A Comparison of Gamma and Proton Radiation Effects in 200 Ghz Sige Hbts Akil K

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2358 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 6, DECEMBER 2005 A Comparison of Gamma and Proton Radiation Effects in 200 GHz SiGe HBTs Akil K. Sutton, Student Member, IEEE, Becca M. Haugerud, Student Member, IEEE, A. P. Gnana Prakash, Member, IEEE, Bongim Jun, Member, IEEE, John D. Cressler, Fellow, IEEE, Cheryl J. Marshall, Member, IEEE, Paul W. Marshall, Member, IEEE, Ray Ladbury, Member, IEEE, Fernando Guarin, Member, IEEE, and Alvin J. Joseph, Member, IEEE

Abstract—We present the results of gamma irradiation on third-generation SiGe HBTs demonstrate an increased proton third-generation, 200 GHz SiGe HBTs. Pre- and post-radiation dc tolerance over previous SiGe technology nodes. In this work we figures-of-merit are used to quantify the tolerance of the raised present the first study of the gamma tolerance of these devices. extrinsic base structure to Co-60 gamma rays for varying device geometries. Additionally, the impact of technology scaling on the The third-generation SiGe HBTs investigated here were fab- observed radiation response is addressed through comparisons to ricated at IBM Microelectronics (IBM 8HP) [5], and achieve a second generation, 120 GHz SiGe HBTs. Comparisons to previous peak cutoff frequency (peak ) of 200 GHz. This improvement proton-induced degradation results in these 200 GHz SiGe HBTs in peak over previous technology nodes [6], [7] was realized are also made, and indicate that the STI isolation oxide of the through fundamental changes in the physical structure of the de- device shows increased degradation following Co-60 irradiation. The EB spacer oxide, on the other hand, demonstrates increased vice, yielding a novel, reduced thermal cycle “raised extrinsic susceptibility to proton damage. Low dose rate proton testing base” structure. The SiGe base region features an uncondition- was also performed and indicate that although there is a proton ally stable, 25% peak Ge, and a C-doped SiGe profile deposited dose rate effect present in these devices, it cannot fully explain the using UHV/CVD epitaxial growth techniques [5]. Conventional observed trends. Similar trends have previously been observed for deep trench (DT) and shallow trench isolation (STI), in addition buried oxides and isolation oxides in several MOS technologies and have been attributed to increased charge yield in these oxides for to an in-situ doped polysilicon emitter, were maintained from 1.2 MeV Co-60 gamma rays when compared to 63 MeV protons. prior technology nodes. Previous investigations identified the emitter-base (EB) and STI oxide interfaces as the areas most Index Terms—EB spacer, ELDRS, gamma radiation, proton radiation, radiation sources, SiGe, SiGe HBT, shallow trench isola- prone to the formation of radiation induced traps. A represen- tion, technology scaling. tative 2D-MEDICI cross-section of the region of interest is depicted in Fig. 1 (after [8]). These 200 GHz SiGe HBTs demonstrate increased proton tol- I. MOTIVATION erance when compared to earlier SiGe HBT technology nodes HE inherent robust total dose tolerance, typically to multi- (first-generation 50 GHz (IBM 5HP) and second-generation 120 T Mrad(Si) levels, of Silicon-Germanium Hetero-junction GHz (IBM 7 HP) SiGe HBTs) [4]. The aim of the present work Bipolar Transistors (SiGe HBTs) make them prime contenders is therefore two-fold; first we seek to analyze the impact of for a variety of terrestrial and space-borne integrated circuit ap- technology scaling on gamma-induced radiation damage, and plications. SiGe HBT performance characteristics continue to secondly, we wish to compare the degradation response mech- match those of III-V materials, while sharing a unique seam- anisms for proton and gamma irradiation. less integration platform with traditional low cost, high yield Si CMOS fabrication [1]. Single event upset (SEU) is a known II. EXPERIMENT concern for high-speed SiGe HBT digital circuits, and a variety A. Sample Preparation of mitigation techniques are currently under investigation to ad- dress these issues [2]. Previous studies [3], [4] have shown that Devices of varying geometry across the second- and third technology generations were chosen for testing. Third-generation devices with an emitter-area of m and Manuscript received July 2005; revised December 2005. This work was sup- ported in part by DTRA under the Radiation Hardened Microelectronics Pro- m were chosen as the primary device geometries gram, in part by NASA-GSFC under the NASA Electronic Parts and Packaging of interest for comparing proton and gamma radiation damage. (NEPP) Program, in part by IBM, in part by DARPA, and in part by the Georgia Additional devices ( m m Electronic Design Center at Georgia Tech. A. K. Sutton, B. M. Haugerud, A. P. G. Prakash, B. Jun, and J. D. Cressler and m ) were included in the examina- are with the School of Electrical and Computer Engineering, Georgia Institute tion of geometrical dependence, proton dose rate effects, and of Technology, Atlanta, GA 30308 USA (e-mail:[email protected]). high temperature annealing. The impact of technology scaling C. J. Marshall and P. W. Marshall are with the NASA-GSFC, Greenbelt, MD 20771 USA. on gamma-induced radiation degradation included second-gen- R. L. Ladbury is with the Muinez Engineering, Houston, TX 77001 USA. eration devices with m and m . F. Guarin is with the IBM Microelectronics, East Fishkill, NY 12533 USA. Samples were received on wafer and were diced and pack- A. J. Joseph is with the IBM Microelectronics, Essex Junction, VT 05452 USA. aged into 28-pin DIPs with two or three devices bonded out per Digital Object Identifier 10.1109/TNS.2005.860728 package. All terminals (emitter, base, collector and substrate) of

TABLE I APPROXIMATE INCREMENTAL DOSE RATE COMPARISON FOR THE PROTON AND GAMMA IRRADIATION OF SECOND AND THIRD-GENERATION DEVICES

TABLE II APPROXIMATE INCREMENTAL DOSE RATE COMPARISON FOR THE PROTON AND GAMMA IRRADIATION OF THIRD-GENERATION DEVICES

Fig. 1. MEDICI simulation cross section showing the EB and CB space charge regions and their proximity to the trap regions of interest. the devices were grounded during the irradiation. Pre-radiation Cumulative dose rates in this case ranged from 100 krad(Si) to characterization was done at the facility and each device irradi- 3Mrad(Si) as depicted in Table II. Sample sizes for this exper- ated to a given dose with measurements being made in between iment ranged from two to four devices per geometry each ex- incremental dose steps (stepped-stress technique). posed in a stepped-stress fashion. The results of several experiments are presented in this work, each with varying sample sizes. The comparisons between the C. Proton Exposure Facility second and third generation technologies as well as the proton The 63.3 MeV proton irradiation was performed at the dose rate experiments featured sample sizes of one transistor Crocker Nuclear Laboratory at the University of California at per geometry and radiation source. Additional experiments fo- Davis. The dosimetry measurements used a five-foil secondary cusing only on the differences in the gamma and proton response emission monitor calibrated against a Faraday cup. The radi- of the third-generation device featured two to four transistors ation source (Ta scattering foils) was located several meters per geometry and radiation source. In such cases results are pre- upstream of the target to establish a beam spatial uniformity of sented as the ensemble average with error bars representing the about 15% over a 2.0 cm radius circular area. Beam currents maximum and minimum data bounds. from about 20 to 100 nA allowed testing with proton fluxes from to proton/cm sec. The dosimetry system B. Gamma Exposure Facility has been previously described [9], [10], and is accurate to about The 1.2 MeV Gamma irradiation was performed using a 10%. Shepard Model 81 Co-60 source at the NASA Goddard Space A total of three proton experiments are reported on in this Flight Center Radiation Effects Facility. The dose rate was work. As with the gamma investigation, samples were exposed held constant at approximately 30 rad(Si)/s and the dose was with all terminals grounded for all irradiations. In the first exper- uniform to within 10% across all test samples, as determined iment, comparing second and third generation devices, proton using an ion chamber probe. In accordance with MIL-STD 883 fluences ranged from p/cm to p/cm , cor- Method 1019.6, a Pb/Al box was used to decrease the flux of responding to equivalent gamma doses of 135 krad(Si) to 6 733 secondary gammas and ensure a monochromatic gamma ray krad(Si) as depicted in Table I. The dose rate was 2 krad(Si)/s spectrum. and one device per geometry was exposed in a stepped-stress A total of two gamma experiments are reported on in this fashion. work. In both cases, all devices were irradiated with all termi- The second experiment focused only on the third-generation nals grounded inside of a black conductive foam, housed in a devices which were exposed to proton fluences ranging from conductive black box. In both experiments, samples were im- p/cm to p/cm corresponding to equiv- mediately measured after each incremental dose and then irra- alent gamma doses of approximately 100 krad(Si) to 3 Mrad(Si) diated to the next dose level. In the first experiment, measure- respectively as depicted in Table II. The dose rate was constant ments were taken at various cumulative gamma doses from 92 at approximately 1 krad(Si)/s. Two to four devices per geometry krad(Si) to 3 792 krad(Si) as depicted in Table I. Second and were exposed in a stepped-stress fashion. third-generation devices were investigated with sample sizes of In the third experiment, the effects of varying proton dose one device per geometry exposed in a stepped-stress fashion. rates on device degradation was considered. Third-generation A second experiment was performed in a similar fashion to devices were exposed to proton fluences ranging from provide additional data for the third-generation devices only. p/cm to p/cm corresponding to equivalent 2360 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 6, DECEMBER 2005

Fig. 2. Gamma post-radiation forward-mode Gummel characteristics for a Fig. 4. Forward-mode t as a function of gamma dose for second- and third-generation SiGe HBT. third-generation devices.

Fig. 3. Gamma post-radiation inverse-mode Gummel characteristics for a t third-generation SiGe HBT. Fig. 5. Inverse-mode as a function of gamma dose for second- and third-generation devices. gamma doses of 50 krad(Si) to 1 Mrad(Si) respectively. The is chosen to avoid high injection effects where large carrier dose rates chosen for investigation included 30 rad(Si)/s, 100 densities severely diminish the G/R effects of radiation in- rad(Si)/s, 300 rad(Si)/s, and 1 000 rad(Si)/s. In this experiment duced traps. Bias levels much lower than V result one device per geometry and radiation dose rate was exposed in in much smaller currents and less reliable measurements of a stepped-stress fashion. . The degradation in the normalized peak current gain is also used to quantify the effects III. TECHNOLOGY SCALING AND GAMMA DEGRADATION of technology scaling on the observed gamma degradation. In this section the results from the first proton and gamma ex- It should be noted that practical circuit bias conditions often periments are presented. The incremental dose levels and corre- require bias levels in the range of V corresponding sponding dose rates are depicted in Table I. to collector current densities just below peak . At these The post-radiation forward- and inverse-mode Gummel levels, and are very small. characteristics of the third-generation devices ( This 200 GHz third-generation SiGe HBT is shown to have m ) are depicted in Figs. 2 and 3. The char- a reduced forward-mode , as depicted in Fig. 4. A similar acteristic increase in the low-injection base current density trend, for the proton response, was observed in [4] and has been is clearly evident in both plots. As has been attributed to the novel raised extrinsic base structure, yielding well-documented in the literature [8], this phenomenon is the re- EB junctions physically further removed from the EB spacer sult of radiation-induced generation-recombination (G/R) traps region. That is, the areas of high radiation-induced trap density near the EB spacer (forward-mode) and STI (inverse-mode) and positive region trapped charge are spatially removed from oxide interface regions as depicted in the 2D-MEDICI cross the the regions of charge transport, reducing their influence on section in Fig. 1. the terminal current-voltage characteristics. The post radiation excess base current density In the case of the third-generation device, the inverse-mode , extracted at V (as shown , illustrated in Fig. 5, is an order of magnitude higher than in Figs. 2 and 3) is used as the primary figure-of-merit for its forward-mode counterpart, indicating that gamma radiation comparing degradation levels across radiation sources and results in more damage in the CB junction for these devices. technology generations. An extraction point of V This is in direct contrast to the second-generation device, where SUTTON et al.: COMPARISON OF GAMMA AND PROTON RADIATION EFFECTS IN 200 GHz SiGe HBTs 2361

Fig. 6. Forward-mode @postAa @preA as a function of gamma dose Fig. 7. Forward-mode t for both protons and gamma as a function of for second- and third-generation devices. equivalent gamma dose for third-generation devices. the damage is comparable at both junctions. This difference between the inverse mode characteristics is qualitatively consis- tent with the use of a slightly thicker oxide for the STI region in the third-generation devices. The STI processing conditions (i.e., interfacial properties) were similar for both technologies. The forward-mode peak degradation as a function of gamma dose is shown in Fig. 6 and again demonstrates the improved gamma response of the EB junction with scaling. These results indicate that the “raised extrinsic base” structure maintains its improved tolerance to gamma radiation (as was observed for protons) when compared to prior technology nodes. The inverse-mode results, however, indicate that the STI region has become more susceptible to damage.

Fig. 8. Inverse-mode t for both protons and gamma as a function of IV. PROTON VERSUS GAMMA COMPARISON equivalent gamma dose for third-generation devices. A. DC Results area of interest. An analysis of the geometric dependencies are In this section the results from the second proton and gamma presented in the next section. experiments are presented. The incremental dose levels and corresponding dose rates are depicted in Table II. The post-radiation calculated from in-situ mea- B. Geometric Effects surements of both the proton and gamma degradation for The proton and gamma radiation induced base current third-generation devices is now discussed. The forward- and density degradation are now examined as a function inverse-mode results are depicted in Figs. 7 and 8, for two of the electrical emitter (forward-mode) perimeter-to-area devices m and m ). ratio as well as the shallow trench (inverse-mode) Both radiation types exhibit a forward that increases as perimeter-to-area ratio . The results presented a function of the equivalent gamma dose, with protons showing here are from the second proton and gamma experiments. The increased degradation at higher dose levels. The inverse mode incremental dose levels and corresponding dose rates are de- exhibits the same functional form, however, in this case picted in Table II. All devices were grounded during irradiation the gamma degradation is consistently at least one order of mag- and exposed to a total cumulative equivalent gamma dose of nitude higher than that observed for protons. This result is not approximately 3 Mrad(Si). The device geometries depicted unprecedented, and has previously been observed in ﬁrst-gen- include m m m eration devices irradiated under similar conditions [11]. SiGe and m for gamma, and m , HBTs are typically biased in either the forward-active or cutoff- and m for protons. The forward and inverse mode modes. The increased Co-60 degradation observed in the in- as a function of the relevant electrical perimeter-to-area verse-mode is important because the increased charge can be ratios are depicted in Figs. 9 and 10 respectively. coupled into the collector node of the device in a given circuit In the case of devices with m , and application and is therefore important from a hardness assurance m at least four identical devices were irradiated and perspective. the average values computed with error bars representing the The variation in the observed forward and inverse mode maximum and minimum values. The results indicate that the ob- is known to be dependent on the perimeter-to-area ratio of the served radiation induced increases with the perimeter-to- 2362 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 6, DECEMBER 2005

Fig. 9. Forward-mode t for both protons and gamma as a function of Fig. 11. Forward-mode t as a function of equivalent gamma dose rate for electrical emitter perimeter-to-area ratio for third-generation devices. proton radiation of third-generation devices.

Fig. 10. Inverse-mode t for both protons and gamma as a function of Fig. 12. Inverse-mode t as a function of equivalent gamma dose rate for electrical emitter perimeter-to-area ratio for third-generation devices. proton radiation of third-generation devices. area ratio for the region of interest. This observation is consis- A similar study examining the dose rate effects for gamma tent with a previous result identifying the interfaces of the EB irradiation will be reported at a later date. spacer and shallow trench isolation oxides are the primary loca- The sample set for this study was comprised of four device tions for radiation-induced trap formation in SiGe HBTs. geometries [ , and m ) and four It should be noted that the dose rates used for the proton ex- dose rates (30 rad(Si)/s, 100 rad(Si)/s, 300 rad(Si)/s, and 1 periments are several orders of magnitude higher than that used krad(Si)/s]. One device of a particular geometry for each dose for the gamma experiments as is evident from Table II. In order rate was exposed in a stepped-stress fashion. Measurements of to asses the impact of dose rate effects on the results, additional the forward- and inverse-mode were taken at cumulative low proton dose rate testing was also performed. These results dose steps of 50 krad(Si), 100 krad(Si), 300 krad(Si), and 1 are presented in the next section. Mrad(Si) at V. The results are shown in Figs. 11 and 12 for a total cumulative dose of 1 Mrad(Si). The forward-mode V. P ROTON DOSE RATE EFFECTS post-radiation is observed to increase slightly with de- A. DC Results creasing dose rate for most of the geometries considered. In the inverse-mode, a similar trend is not observed. Moreover, the in- It is well known that Si-based bipolar technologies can verse-mode values are again several orders of magnitude undergo enhanced degradation when irradiated under low dose larger than their forward-mode counterparts. rates. This phenomenon, known as “Enhanced Low Dose Rate Sensitivity” (ELDRS) [13], has been studied extensively by many groups using a wide variety of dose rates and different B. Self-Annealing Characteristics sources [14]–[17]. In this work, there was signiﬁcant variation The presence of a true proton dose rate behavior is further (two orders of magnitude) between the incremental dose rates investigated by analyzing the degradation in the forward- and of the gamma source and the proton source, as depicted in Table inverse-mode as a function of the total time (irradiation II. It is therefore logical to wonder what role dose rate effects time + anneal time) after exposure. This approach has previ- play, if any, in the results obtained thus far. To investigate ously been used to prove the existence a dose rate mechanism this, dose rate studies for proton irradiation were performed. as discussed in [18], [19]. SUTTON et al.: COMPARISON OF GAMMA AND PROTON RADIATION EFFECTS IN 200 GHz SiGe HBTs 2363

Fig. 13. Forward-mode t as a function of total time @irrdition C Fig. 15. Forward-mode t as a function of equivalent gamma dose rate for nnelA after exposure for third-generation devices. proton radiation of third-generation devices.

Fig. 16. Inverse-mode t as a function of equivalent gamma dose rate for Fig. 14. Inverse-mode t as a function of total time @irrdition C proton radiation of third-generation devices. nnelA after exposure for third-generation devices. dose rate data tracks more closely with the 1 Krad(Si)/s proton data. The inverse-mode proton data follows a similar trend. The forward-mode as a function of total time is depicted These results indicate that although a proton dose rate effect in Fig. 13. There is an initial decrease in , as a function of time, across all dose rates, indicative of a self-annealing mech- has been observed in the forward-mode operation of the device anism. After some time, a continued build up of oxide charge it does not fully account for the variation in tolerance of the EB and CB junctions of the device to proton and gamma radiation and interface traps results in an increase in with time. The sources. observed increases in , as a function of total time, decrease for increasing dose rate. Evidently the shallow trench isolation oxides exhibit signif- The inverse-mode results also depict an initial decrease in icantly increased degradation following Co-60 gamma irradiation. Similar results have been previously reported and have followed by substantial increases thereafter. However, in this case, there does not appear to be a strong correlation been attributed to increased charge yield following high energy between the proton dose rate and the observed damage levels gamma irradiation resulting from the creation of secondary elec- [300 rad(Si)/s case is much worse than 100 rad(Si)/s and 1 trons [21], [22]. Krad(Si)/s]. The variation in the annealing and subsequent VI. ANNEALING damage characteristics as a function of total time (at least in the forward-mode) suggest the presence of a true dose rate The samples described in this section were irradiated in the mechanism on the EB spacer side of the device. The results for ﬁrst group of proton and gamma experiments. Both proton and the CB junction are inconclusive and require further study. gamma irradiated third-generation devices were annealed at a The low dose rate proton induced degradation is now com- temperature of 573 K for varying time steps in an isothermal pared with the standard proton [l krad(Si)/s] and gamma [30 fashion AET Rapid Thermal Annealing system. The chamber rad(Si)/s] results from the second experiments. The forward- was ﬁrst purged with 2 sccm of for a total of two minutes. and inverse-mode are displayed in Figs. 15 and 16 respec- Next, the ambient gas was switched to forming gas (N H ) and tively for a third-generation device with m . the temperature ramped to 573 K at a rate of 100 C/min. At the In the forward-mode, the low dose rate proton data tracks more end of the anneal, the high temperature lamps were switched off closely with the 30 rad(Si)/s gamma data at low values of total and the devices were left to cool. Once the temperature dropped equivalent gamma dose. Approaching 1 Mrad total dose, the low to 100 C the devices were removed. Following the anneal, 2364 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 6, DECEMBER 2005

700 m wafer thickness to a depth far greater than the 0.4–0.6 m depth of the shallow trench isolation. Bulk damage does indeed occur, particularly in the collector and sub-collector regions of the device; however, in this study we are primarily concerned with the trap states and charge densities that are physically close to the EB and CB junctions of the device, where they affect the measured current-voltage characteristics. A proton dose rate mechanism in SiGe HBTs has been demonstrated in the forward mode over the dose rates 30 rad(Si)/s to 1 krad(Si)/s. Previous investigations into dose rate effects for SiGe HBTs were performed by Banerjee et al. [15]. In these studies, devices were exposed to 1.43 MeV Cobalt-60 sources up to a cumulative dose of 50 krad(Si) at dose rates of 0.1 rad(Si)/s and 300 rad(Si)/s. The results from that study Fig. 17. Ratio of the post-anneal base current to post-radiation base current in the forward mode for different annealing time steps at 300 C. indicated that the observed dose rate effects were highly technology dependent and varied significantly depending on the regime over which dose rates were chosen, the technology being irradiated, and the source of radiation. Specifically, it was determined that SiGe HBTs exhibited minimal dose rate dependence over the 0.1 rad(Si)/s to 300 rad(Si)/s range, especially when compared to the results from Nowlin et al. [16]. There are still several open issues on the ELDRS topic. Sev- eral physical mechanisms have been proposed in attempts to explain the observed effects. Pershenkov et al. [23] demonstrated that the emitter-base junction bias is extremely influential in the low-dose rate transistor response. Transistors biased in the forward mode exhibited an enhancement of the dose rate effect by a factor of 1.5 for npn and 3 for pnp devices for ionizing radiation using Cr/Y and x-ray sources. Another model involving the interaction of fringing electric fields at the screen oxide was pro- Fig. 18. Ratio of the post-anneal base current to post-radiation base current in posed to account for the experimental observations. The pres- the inverse mode for different annealing time steps at 300 C. ence of shallow electron traps in the bulk oxides was proposed as another damage mechanism by the same group [24] in a study samples were remeasured and then annealed again for another concentrating on MOSFETs. length of time at the same temperature. It should be noted that Proton dose rate studies demonstrated some dose rate sen- these samples were annealed at high temperature after a signifi- sitivity; however, it does not completely explain the enhanced cant period of room temperature self-annealing for both proton sensitivity of the STI region to gamma rays. Experiments (three months) and gamma (six months). There was still consid- performed by Schwank et al. [21] have yielded similar trends, erable damage after this period of room temperature annealing. showing increased degradation in buried and isolation field The base current ratio for annealed samples was compared oxides for Co-60 gamma radiation compared to proton irra- for both proton and gamma irradiation for several device ge- diation. The observed trends were attributed to the secondary ometries. The forward and inverse mode results are illustrated in electrons generated via Co-60 gamma photons. These electrons, Figs. 17 and 18, respectively, and show significant recovery for of low-stopping power, are believed to generate electron-hole both the forward- and inverse-mode characteristics, in both the pairs with large spatial separation, leading to large numbers proton and gamma irradiated samples. In the case of the smallest of charge carriers that are able to escape recombination [25]. device, ( m ), there is twice as much recovery This increased charge yield for Co-60 gamma rays has been in the proton irradiated device compared with the gamma irradi- observed in the literature extensively for MOS devices ([26], ated device. As is expected and evident in Fig. 18, in the inverse [27], [22]), and is a very plausible explanation for the effects mode there is no functional dependence of the device geometry observed in these third-generation 200 GHz SiGe HBTs. on the radiation induced damage or the post-anneal base current ratio. The annealing of the proton and gamma irradiated devices exhibit very similar gradients when plotted as post-anneal base VIII. SUMMARY current ratio as a function of time. The gamma tolerance of third-generation 200 GHz SiGe HBTs has been investigated for the first time. The influence VII. DISCUSSION of technology scaling and device geometry on the observed The experimental results discussed thus far raise several response was quantitatively assessed and comparisons were important questions for hardness assurance. SRIM simulations made to previous proton radiation results. The third-generation predict that protons are able to penetrate straight through the devices exhibited a significantly reduced forward-mode peak SUTTON et al.: COMPARISON OF GAMMA AND PROTON RADIATION EFFECTS IN 200 GHz SiGe HBTs 2365

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