types of radiation [17 20] has been noted many times. However, most papers considered the radiation-induced alteration of the Gnniniol's plots (the dependences of the collector current Ic and the base current lb on the voltage across the forward-biased base-emitter junction Ube), and also the dependence of the gain ft = f (Ie)
The aim of this article is to consider 60 Co gamma ray effect on SiGe BT characteristics, which determine the static parameters of the analog ICs (operational amplifiers (OA), voltage regulators, etc.).
1 The Samples under Investigation, the Instrument and the Experimental Technique
The test chip SGB25V_016P. consisting of two n-p-n-transistors of n-p-n H type connected in parallel, was studied. Each transistor contained 16 emitters with dimensions of 0.42x3.36 ^m2, arranged in the form of 8x2 matrix. The test chip was produced using the technology of 0.25 ^m SiGe BiCMOS of SGB25V type and assembled into the package 5140.8-AH3 with the capacity of current-carrying elements not higher than 0.3 pF. The standard structure of the transistor is shown in Fig. 1. and its main parameters are presented in Table 1 [21].
Polysilicon emitter Gate polysilicon
Fig. 1. The standard structure of n-p-nH transistor on IHP's SGB25V technology [21].
Table 1 The main parameters of n-p-nH transistor
Name of parameter Valne
Size of emitter 0.42x0.84 mm2
Peak cut-off frequency 25 GHz
Collector-to-emitter breakdown voltage 7.0 V
Collector-to-base breakdown voltage > 20 V
Early's voltage > 100 V
The measurements were conducted by IPPP-1 tool [22]. Besides, special attention was given to the connecting circuit of the measured transistors in order to exclude self-excitation [1].
The irradiation of the samples with 60Co gamma-quanta was carried out with gamma dose rate of 12.4 rad/'s. After the irradiation sessions the total absorbed dose (A3) was the following: 0.05; 0.1; 0.2; 0.5; 1.1; 2.1 Mrad. The irradiation of the samples with the short-circnitcd pins was conducted at the temperature of about 300 K. The measurements were carried ont immediately after the irradiation. The duration of the measurement parameters did not exceed 1 hour.
2 The Measurement Results
A preliminary study of the storage time effect of one of BT samples on ft value was made after the irradiation at room temperature (anneal time).
It was established that the worst case is the measurement of BT parameters immediately after the exposure of gamma rays (Fig. 2).
P
le, A
Fig. 2. The dependence of ft = f (Ie) when the collector-to-base voltage Ucb = 1 V and the absorb ed dose Dq 2.1 Mrad for the various anneal time: 1 without anneal. 2-4 hours, 3-72 hours.
It was in this mode (without anneal) that four BT samples were measured, which showed close results (Fig. 3).
P
250 240 230 220 210 200 190 180 170 160 150
5„0E+4 5„0E+5 5„0E+6
C&PaA
Fig. 3. The dependence of ft on Dq absorbed dose for four BT samples, when = IV, Ie =1 mA.
The graphs of changes of ft gain at various doses of gamma rays for BT sample No.3 with the dependence ft = f (Ie), which was the closest to the average (Fig. ), are given in Fig. 4.
P
Ie, A
Fig. 4. The dependence of ft = f (/e), when Ucb = 1 V (without anneal): 1 — before the irradiation; 2 - if Dq = 50 Krad, 3 - = 200 Krad, 4 — Dq = 500 Krad, 5 — .Dg = 1.1 Mrad, 6 — Dq = 2,1 Mrad.
The output characteristic of BTs in the conimon-emiter configuration at various Dq is shown in Fig. . This dependence enables to determine the changes of the output differential resistance of the transistor
(Rout) and its Early's voltage (Ua) „ = ^Uee ^
Kout = A/c ~ Ic ■
1100m-
(1)
800m
400m
0
1,0
2,0 Uce, B
3,0
4,0
Fig. 5. The normalized dependence of Ic on the collector-to-emitter voltage Uce at Ib=8 ^A: the block curve - when Dq = 0; the broken curve
- /c/1.508 mA, when Dq = 2.1 Mrad.
8,J60E-4
(\0E-t-0 2„0E-3
1.0E-2 \2E-2
Fig. 6. The effect of the absorbed dose Dq=2.1 Mrad on the changes of the emitter-to-base voltage of BTs relating to the normal conditions (Dq=0) at various emitter currents (Ie).
The dependence of changes of the emitter-to-base voltage of BTs on the absorbed dose of gamma-qnanta Dq=2A Mrad is shown in Fig. .
The analysis of the obtained results enabled us to establish the following features of characteristics of SiGe n-p-n-transistors of SGB25V technology:
1. When Dq = 2.1 Mrad, gamma rays practically don't affect Ube value at Ie = Const. Thus, when
Ie = 100 ^A, Ube decreased (in comparison with the normal conditions) by 1.03 mV; when Ie = 1 mA — by 0. 95 mV; when Ie = 10 mA — by 0.88 mV. On this basis we can assume that the parameters of the analog ICs, determined by Ube value (the offset voltage of OA, the coordinates of the steady-state behavior, etc), will be low-sensitive to the effect of gamma rays.
2. Gamma rays strongly affect ft value only in the area of small emitter currents. Therefore, providing the operating mode of BTs with the heavy density of the emitter current, it is possible to reduce the radiation-indnced alteration of ft and, thus, the effect of gamma rays on such parameters as the input resistance of the emitter followers, the transfer ratio of the ''current mirrors", etc.
3. Usually, to increase the voltage gain in the analog ICs the active loads (ALs) are nsed on the transistors with a high valne of the output small-signal resistance. It follows from Fig. 5, gamma rays have little effect on output characteristic of BT in the common-emitter configuration. Consequently, the voltage gain of stages with AL will practically not change when exposure to gamma rays.
3 The Methods of Decreasing the Effect of ft Degradation of Transistors on the Main Parameters of the Differential Amplifiers
To decrease the effect of gamma rays on the steady-state behavior of the analog ICs, for example, for the zero level of the OA with one high-impedance node, it is reasonable to apply the special circuit techniques, developed in [23,24].
If we assume that all the n-p-n and p-n-p transistors of OA of Fig. 7, containing identical input differential stages (DS1, DS2), current mirror (CM1) and buffer amplifier (BA1), operate at the same emitter current, equal to some quantum I0, then the errors of the output current coordinates of each functional node of OA (their differences from the ideal value of I0) can be described by the following combined equation
I\.i = Io + Api.ilbp + Ani.ilbn, ¡2.1 = Io + Ap2.llbp + An2.llbn, I1.2 = Io + Api.2lbp + Ani.2hn, I2.2 = Io + Ap2.2lbp + An2.2lbn,
1cm 2 = Il.l + 11.2 + iplhp + inlhn
(2)
(3)
0
where hp, hn — base currents of the n-p-n and p-n-p transistors of DSI, DS2, CM1 at the emitter current Ie = J0; I1.u I2a-, h.2-, I2a-, I2.2 — output currents of DSI, DS2 (Fig. ); Icm2 — output current of CM1;
Fig. 7. The method of decreasing zero level of OA with one high-impedance node Si.
Api.j, Ani.j — positive and negative integers, which characterize a weak current unbalance of the input stages DS1, DS2, connected with the effect of ft gain on their operation [ , ]; 1 — coefficients of the
weak current unbalance of the current mirror CM1, regarding zero suppression of CM1 due to the base current effect of transistors [23.24].
For S1 highimpedance node the following Ki-rchhoff's combined equation, which takes into account ft effect on the operation of the functional nodes, is valid:
Ip = Icm 2 — Xplbp — XnInp — I<2.2 — I2.i, (4)
where Ip — independent parameter, which determines the zero level of OA [ ], Xp, Xn — parameters of the buffer amplifiers BA1, which characterize the direction and the absolute values of components of its input current.
To minimize the systematic component of the offset voltage of OA of Fig. (Vio) it is necessary to meet the following conditions
Api.i + Api.2 + = AP2.i + AP2.2 + Xp, (5)
construction of reference current sources [25], adjusting the static mode of the input transistors to the specified level of the gamma radiation, is necessary.
Summary
For many tasks of tool engineering, it is reasonable to realize the production of analog ICs, processing the signals of the low-ohmic sources in conditions of simultaneous effect of low temperatures and penetrating radiation, on SiGe BTs.
SiGe n-p-n-transistors of SGB25V technology retain their amplifying properties at the temperature of liquid nitrogen and at the effect of gamma rays with the absorbed dose up to 2.1 Mrad. Besides,
- the voltage on the forward-biased base-emitter junction (the test chip SGB25V_01GP) decreases in comparison with normal conditions by less than 1.03 niV within the range of the emitter currents from 100 ^A up to 10 mA;
- the output small-signal resistance in the common-emitter configuration doesn't practically change:
ft
area of small emitter currents. Therefore, providing the operating mode of the analog IC with heavy density of the emitter current, it is possible to reduce significantly the radiation-induced ft
- there is a number of circuitry techniques to riiini-
ft
frorii the exposure of gamma-quanta on the circuit functions, e.g. the systematic component of the offset voltage of OA [23, 24], the voltage gain [25].
Acknowledgments
The research is carried out at the expense of the Grant of the Russian Science Foundation (Project No 16-19-00122).
+ An2.2 + Xn. (6)
Thus, the circuit synthesis of OA with the architecture of Fig. , which should have low Vio, reduces to the rational choice of the main functional nodes of OA (DS1, DS2, CM1, BA), the coefficients of the weak current unbalance which should satisfy equations (5), (6). The versions of designing these functional nodes of OA (DS1, DS2, CM1, BA) are given in [23,24].
Similarly, to reduce the radiation degradation of the voltage gain of classical differential cascades, special
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Вплив гамма- випромшювання на основы! статичш характеристики SiGe транзистор!в
Двортков О. В., Дятлов В. Л., Прокопенко М. М., Чеховський В. О.
Розглянуто вплив гамма-вииромшювання радюну-кл!да 60 Со на найбшын важлив! для аналогових мшро-схем характеристики SiGe n-p-n транзистор!в техпро-цесу SGB25V: напруга на прямозмщеному емиерному переход!, залежшсть статичного коефщ!ента передач! струму бази в схем! з сшльним ем!тером (,б) в!д ем!тер-ного струму, вих!дна характеристика в схем! з сшльним емиером.
Ключовг слова: рад!ацшна стшкють; SiGe-транзистори; аналогов! мшросхеми; гамма випромшю-вання; основш статичш характеристики транзистора
Влияние гамма-излучения на основные статические характеристики SiGe транзисторов
Дворников О. В., Дятлов В. Л., Прокопенко П. П., Чеховский В. А.
Рассмотрено воздействие гамма-излучения радио-
60
микросхем характеристики SiGe n-p-n транзисторов техпроцесса SGB25V: напряжение на прямосмещенном эмиттерном переходе, зависимость статического коэффициента передачи тока базы в схеме с общим эмиттером {р) от эмиттерного тока, выходная характеристика в схеме с общим эмиттером.
Ключевые слова: радиационная стойкость; SiGe-транзисторы; аналоговые микросхемы; гамма излучение; основные статические характеристики транзистора
