- Email: [email protected]
DLTS Studies of bias dependence of defects in silicon NPN bipolar junction transistor irradiated by heavy ions
Nuclear Instruments and Methods in Physics Research A 688 (2012) 7–10
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Technical Notes
DLTS Studies of bias dependence of defects in silicon NPN bipolar junction transistor irradiated by heavy ions Chaoming Liu a, Xingji Li a,n, Hongbin Geng a, Erming Rui a, Jianqun Yang a, Liyi Xiao b a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Microelectronics Center, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
abstract
Article history: Received 28 February 2012 Received in revised form 6 May 2012 Accepted 20 May 2012 Available online 1 June 2012
The characteristic degradation in silicon NPN bipolar junction transistors (BJTs) of 3DG130 type is examined under the irradiation with 35 MeV silicon (Si) ions under forward, grounded and reverse bias conditions, respectively. Different electrical parameters were in-situ measured during the exposure under each bias condition. Using deep level transient spectroscopy (DLTS), deep level defects in the base-collector junction of 3DG130 transistors under various bias conditions are measured after irradiation. The activation energy, capture cross section and concentration of observed deep level defects are measured using DLTS technique. Based on the in situ electrical measurement and DLTS spectra, it is clearly that the bias conditions could affect the concentration of deep level defects, and the displacement damage induced by heavy ions. & 2012 Elsevier B.V. All rights reserved.
Keywords: Bipolar junction transistors Radiation damage Deep level transient spectroscopy Gain degradation
1. Introduction Bipolar junction transistors (BJTs) have important applications in analog or mixed-signal integrated circuits (ICs) and BiCMOS (Bipolar Complementary Metal Oxide Semiconductor) circuits because of their current drive capability, linearity and excellent matching characteristics, some of which are employed for space application [1–7]. It is valuable to research the radiation response of bipolar junction transistors to find better design strategies before employing them for specific applications. In space systems, electronic components are susceptible to radiation environment composed of cosmic rays, protons, electrons and other particles [8–14]. In the recent years, bipolar junction transistors have been studied for damage mechanisms by heavy ion irradiation [15,16]. However, there is little work on heavy ion induced effects and consequent characterization of defects and traps by deep level transient spectroscopy (DLTS). The bias condition during the irradiation or the operation is an important factor for bipolar devices. Some researches focus on the bias dependence of the ionizing effects induced by electrons and g-rays [17,18], and the former work found that the bias conditions also could affect the displacement effects [19]. However, the bias dependence to combined effects of the ionizing and displacement damage induced by incident particles is not known clearly. It is valuable to research the bias dependence to the
n
Corresponding author. Tel. þ 86 451 86414445; fax: þ86 451 86415168. E-mail address: [email protected] (X. Li).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.05.073
combined effects of ionizing and displacement damage induced by heavy ions. Therefore, the results of in-situ electrical measurements and DLTS studies are presented to gain an insight into the mechanism of transistor gain degradation under various bias conditions in this paper.
2. Experimental Details The 3DG112 bipolar junction transistors were used as samples in this study, which are a type of the high radio frequency and low power silicon (Si) NPN transistor made by a single manufacturer from a single diffusion lot. The thickness is about 600 nm, 1.2, 1.3 and 12 mm for the insulating silicon dioxide (SiO2), the emitter (n þ), the base (p þ) and the epitaxial layer (n-) of the 3DG112 BJTs, respectively. The collector was doped with phosphorus to a level of 7 1015 cm 3. The base was boron doped to about 1 1018 cm 3. Doping levels were determined from CV measurements of the transistor diodes. The heavy ions irradiations were performed using the EN Tandem Accelerator in the State Key Laboratory of Nuclear Physics and Technology, Peking University, China. The irradiations were carried out in a vacuum chamber with a specially designed Faraday cup, which is used to measure the heavy ions beam current. From these measurements, the flux and fluences were determined for the irradiation experiments. The samples were mounted inside a package with removable upper lid for irradiation. In order to research the degradation of transistors irradiated
8
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 688 (2012) 7–10
by 35 MeV Si ions under various bias conditions, the following cases were performed: (1) Zero bias case: VBE ¼VBC ¼0 V (2) Forward bias case: VBE ¼0.7 V, VBC ¼0 V (3) Reverse bias case: VBE ¼-4 V, VBC ¼0 V Electrical parameters of the NPN BJTs were measured in-situ using a semiconductor characterization system that consists of KEITHLEY 4200-SCS. The turn-around time between irradiation and device measurements is approximately less than 5 s. The irradiations and electrical measurements were performed at room temperature. DLTS measurements were performed using a PhysTech HERA-DLTS (High Energy Resolution Analysis Deep Level Transient Spectroscopy) system. This system uses maximum analysis on the capacitance transients to generate 28 DLTS spectra during one temperature scan. The amplitude of a DLTS peak is directly proportional to the defect concentration, and time constant can be used to calculate the activation energy. Fig. 2. The change in the reciprocal of current gain (D(1/b)) at a base-emitter voltage of 0.65 V as a function of fluence under various bias conditions.
3. Results and Discussion 3.1. In situ Measurements of Electrical Characteristics Gummel characteristics were measured for the NPN transistors before and after irradiation in a common-emitter configuration, applying a sweep in VE from 0 to 1.2 V and keeping VB ¼VC ¼VCB ¼0 V. Fig. 1 shows the variations of collector current (IC) and base current (IB) with base-emitter voltage (VBE) for the 3DG130 transistors irradiated by 35 MeV Si ions with different fluences, in which the case of base-emitter junction is zero bias condition during exposures. The plots under forward and reverse bias cases are similar as Fig. 1. As shown in Fig. 1, the collector current (IC) keeps invariably with the increasing fluence, while the base current (IB) increases. These phenomena indicate that the base current (IB) is more sensitive to the radiation damage caused by 35 MeV Si ions, while the collector current is only slightly affected by irradiation at any given base-emitter voltage value (VBE). As a consequence, for these fluences, the gain degradation is mostly affected by the behavior of the base current. The change in the reciprocal of the gain variation (D(1/b)) is defined as the value after irradiation minus the initial one, that is D(1/b) ¼1/b–1/bpre-rad. Fig. 2 shows the change in the reciprocal of current gain (D(1/b)) at a base-emitter voltage of 0.65 V as a
Fig. 3. DLTS spectra of Si ions irradiated transistors for different bias conditions.
function of fluence under the three bias conditions. It is shown that D(1/b) in three bias cases show an approximate linear behavior with irradiation fluence. The radiation damage under zero bias condition is greatest, forward bias condition is lowest, and the damage under reverse bias case is between the other two bias cases. 3.2. The DLTS Measurements
Fig. 1. The IC and IB with VBE for the 3DG130 transistors irradiated by 35 MeV Si ions with different fluences under zero bias case.
The deep level defects introduced by Si ions irradiation are characterized using DLTS technique. Temperature scans were made between 50 and 300 K using a liquid helium cryostat. In the DLTS characterization, the capacitance transients of base-collector junction at different temperatures are recorded. Typically, the DLTS scans were performed with a reverse bias voltage (VR) of 5 V and a pulse voltage (VP) of 1 V (i.e., a fill pulse of 4 V). A fill pulse width (tP) of 100 ms was chosen to ensure complete trap filling. DLTS spectrums of the base-collector junction of 3DG130 transistors under various bias conditions at the same fluence are shown in Fig. 3. There major peaks are apparent in the spectra that correspond to known defects [20–22]. The vacancy-oxygen (V-O) traps produce a level at 0.13 eV below EC and a broad DLTS
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 688 (2012) 7–10
9
Table 1 Data obtained from DLTS analysis of irradiated transistor. Bias case
Defect label
Defect name
Activation energy (EC ET) (eV)
Trap concentration NT(cm 3)
Capture cross section (cm2)
Zero bias case
EZ1 EZ2 EZ3
E center (V-P) Di-vacancy (V2(¼ / )) A center (V-O)
0.397 0.231 0.129
3.44Eþ 14 1.27Eþ 14 1.82Eþ 13
9.55E 16 1.82E 15 4.34E 16
Forward bias case
EF1 EF2 EF3
E center (V-P) Di-vacancy (V2 (¼ / )) A center (V-O)
0.390 0.241 0.124
3.00E þ 14 2.25Eþ 14 6.92Eþ 13
6.28E 16 8.80E 15 5.95E 18
Reverse bias case
ER1 ER2 ER3
E center (V-P) Di-vacancy (V2 (¼ / )) A center (V-O)
0.393 0.239 0.130
3.38Eþ 14 2.13Eþ 14 3.09Eþ 13
7.63E 16 5.91E 14 7.61E 16
peak at 70 K. The divacancy (V2) trap has a level at 0.23 eV below the conduction band EC corresponding to ¼/ (125 K) transitions. The 220 K peak is the sum of several other defects including the E center (V-P) and a complex defect (or defects) thought to be part of the damage cascade. The concentration of the defects, activation energy and capture cross section of the different defects are calculated and tabulated in Table 1. Based on the results of the DLTS spectra, the bias conditions during irradiations could affect the generation and recombination of deep level defects. In DLTS spectrum, the peak height represents the trap concentration. As shown in Fig. 3, the concentration of V-P centers in the BJT under the ground bias condition during irradiations is higher than the BJTs under other two bias conditions. Therefore, the bias conditions could affect the concentration of deep level defects, and the displacement damage induced by heavy ions.
[25]. The free charge carriers may be introduced in several ways, including electrical injection into devices. When the base-emitter junction is forward bias, extra minority-carriers would be infused into the base region. While the base-emitter junction is reverse biased or all terminals grounded, there is no extra minoritycarriers infused into the base. Therefore, the electrical injection results in less degradation of current gain in the forward bias case than other bias cases. Based on the former experimental results [5–7,19], the trends of the D(1/b) curves in this experiment is not the same as the phenomenon above in the individual bias effect of ionizing or displacement damage. Therefore, there might be some complex interactions between the ionizing and displacement damage, leading to that the NPN BJT under the reverse bias case has lower radiation damage induced by 35 MeV Si ions. This mechanism is valuable and needs to be further studied in the future.
4. Discussion
5. Conclusions
Heavy ions can induce both of ionization damage and displacement damage in the BJTs. Bipolar junction transistors are sensitive to both ionization and displacement damage, leading to current gain degradation. The ionization damage could cause interface traps and net positive charges in the oxide overlying the base-emitter junction, leading to an increase in the base current and the current gain degradation. Based on the DLTS results, the displacement damage can produce vacancy defect complexes that are effective recombination and trapping centers, leading to a decrease in minority carrier lifetime. The degradation of minority carrier lifetime results in the current gain degradation for the BJTs. The radiation damage will increase recombination in both the neutral base region and the emitter-base depletion region. As mentioned in Ref. [23], it is usually assumed that recombination current in emitter-base junctions varies as exp(qVBE/nkT), where q is the charge of electron; n is the ideality factor; k is the Boltzmann constant; T is the absolute temperature. The recombination in the depletion region has an ideality factor of two, while that in the neutral base is one. Based on the Gummel curves, it appears that the degradation is dominated by recombination in the depletion region. Based on the results in Fig. 2, the D(1/b) under various bias conditions show a linear behavior that follows the Messenger– Spratt equation [24] for the heavy ions irradiations. It is clear that the change in reciprocal of the current gain (D(1/b)) of the device in the forward bias case varies less, while the D(1/b) in the zero bias case changes severely at a given fluence. Base-emitter bias conditions could affect the displacement effects to some extent in the BJTs. The injection annealing is an enhancement of displacement defect reordering by the presence of free charge carriers
In this paper, the characteristic degradation in Si NPN BJTs of 3DG130 type is examined under the irradiation with 35 MeV Si ions. The experimental data shows that the collector current (IC) remains almost unvaried with the increasing fluence, while the base current (IB) increases. The gain degradation is mostly affected by the behavior of the base current. Deep level defects introduced in the base-collector junction of the transistor are identified as E center, divacancy and A center. The activation energy, capture cross section and concentration of observed deep level defects are measured using the DLTS technique. Based on the in situ electrical measurement and DLTS spectra, it is clearly that the bias conditions could affect the concentration of deep level defects, and the displacement damage induced by heavy ions. References [1] H.J. Barnaby, R.D. Schrimpf, A.L. Sternberg, V. Berthe, C.R. Cirba, R.L. Pease, IEEE Transactions on Nuclear Science 48 (6) (2001) 2074. [2] H.J. Barnaby, R.D. Schrimpf, R.L. Pease, P. Cole, T. Turflinger, J. Krieg, J. Titus, D. Emily, M. Gehlhausen, S.C. Witczak, M.C. Maher, D. Van Nort, IEEE Transactions on Nuclear Science 46 (6) (1999) 1666. [3] J. Beaucour, T. Carriere, A. Gach, P. Poirot, IEEE Transactions on Nuclear Science 41 (6) (1994) 2420. [4] A.H. Johnston, R.E. Plaag, IEEE Transactions on Nuclear Science NS-34 (6) (1987) 1474. [5] X.J. Li, H.B. Geng, C.M. Liu, Z.M. Zhao, M.J. Lan, D.Z. Yang, S.Y. He, IEEE Transactions on Nuclear Science 57 (2) (2010) 831. [6] X.J. Li, J.D. Xiao, C.M. Liu, Z.M. Zhao, H.B. Geng, M.J. Lan, D.Z. Yang, S.Y. He, Nuclear Instruments and Methods in Physics Research Section A 621 (2010) 707. [7] X.J. Li, H.B. Geng, C.M. Liu, Z.M. Zhao, M.J. Lan, D.Z. Yang, S.Y. He, Nuclear Instruments and Methods in Physics Research Section A 612 (2009) 171. [8] G.P. Summers, E.A. Burke, C.J. Dale, E.A. Wolicki, P.W. Marshall, M.A. Gehlhausen, IEEE Transactions on Nuclear Science 34 (6) (1987) 1134. [9] C.M. Dinesh, Ramani, M.C. Radhakrishna, R.N. Dutt, S.A. Khan, D. Kanjilal, Nuclear Instruments and Methods in Physics Research Section B 266 (2008) 1713.
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[10] S.L. Kosier, R.D. Schrimpf, R.N. Nowlin, D.M. Fleetwood, M. DeLaus, R.L. Pease, W.E. Combs, A. Wei, F. Chai, IEEE Transactions on Nuclear Science 40 (6) (1993) 1276. [11] S.A. Kamh, F.A.S. Solman, Nuclear Instruments and Methods in Physics Research Section A 564 (1) (2006) 463. [12] A.H. Johnston, G.M. Swift, B.G. Rax, IEEE Transactions on Nuclear Science 41 (6) (1994) 2427. [13] M. Manghisoni, L. Ratti, V. Re, V. Speziali, G. Traversi, G. Fallica, Nuclear Instruments and Methods in Physics Research Section A 518 (2004) 477. [14] A. Al-Mohamad, M. Chahoud, Nuclear Instruments and Methods in Physics Research Section A 538 (2005) 703. [15] H.J. Barnaby, S.K. Smith, R.D. Schrimpf, D.M. Fleetwood, R.L. Pease, IEEE Transactions on Nuclear Science 49 (6) (2002) 2643. [16] V.S. Pershenkov, K.A. Chumakov, A.Y. Nikiforov, A.I. Chumakov, V.N. Ulimov, A.A. Romanenko, Proceedings of the IEEE 9th Radiation and its Effects on Components and Systems, 2007, 1. [17] S.L. Kosier, W.E. Combs, A. Wei, R.D. Schrimpf, D.M. Fleetwood, M. DeLaus, R.L. Pease, IEEE Transactions on Nuclear Science 41 (6) (1994) 1864.
[18] V.S. Pershenkov, V.B. Maslov, S.V. Cherepko, I.N. Shvetzov-Shilovsky, V.V. Belyakov, A.V. Sogoyan, V.I. Rusanovsky, V.N. Ulimov, V.V. Emelianov, V.S. Nasibullin, IEEE Transactions on Nuclear Science 44 (6) (1997) 1840. [19] C.M. Liu, X.J. Li, H.B. Geng, Z.M. Zhao, D.Z. Yang, S.Y. He, Nuclear Instruments and Methods in Physics Research Section A 624 (2010) 671. [20] B.G. Svensson, in: Robert Hull (Ed.), Properties of Crystalline Silicon, INSPEC: The Insitution of Electrical Engineers, London, 1999, p. 763. [21] G.D. Watkins, in: S.T. Pantelides (Ed.), Deep Centers in Semiconductors, Gordon and Breach, New York, 1992, p. 177. [22] G.D. Watkins, in: Hull Robert (Ed.), Properties of Crystalline Silicon, INSPEC.: The Institution of Electrial Engineers, London, 1999, p. 643. [23] S.L. Kosier, R.D. Schrimpf, R.N. Nowlin, D.M. Fleetwood, M. Delaus, R.L. Pease, W.E. Combs, A. Wei, F. Chai, IEEE Transactions on Nuclear Science 40 (6) (1993) 1276. [24] G.C. Messenger, M.S. Ash, The Effects of Radiation on Electronic Systems, second ed., Van Nostrand Reinhold, New York, 1992. [25] J.R. Srour, C.J. Marshall, P.W. Marshall, IEEE Transactions on Nuclear Science 50 (3) (2003) 653.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Technical Notes
DLTS Studies of bias dependence of defects in silicon NPN bipolar junction transistor irradiated by heavy ions Chaoming Liu a, Xingji Li a,n, Hongbin Geng a, Erming Rui a, Jianqun Yang a, Liyi Xiao b a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Microelectronics Center, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
abstract
Article history: Received 28 February 2012 Received in revised form 6 May 2012 Accepted 20 May 2012 Available online 1 June 2012
The characteristic degradation in silicon NPN bipolar junction transistors (BJTs) of 3DG130 type is examined under the irradiation with 35 MeV silicon (Si) ions under forward, grounded and reverse bias conditions, respectively. Different electrical parameters were in-situ measured during the exposure under each bias condition. Using deep level transient spectroscopy (DLTS), deep level defects in the base-collector junction of 3DG130 transistors under various bias conditions are measured after irradiation. The activation energy, capture cross section and concentration of observed deep level defects are measured using DLTS technique. Based on the in situ electrical measurement and DLTS spectra, it is clearly that the bias conditions could affect the concentration of deep level defects, and the displacement damage induced by heavy ions. & 2012 Elsevier B.V. All rights reserved.
Keywords: Bipolar junction transistors Radiation damage Deep level transient spectroscopy Gain degradation
1. Introduction Bipolar junction transistors (BJTs) have important applications in analog or mixed-signal integrated circuits (ICs) and BiCMOS (Bipolar Complementary Metal Oxide Semiconductor) circuits because of their current drive capability, linearity and excellent matching characteristics, some of which are employed for space application [1–7]. It is valuable to research the radiation response of bipolar junction transistors to find better design strategies before employing them for specific applications. In space systems, electronic components are susceptible to radiation environment composed of cosmic rays, protons, electrons and other particles [8–14]. In the recent years, bipolar junction transistors have been studied for damage mechanisms by heavy ion irradiation [15,16]. However, there is little work on heavy ion induced effects and consequent characterization of defects and traps by deep level transient spectroscopy (DLTS). The bias condition during the irradiation or the operation is an important factor for bipolar devices. Some researches focus on the bias dependence of the ionizing effects induced by electrons and g-rays [17,18], and the former work found that the bias conditions also could affect the displacement effects [19]. However, the bias dependence to combined effects of the ionizing and displacement damage induced by incident particles is not known clearly. It is valuable to research the bias dependence to the
n
Corresponding author. Tel. þ 86 451 86414445; fax: þ86 451 86415168. E-mail address: [email protected] (X. Li).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.05.073
combined effects of ionizing and displacement damage induced by heavy ions. Therefore, the results of in-situ electrical measurements and DLTS studies are presented to gain an insight into the mechanism of transistor gain degradation under various bias conditions in this paper.
2. Experimental Details The 3DG112 bipolar junction transistors were used as samples in this study, which are a type of the high radio frequency and low power silicon (Si) NPN transistor made by a single manufacturer from a single diffusion lot. The thickness is about 600 nm, 1.2, 1.3 and 12 mm for the insulating silicon dioxide (SiO2), the emitter (n þ), the base (p þ) and the epitaxial layer (n-) of the 3DG112 BJTs, respectively. The collector was doped with phosphorus to a level of 7 1015 cm 3. The base was boron doped to about 1 1018 cm 3. Doping levels were determined from CV measurements of the transistor diodes. The heavy ions irradiations were performed using the EN Tandem Accelerator in the State Key Laboratory of Nuclear Physics and Technology, Peking University, China. The irradiations were carried out in a vacuum chamber with a specially designed Faraday cup, which is used to measure the heavy ions beam current. From these measurements, the flux and fluences were determined for the irradiation experiments. The samples were mounted inside a package with removable upper lid for irradiation. In order to research the degradation of transistors irradiated
8
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 688 (2012) 7–10
by 35 MeV Si ions under various bias conditions, the following cases were performed: (1) Zero bias case: VBE ¼VBC ¼0 V (2) Forward bias case: VBE ¼0.7 V, VBC ¼0 V (3) Reverse bias case: VBE ¼-4 V, VBC ¼0 V Electrical parameters of the NPN BJTs were measured in-situ using a semiconductor characterization system that consists of KEITHLEY 4200-SCS. The turn-around time between irradiation and device measurements is approximately less than 5 s. The irradiations and electrical measurements were performed at room temperature. DLTS measurements were performed using a PhysTech HERA-DLTS (High Energy Resolution Analysis Deep Level Transient Spectroscopy) system. This system uses maximum analysis on the capacitance transients to generate 28 DLTS spectra during one temperature scan. The amplitude of a DLTS peak is directly proportional to the defect concentration, and time constant can be used to calculate the activation energy. Fig. 2. The change in the reciprocal of current gain (D(1/b)) at a base-emitter voltage of 0.65 V as a function of fluence under various bias conditions.
3. Results and Discussion 3.1. In situ Measurements of Electrical Characteristics Gummel characteristics were measured for the NPN transistors before and after irradiation in a common-emitter configuration, applying a sweep in VE from 0 to 1.2 V and keeping VB ¼VC ¼VCB ¼0 V. Fig. 1 shows the variations of collector current (IC) and base current (IB) with base-emitter voltage (VBE) for the 3DG130 transistors irradiated by 35 MeV Si ions with different fluences, in which the case of base-emitter junction is zero bias condition during exposures. The plots under forward and reverse bias cases are similar as Fig. 1. As shown in Fig. 1, the collector current (IC) keeps invariably with the increasing fluence, while the base current (IB) increases. These phenomena indicate that the base current (IB) is more sensitive to the radiation damage caused by 35 MeV Si ions, while the collector current is only slightly affected by irradiation at any given base-emitter voltage value (VBE). As a consequence, for these fluences, the gain degradation is mostly affected by the behavior of the base current. The change in the reciprocal of the gain variation (D(1/b)) is defined as the value after irradiation minus the initial one, that is D(1/b) ¼1/b–1/bpre-rad. Fig. 2 shows the change in the reciprocal of current gain (D(1/b)) at a base-emitter voltage of 0.65 V as a
Fig. 3. DLTS spectra of Si ions irradiated transistors for different bias conditions.
function of fluence under the three bias conditions. It is shown that D(1/b) in three bias cases show an approximate linear behavior with irradiation fluence. The radiation damage under zero bias condition is greatest, forward bias condition is lowest, and the damage under reverse bias case is between the other two bias cases. 3.2. The DLTS Measurements
Fig. 1. The IC and IB with VBE for the 3DG130 transistors irradiated by 35 MeV Si ions with different fluences under zero bias case.
The deep level defects introduced by Si ions irradiation are characterized using DLTS technique. Temperature scans were made between 50 and 300 K using a liquid helium cryostat. In the DLTS characterization, the capacitance transients of base-collector junction at different temperatures are recorded. Typically, the DLTS scans were performed with a reverse bias voltage (VR) of 5 V and a pulse voltage (VP) of 1 V (i.e., a fill pulse of 4 V). A fill pulse width (tP) of 100 ms was chosen to ensure complete trap filling. DLTS spectrums of the base-collector junction of 3DG130 transistors under various bias conditions at the same fluence are shown in Fig. 3. There major peaks are apparent in the spectra that correspond to known defects [20–22]. The vacancy-oxygen (V-O) traps produce a level at 0.13 eV below EC and a broad DLTS
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 688 (2012) 7–10
9
Table 1 Data obtained from DLTS analysis of irradiated transistor. Bias case
Defect label
Defect name
Activation energy (EC ET) (eV)
Trap concentration NT(cm 3)
Capture cross section (cm2)
Zero bias case
EZ1 EZ2 EZ3
E center (V-P) Di-vacancy (V2(¼ / )) A center (V-O)
0.397 0.231 0.129
3.44Eþ 14 1.27Eþ 14 1.82Eþ 13
9.55E 16 1.82E 15 4.34E 16
Forward bias case
EF1 EF2 EF3
E center (V-P) Di-vacancy (V2 (¼ / )) A center (V-O)
0.390 0.241 0.124
3.00E þ 14 2.25Eþ 14 6.92Eþ 13
6.28E 16 8.80E 15 5.95E 18
Reverse bias case
ER1 ER2 ER3
E center (V-P) Di-vacancy (V2 (¼ / )) A center (V-O)
0.393 0.239 0.130
3.38Eþ 14 2.13Eþ 14 3.09Eþ 13
7.63E 16 5.91E 14 7.61E 16
peak at 70 K. The divacancy (V2) trap has a level at 0.23 eV below the conduction band EC corresponding to ¼/ (125 K) transitions. The 220 K peak is the sum of several other defects including the E center (V-P) and a complex defect (or defects) thought to be part of the damage cascade. The concentration of the defects, activation energy and capture cross section of the different defects are calculated and tabulated in Table 1. Based on the results of the DLTS spectra, the bias conditions during irradiations could affect the generation and recombination of deep level defects. In DLTS spectrum, the peak height represents the trap concentration. As shown in Fig. 3, the concentration of V-P centers in the BJT under the ground bias condition during irradiations is higher than the BJTs under other two bias conditions. Therefore, the bias conditions could affect the concentration of deep level defects, and the displacement damage induced by heavy ions.
[25]. The free charge carriers may be introduced in several ways, including electrical injection into devices. When the base-emitter junction is forward bias, extra minority-carriers would be infused into the base region. While the base-emitter junction is reverse biased or all terminals grounded, there is no extra minoritycarriers infused into the base. Therefore, the electrical injection results in less degradation of current gain in the forward bias case than other bias cases. Based on the former experimental results [5–7,19], the trends of the D(1/b) curves in this experiment is not the same as the phenomenon above in the individual bias effect of ionizing or displacement damage. Therefore, there might be some complex interactions between the ionizing and displacement damage, leading to that the NPN BJT under the reverse bias case has lower radiation damage induced by 35 MeV Si ions. This mechanism is valuable and needs to be further studied in the future.
4. Discussion
5. Conclusions
Heavy ions can induce both of ionization damage and displacement damage in the BJTs. Bipolar junction transistors are sensitive to both ionization and displacement damage, leading to current gain degradation. The ionization damage could cause interface traps and net positive charges in the oxide overlying the base-emitter junction, leading to an increase in the base current and the current gain degradation. Based on the DLTS results, the displacement damage can produce vacancy defect complexes that are effective recombination and trapping centers, leading to a decrease in minority carrier lifetime. The degradation of minority carrier lifetime results in the current gain degradation for the BJTs. The radiation damage will increase recombination in both the neutral base region and the emitter-base depletion region. As mentioned in Ref. [23], it is usually assumed that recombination current in emitter-base junctions varies as exp(qVBE/nkT), where q is the charge of electron; n is the ideality factor; k is the Boltzmann constant; T is the absolute temperature. The recombination in the depletion region has an ideality factor of two, while that in the neutral base is one. Based on the Gummel curves, it appears that the degradation is dominated by recombination in the depletion region. Based on the results in Fig. 2, the D(1/b) under various bias conditions show a linear behavior that follows the Messenger– Spratt equation [24] for the heavy ions irradiations. It is clear that the change in reciprocal of the current gain (D(1/b)) of the device in the forward bias case varies less, while the D(1/b) in the zero bias case changes severely at a given fluence. Base-emitter bias conditions could affect the displacement effects to some extent in the BJTs. The injection annealing is an enhancement of displacement defect reordering by the presence of free charge carriers
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