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: email@example.com 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. 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