| 1. |
- Bergsten, Johan, et al.
(author)
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Impact of AlGaN/GaN interface sharpness on HEMT performance
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Other publication (other academic/artistic)abstract
- The impact of the design and sharpness of the AlGaN/GaN interface in GaN-based HEMTs is investigated. Three structures with different AlGaN/GaN interface properties were grown with hot-wall MOCVD. One structure has a 2-nmthick AlN exclusion layer in between the AlGaN and the GaN, while the other two differ in their sharpness of the Al transition at the AlGaN/GaN interface. The structures with AlN exclusion layer and optimized sharpness of the interface show similar electron mobilities (1760 and 1740 cm2/Vs). HEMTs were processed and evaluated. Gated Hall-measurements indicate that the sharper interface maintains a higher mobility when the electrons are close to the interface compared both to the AlNexclusion layer and the non-optimized structure. The higher mobility manifests as lower parasitic resistance yielding better DC and high frequency performance. Pulsed IV measurements indicate that the sharper interface provide less dispersive effects compared both to the AlN exclusion layer and the optimized interface.
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| 2. |
- Bergsten, Johan, et al.
(author)
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Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process
- 2016
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In: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0018-9383 .- 1557-9646. ; 63:1, s. 333-338
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Journal article (peer-reviewed)abstract
- The impact of the sharpness of the AlGaN/GaN interface in high-electron mobility transistors (HEMTs) is investigated. Two structures, one with an optimized AlGaN/GaN interface and another with an unoptimized, were grown using hot-wall metal-organic chemical vapor deposition. The structure with optimized sharpness of the interface shows electron mobility of 1760 cm(2)/V . s as compared with 1660 cm(2)/V . s for the nonoptimized interface. Gated Hall measurements indicate that the sharper interface maintains higher mobility when the electrons are close to the interface compared with the nonoptimized structure, indicating less scattering due to alloy disorder and interface roughness. HEMTs were processed and evaluated. The higher mobility manifests as lower parasitic resistance yielding a better dc and high-frequency performance. A small-signal equivalent model is extracted. The results indicate a lower electron penetration into the buffer in the optimized sample. Pulsed-IV measurements imply that the sharper interface provides less dispersive effects at large drain biases. We speculate that the mobility enhancement seen AlGaN/AlN/GaN structures compared with the AlGaN/GaN case is not only related to the larger conduction band offset but also due to a more welldefined interface minimizing scattering due to alloy disorder and interface roughness.
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| 3. |
- Bergsten, Johan, 1988, et al.
(author)
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Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process
- 2016
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In: IEEE Transactions on Electron Devices. - : Institute of Electrical and Electronics Engineers (IEEE). - 1557-9646 .- 0018-9383. ; 63:1, s. 333-338
-
Journal article (peer-reviewed)abstract
- The impact of the sharpness of the AlGaN/GaN interface in high-electron mobility transistors (HEMTs) is investigated. Two structures, one with an optimized AlGaN/GaN interface and another with an unoptimized, were grown using hot-wall metal-organic chemical vapor deposition. The structure with optimized sharpness of the interface shows electron mobility of 1760 cm(2)/V . s as compared with 1660 cm(2)/V . s for the nonoptimized interface. Gated Hall measurements indicate that the sharper interface maintains higher mobility when the electrons are close to the interface compared with the nonoptimized structure, indicating less scattering due to alloy disorder and interface roughness. HEMTs were processed and evaluated. The higher mobility manifests as lower parasitic resistance yielding a better dc and high-frequency performance. A small-signal equivalent model is extracted. The results indicate a lower electron penetration into the buffer in the optimized sample. Pulsed-IV measurements imply that the sharper interface provides less dispersive effects at large drain biases. We speculate that the mobility enhancement seen AlGaN/AlN/GaN structures compared with the AlGaN/GaN case is not only related to the larger conduction band offset but also due to a more welldefined interface minimizing scattering due to alloy disorder and interface roughness.
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| 4. |
- Eckhardt, Martin, 1974-
(author)
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The Bumpy Road to Universal Health Coverage : Access to Primary and Emergency Care in Rural Tropical Ecuador
- 2018
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Doctoral thesis (other academic/artistic)abstract
- Background: By the turn of the new millennium 84% of families in rural coastal Ecuador had difficulties to access health services. A health reform some years earlier to improve this situation had not been implemented. In 2001, the non-governmental organization (NGO) Foundation Human Nature together with a rural population established a primary health care center in North-Western Ecuador. A public private partnership with the Ministry of Public Health was formed. Services depended on out-of-pocket payments, restricting the poor’s access to care. In order to increase access to primary and emergency care, Foundation Human Nature planned to establish a community-based health insurance. In the meantime, a major health reform was initiated by a new government in 2008. It aimed at universal health coverage, providing qualitative services for all based on primary health care, while ensuring protection from financial hardship. The aims of this thesis were to appraise the feasibility of community- based health insurance in Ecuador; to study how rural stakeholders perceived the 2008 reform and its effects on rural health services; to explore the local population’s perception of the NGO in service delivery; and to measure the scope and describe the nature of perceived emergencies, the related health care seeking behavior and health expenditure. Methods: Quantitative and qualitative methods were applied to tackle the research objectives. Data collection for the health insurance study and the study of perceived emergencies was carried out through cross-sectional household surveys. For each of the studies 210 households were sampled with two-stage cluster sampling. Structured questionnaires were used with on the spot household interviews. Focus group discussions with local stakeholders were performed to explore their perceived effects of the 2008 health reform. The population’s perception of the NGO was also studied through focus group discussions, which were complemented by key-informant interviews with local stakeholders. Inductive qualitative content analysis, focusing on the manifest content was applied. Results: 69% of interviewees were willing to join the presented community-based health insurance scheme for 30 US$ per household and year. Attitudes towards the scheme were positive and 92% of interviewees stated they would increase their health service utilization with affiliation. The implementation of the 2008 health reform was perceived as top-down, lacking communication. However, the reform’s effects were mainly perceived as positive with free medical attendance and drugs. Increased service utilization was described as leading to a relative shortage of drugs and appointments. Access problems for remote dwellers were found, who were described of having to seek private care, also in emergencies. The NGO and its services were perceived positively by the population due to health care improvements in the region. The structure of the public private partnership was unclear, leading to dissatisfaction. Community participation was found to be rather weak. Perceived emergencies occurred to at least 90/1,000 inhabitants in the past year. Fever, traumatic injury and abdominal pain were the most frequent chief complaints. The first contacted providers in 57% of all cases were private for-profit providers, including traditional healers. Public health services treated one third of all cases. Health expenditure was found to be high and catastrophic health expenditure occurred in 24% of all cases. Conclusions: Prior to the 2008 reform community-based health insurance was found to be feasible in the study region. This financing instrument may have a role in the post-reform system, to cover services that the government does not yet sufficiently provide. The effects of the 2008 reform were mainly perceived positively, but an adjustment of the system is needed to improve the relative lack of drugs and appointments, especially for remote dwellers. Free health services may not be sufficient to reach universal health coverage for patients with perceived emergencies. Changes in public emergency departments and improved financial protection for emergency patients may improve the situation. The NGO’s role was perceived positively by the population. A lack of communication about the public private partnership and relatively weak community participation restricted the NGO’s full potential and should be improved.
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| 5. |
- Forsberg, Tobias, et al.
(author)
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Condition Monitoring of Train Ride Stability
- 2007
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In: Proceedings of the 7th International Conference on Railway Bogies and Running Gears, BOGIE'07, Budapest, September 3-6, 2007. ; , s. 271-280
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Conference paper (peer-reviewed)
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| 6. |
- Gustafsson, Sebastian, et al.
(author)
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Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer
- 2015
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In: IEEE Transactions on Electron Devices. - : IEEE Press. - 0018-9383 .- 1557-9646. ; 62:7, s. 2162-2169
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Journal article (peer-reviewed)abstract
- Aluminium gallium nitride (AlGaN)/GaN high-electron mobility transistor performance is to a large extent affected by the buffer design, which, in this paper, is varied using different levels of carbon incorporation. Three epitaxial structures have been fabricated: 1) two with uniform carbon doping profile but different carbon concentration and 2) one with a stepped doping profile. The epitaxial structures have been grown on 4H-SiC using hot-wall metal-organic chemical vapor deposition with residual carbon doping. The leakage currents in OFF-state at 10 V drain voltage were in the same order of magnitude (10-4 A/mm) for the high-doped and stepped-doped buffer. The high-doped material had a current collapse (CC) of 78.8% compared with 16.1% for the stepped-doped material under dynamic I-V conditions. The low-doped material had low CC (5.2%) but poor buffer isolation. Trap characterization revealed that the high-doped material had two trap levels at 0.15 and 0.59 eV, and the low-doped material had one trap level at 0.59 eV.
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| 7. |
- Gustafsson, Sebastian, 1990, et al.
(author)
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Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer
- 2015
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In: IEEE Transactions on Electron Devices. - : Institute of Electrical and Electronics Engineers (IEEE). - 1557-9646 .- 0018-9383. ; 62:7, s. 2162-2169
-
Journal article (peer-reviewed)abstract
- Aluminium gallium nitride (AlGaN)/GaN high-electron mobility transistor performance is to a large extent affected by the buffer design, which, in this paper, is varied using different levels of carbon incorporation. Three epitaxial structures have been fabricated: 1) two with uniform carbon doping profile but different carbon concentration and 2) one with a stepped doping profile. The epitaxial structures have been grown on 4H-SiC using hot-wall metal-organic chemical vapor deposition with residual carbon doping. The leakage currents in OFF-state at 10 V drain voltage were in the same order of magnitude (10(-4) A/mm) for the high-doped and stepped-doped buffer. The high-doped material had a current collapse (CC) of 78.8% compared with 16.1% for the stepped-doped material under dynamic I-V conditions. The low-doped material had low CC (5.2%) but poor buffer isolation. Trap characterization revealed that the high-doped material had two trap levels at 0.15 and 0.59 eV, and the low-doped material had one trap level at 0.59 eV.
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| 8. |
- Hedwig, Sebastian, et al.
(author)
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From Trace to Pure : Pilot-Scale Scandium Recovery from TiO2 Acid Waste
- 2023
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In: ACS Sustainable Chemistry and Engineering. - : American Chemical Society (ACS). - 2168-0485.
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Journal article (peer-reviewed)abstract
- Scandium (Sc), declared a critical raw material in the European Union (EU), could face further supply issues as the EU depends almost entirely on imports from China, Russia, and Ukraine. In this study, a tandem nanofiltration-solvent extraction procedure for Sc recovery from titania (TiO2) acid waste was piloted and then augmented by antisolvent crystallization. The new process, comprising advanced filtration (hydroxide precipitation, micro-, ultra-, and nanofiltration), solvent extraction, and antisolvent crystallization, was assessed in relation to material and energy inputs and benchmarked on ScF3 production. From ∼1 m3 of European acid waste containing traces of Sc (81 mg L–1), ∼13 g of Sc (43% yield, nine stages) was recovered as (NH4)3ScF6 with a purity of approximately 95%, demonstrating the technical feasibility of the approach. The production costs per kilogram of ScF3 were lower than reported market prices, which underscores a competitive process at scale. Although a few technical bottlenecks (e.g., S/L separation and electricity consumption) need to be overcome, combining advanced filtration with solvent extraction and antisolvent crystallization promises a future supply of this critical raw material from European secondary sources.
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| 9. |
- Holmstrom, Jesper, et al.
(author)
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Do we Read what we Share? Analyzing the Click Dynamic of News Articles Shared on Twitter
- 2019
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In: PROCEEDINGS OF THE 2019 IEEE/ACM INTERNATIONAL CONFERENCE ON ADVANCES IN SOCIAL NETWORKS ANALYSIS AND MINING (ASONAM 2019). - New York, NY, USA : Institute of Electrical and Electronics Engineers (IEEE). - 9781450368681 ; , s. 420-425
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Conference paper (peer-reviewed)abstract
- News and information spread over social media can have big impact on thoughts, beliefs, and opinions. It is therefore important to understand the sharing dynamics on these forums. However, most studies trying to capture these dynamics rely only on Twitters open APIs to measure how frequently articles are shared/retweeted, and therefore do not capture how many users actually read the articles linked in these tweets. To address this problem, in this paper, we first develop a novel measurement methodology, which combines the Twitter steaming API, the Bitly API, and careful sample rate selection to simultaneously collect and analyze the timeline of both the number of retweets and clicks generated by news article links. Second, we present a temporal analysis of the news cycle based on five-day-long traces (containing both clicks and retweet over time) for the news article links discovered during a seven-day period. Among other things, our analysis highlights differences in the relative timelines observed for clicks and retweets (e.g., retweet data often lags and underestimates the bias towards reading popular links/articles), and helps answer important questions regarding differences in how age-based biases and churn affect how frequently news articles shared on Twitter are accessed over time.
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| 10. |
- Holmström, Anna, et al.
(author)
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YopK of Yersinia pseudotuberculosis controls translocation of Yop effectors across the eukaryotic cell membrane
- 1997
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In: Molecular Microbiology. - : Wiley. - 0950-382X .- 1365-2958. ; 24:1, s. 73-91
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Journal article (peer-reviewed)abstract
- Introduction of anti-host factors into eukaryotic cells by extracellular bacteria is a strategy evolved by several Gram-negative pathogens. In these pathogens, the transport of virulence proteins across the bacterial membranes is governed by closely related type III secretion systems. For pathogenic Yersinia, the protein transport across the eukaryotic cell membrane occurs by a polarized mechanism requiring two secreted proteins, YopB and YopD. YopB was recently shown to induce the formation of a pore in the eukaryotic cell membrane, and through this pore, translocation of Yop effectors is believed to occur (Håkansson et al., 1996b). We have previously shown that YopK of Yersinia pseudotuberculosis is required for the development of a systemic infection in mice. Here, we have analysed the role of YopK in the virulence process in more detail. A yopK-mutant strain was found to induce a more rapid YopE-mediated cytotoxic response in HeLa cells as well as in MDCK-1 cells compared to the wild-type strain. We found that this was the result of a cell-contact-dependent increase in translocation of YopE into HeLa cells. In contrast, overexpression of YopK resulted in impaired translocation. In addition, we found that YopK also influenced the YopB-dependent lytic effect on sheep erythrocytes as well as on HeLa cells. A yopK-mutant strain showed a higher lytic activity and the induced pore was larger compared to the corresponding wild-type strain, whereas a strain overexpressing YopK reduced the lytic activity and the apparent pore size was smaller. The secreted YopK protein was found not to be translocated but, similar to YopB, localized to cell-associated bacteria during infection of HeLa cells. Based on these results, we propose a model where YopK controls the translocation of Yop effectors into eukaryotic cells.
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