| 1. |
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| 2. |
- Bremer, Johan, 1991, et al.
(author)
-
Analysis of Lateral Thermal Coupling for GaN MMIC Technologies
- 2018
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In: IEEE Transactions on Microwave Theory and Techniques. - 0018-9480 .- 1557-9670. ; 66:10, s. 4430-4438
-
Journal article (peer-reviewed)abstract
- This paper presents a study of the lateral heat propagation in an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure grown on a silicon carbide substrate. The study is enabled by the design of a temperature sensor that utilizes the temperature-dependent I-V characteristic of a semiconductor resistor, making it suitable for integration in GaN monolithic microwave integrated circuit technologies. Using the sensor, we are able to characterize the thermal transient response and extract lateral thermal time constants from the measurements. Time constants in the range from 25 mu s to 1.2 ms are identified. Furthermore, the heat propagation properties are characterized for heat source-to-sensor distances of 86-484 mu m, resulting in delay times from 3.5 to 111 mu s. It is shown that both the time constants and propagation delay increase with temperature. An empirical model of the sensor current versus temperature and voltage is proposed and used to predict the junction temperature of the sensor. The study provides knowledge for heat management design and proposes an integrated temperature measurement solution for future highly integrated GaN applications.
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| 3. |
- Fager, Christian, 1974, et al.
(author)
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Analysis of Thermal Effects in Integrated Radio Transmitters
- 2018
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In: 2018 INTERNATIONAL SYMPOSIUM ON VLSI TECHNOLOGY, SYSTEMS AND APPLICATION (VLSI-TSA). - 1930-8868. - 9781538648254 - 9781538648254
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Conference paper (peer-reviewed)abstract
- The combination of compact size and low efficiency at mm-waves has turned heat dissipation into a fundamental constraint for design of multi-antenna radios. This paper describes methods for analysis of thermal effects at both at the circuit, system and component level. The first part describes how thermal analysis can be combined with advanced RF modeling techniques to predict self-heating and thermal coupling in multi-antenna transmitter systems. Experimental methods are then used to determine thermal coupling effects occurring at chip level. Various experimental and theoretical results, using MIMO amplifiers and GaN HEMTs, are used to demonstrate the methods in realistic application scenarios.
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| 4. |
- Houshuai, Wang, et al.
(author)
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Molecular phylogeny of Lymantriinae (Lepidoptera, Noctuoidea, Erebidae) inferred from eight gene regions
- 2015
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In: Cladistics. - : Wiley. - 0748-3007 .- 1096-0031. ; 31:6, s. 579-592
-
Journal article (peer-reviewed)abstract
- To understand the evolutionary history of Lymantriinae and test the present higher-level classification, we performed the first broad-scale molecular phylogenetic analysis of the subfamily, based on 154 exemplars representing all recognized tribes and drawn from all major biogeographical regions. We used two mitochondrial genes (cytochrome c oxidase subunit I and 16S ribosomal RNA) and six nuclear genes (elongation factor-1α, carbamoylphosphate synthase domain protein, ribosomal protein S5, cytosolic malate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase and wingless). Data matrices (in total 5424 bp) were analysed by parsimony and model-based evolutionary methods (maximum likelihood and Bayesian inference). Based on the results of the analyses, we present a new phylogenetic classification for Lymantriinae composed of seven well-supported tribes, two of which are proposed here as new: Arctornithini, Leucomini, Lymantriini, Orgyiini, Nygmiini, Daplasini trib. nov. and Locharnini trib. nov. We discuss the internal structure of each of these tribes and address some of the more complex problems with the genus-level classification, particularly within Orgyiini and Nygmiini.
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| 5. |
- Ihre, Johan, 1707-1780
(author)
-
Dissertatio de inæquali moralitatis mensura, quam, ... præside, ... Johanne Ihre, ... pro laurea, publicæ disquisitioni subjicit, stipendiarius regius, Ericus Bergsten, Nericius, in Auditor. Carol. maj. die VII Junii, MDCCLXXIII, horis ante meridiem solitis.
- 1773
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Doctoral thesis (other academic/artistic)
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| 6. |
- Kroon, Johan, et al.
(author)
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Increasing production value in Scots pine plantation through mixing with lodgepole pine
- 2019
-
In: Scandinavian Journal of Forest Research. - : Taylor & Francis Group. - 0282-7581 .- 1651-1891. ; 34:8, s. 689-698
-
Journal article (peer-reviewed)abstract
- Mixing tree species could be a silviculture model that allows early harvest of short-rotation trees, while longer-rotation crop trees remain in the stand. We examined the effects on growth and tree characteristics in a planted experiment with lodgepole pine (LP) and elite-bred Scots pine (SP) in mixed (50/50) and monospecific plots in three different spacings (at 28 years of age after planting). The future development under different thinning regimes, including net present value for one rotation, was analyzed using the Heureka simulation software. As expected, LP had higher survival and initially more rapid growth than SP, with highest stand productivity and biomass production in LP monoculture during a rotation period as a result. However, intimate mixtures of SP and LP at the two widest spacings could give greater production and economic benefits, compared to SP in monoculture. It seems that elite-bred SP will differ in competitiveness against LP, depending on spacing for growth and some quality traits (branch and bark thickness, height of green crown). The findings support developing management systems for combining sparsely planted, and expensive, elite-bred SP in mixture with other trees that maintains high stem volume production and secures certain properties of trees and stands.
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| 7. |
- Nilsson, R. Henrik, 1976, et al.
(author)
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Five simple guidelines for establishing basic authenticity and reliability of newly generated fungal ITS sequences
- 2012
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In: MycoKeys. - : Pensoft Publishers. - 1314-4057 .- 1314-4049. ; 4, s. 37-63
-
Journal article (peer-reviewed)abstract
- Molecular data form an important research tool in most branches of mycology. A non-trivial proportion of the public fungal DNA sequences are, however, compromised in terms of quality and reliability, contributing noise and bias to sequence-borne inferences such as phylogenetic analysis, diversity assessment, and barcoding. In this paper we discuss various aspects and pitfalls of sequence quality assessment. Based on our observations, we provide a set of guidelines to assist in manual quality management of newly generated, near-full-length (Sanger-derived) fungal ITS sequences and to some extent also sequences of shorter read lengths, other genes or markers, and groups of organisms. The guidelines are intentionally non-technical and do not require substantial bioinformatics skills or significant computational power. Despite their simple nature, we feel they would have caught the vast majority of the severely compromised ITS sequences in the public corpus. Our guidelines are nevertheless not infallible, and common sense and intuition remain important elements in the pursuit of compromised sequence data. The guidelines focus on basic sequence authenticity and reliability of the newly generated sequences, and the user may want to consider additional resources and steps to accomplish the best possible quality control. A discussion on the technical resources for further sequence quality management is therefore provided in the supplementary material.
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| 8. |
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| 9. |
- Baldini, Francesco, et al.
(author)
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Miniaturised Optical Fibre Sensor for Dew Detection Inside Organ Pipes
- 2008
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In: Journal of Sensors. - 1687-7268. ; 2008
-
Journal article (peer-reviewed)abstract
- A new optical sensor for the continuous monitoring of the dew formation inside organ pipes was designed. This aspect is particularly critical for the conservation of organs in unheated churches since the dew formation or the condensation on the pipe surfaces can contribute to many kinds of physical and chemical disruptive mechanisms. The working principle is based on the change in the reflectivity which is observed on the surface of the fibre tip, when a water layer is formed on its distal end. Intensity changes of the order of 35% were measured, following the formation of the water layer on the distal end of a 400/430μm optical fibre. Long-term tests carried out placing the fibre tip inside the base of an in-house-made metallic foot of an organ pipe located in an external environment revealed the consistency of the proposed system.
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| 10. |
- Bergqvist, Ewa, 1971-
(author)
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Mathematics and mathematics education - two sides of the same coin : creative reasoning in university exams in mathematics
- 2006
-
Doctoral thesis (other academic/artistic)abstract
- Avhandlingen består av två ganska olika delar som ändå har en del gemensamt. Del A är baserad på två artiklar i matematik och del B är baserad på två matematikdidaktiska artiklar. De matematiska artiklarna utgår från ett begrepp som heter polynomkonvexitet. Grundidén är att man skulle kunna se vissa ytor som en sorts ”tak” (tänk på taket till en carport). Alla punkter, eller positioner, ”under taket” (ungefär som de platser som skyddas från regn av carporttaket) ligger i något som kallas ”polynomkonvexa höljet.” Tidigare forskning har visat att för ett givet tak och en given punkt så finns det ett sätt att avgöra om punkten ligger ”under taket”. Det finns nämligen i så fall alltid en sorts matematisk funktion med vissa egenskaper. Finns det ingen sådan funktion så ligger inte punkten under taket och tvärt om; ligger punkten utanför taket så finns det heller ingen sådan funktion. Jag visar i min första artikel att det kan finnas flera olika sådana funktioner till en punkt som ligger under taket. I den andra artikeln visar jag några exempel på hur man kan konstruera sådana funktioner när man vet hur taket ser ut och var under taket punkten ligger. De matematikdidaktiska artiklarna i avhandlingen handlar om vad som krävs av studenterna när de gör universitetstentor i matematik. Vissa uppgifter kan gå att lösa genom att studenterna lär sig någonting utantill ur läroboken och sen skriver ner det på tentan. Andra går kanske att lösa med hjälp en algoritm, ett ”recept,” som studenterna har övat på att använda. Båda dessa sätt att resonera kallas imitativt resonemang. Om uppgiften kräver att studenterna ”tänker själva” och skapar en (för dem) ny lösning, så kallas det kreativt resonemang. Forskning visar att elever i stor utsträckning väljer att jobba med imitativt resonemang, även när uppgifterna inte går att lösa på det sättet. Mycket pekar också på att de svårigheter med att lära sig matematik som elever ofta har är nära kopplat till detta arbetssätt. Det är därför viktigt att undersöka i vilken utsträckning de möter olika typer av resonemang i undervisningen. Den första artikeln består av en genomgång av tentauppgifter där det noggrant avgörs vilken typ av resonemang som de kräver av studenterna. Resultatet visar att studenterna kunde bli godkända på nästan alla tentorna med hjälp av imitativt resonemang. Den andra artikeln baserades på intervjuer med sex av de lärare som konstruerat tentorna. Syftet var att ta reda på varför tentorna såg ut som de gjorde och varför det räckte med imitativt resonemang för att klara dem. Det visade sig att lärarna kopplade uppgifternas svårighetsgrad till resonemangstypen. De ansåg att om uppgiften krävde kreativt resonemang så var den svår och att de uppgifter som gick att lösa med imitativt resonemang var lättare. Lärarna menade att under rådande omständigheter, t.ex. studenternas försämrade förkunskaper, så är det inte rimligt att kräva mer kreativt resonemang vid tentamenstillfället.
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| 11. |
- Bergsten, Christer, 1949-, et al.
(author)
-
Algebra för alla
- 1997. - 1
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Book (other academic/artistic)abstract
- Bakom titeln Algebra för alla ligger tanken att alla lärare som arbetar med matematik i grundskola och gymnasieskola ska ha glädje av innehållet och målet att alla elever ska möta matematikens generaliserande kraft. Syftet med materialet är att avdramatisera arbetet med skolalgebra samt att uppmärksamma och stimulera möjligheterna att arbeta med olika uttrycksformer. I boken ingår ett stort antal elevaktiviteter och studieuppgifter för lärare samt litteraturförslag.
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| 12. |
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| 13. |
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| 14. |
- Bergsten, Carl Johan, 1953
(author)
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Att rädda Europas orglar
- 2007
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In: Kyrkomusikernas Tidning. - 0281-286X. ; :3 (Mars), s. 6-8
-
Journal article (other academic/artistic)
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| 15. |
- Bergsten, Carl Johan, 1953
(author)
-
COLLAPSE
- 2009
-
In: Preserving our heritage, Improving our environment [Vol. 2]. - Luxembourg : Publications Office of the European Union. - 9789279090295 ; , s. 72-73
-
Book chapter (other academic/artistic)
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| 16. |
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| 17. |
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| 18. |
- Bergsten, Carl Johan, 1953
(author)
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Conservation of Pipe Organs : La conservazione degli organi
- 2006
-
In: Swerling, G. & Travaglini, B. (eds). Il riscaldamento nelle chiese e la conservazione dei beni culturali: guida all'analisi dei pro e dei contro dei vari sistemi di riscaldamento = Church heating and the preservation of the cultural heritage : guide to the analysis of the pros and cons of various heating systems. - Milano : Electa. - 8837050356 ; , s. 57-61
-
Book chapter (other academic/artistic)
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| 19. |
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| 20. |
- Bergsten, Carl Johan, 1953
(author)
-
Organs in harmful environments
- 2011
-
In: Analysis and description of music instruments using engineering methods. - Halle : Stiftung Händel-Haus. - 9783943095005 ; , s. 29-33
-
Book chapter (other academic/artistic)
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| 21. |
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| 22. |
- Bergsten, Carl Johan, 1953
(author)
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SENSORGAN
- 2009
-
In: Preserving our heritage, Improving our environment [Vol. 2]. - Luxembourg : Publications Office of the European Union. - 9789279090295 ; , s. 108-109
-
Book chapter (other academic/artistic)
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| 23. |
- Bergsten, Carl Johan, 1953
(author)
-
The Düben Database structure
- 2010
-
In: The Dissemination of Music in Seventeenth-Century Europe. Celebrating the Düben Collection. Proceedings from the International Conference at Uppsala University 2006. - Bern (CH) : Peter Lang. - 9783034300575 ; , s. 329-334
-
Book chapter (other academic/artistic)
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| 24. |
- Bergsten, Carl Johan, 1953, et al.
(author)
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The Field Study Organs
- 2011
-
In: The Collapse Project: corrosion of organ pipes - causes and recommendations. - Brusels : European Union. - 9789279170799 ; , s. 24-55
-
Book chapter (other academic/artistic)
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| 25. |
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| 26. |
- Bergsten, Eva L., 1969-, et al.
(author)
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Effects of relocation to activity-based workplaces on perceived productivity : importance of change-oriented leadership
- 2021
-
In: Applied Ergonomics. - : Elsevier. - 0003-6870 .- 1872-9126. ; 93, s. 10-16
-
Journal article (peer-reviewed)abstract
- Activity-based workplaces (ABWs) are becoming popular in Western countries and were implemented at four office sites of a large Swedish government agency. A fifth office was used as a control group. The study aim was to examine the effects of relocation to ABW on perceived productivity among employees and to determine if perceived change-oriented leadership behavior prior to relocation moderates potential effects. Data were collected three months prior to relocation, and three and 12 months after. 407 respondents were included in linear mixed regression models. Perceived productivity decreased significantly after relocation compared to the control group and these effects persisted 12 months after the relocation. However, the decrease in perceived productivity was significantly smaller among employees perceiving high change-oriented leadership before relocation. Our results point out the importance of a change-oriented leadership behavior during the implementation to avoid productivity loss among employees when implementing ABWs.
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| 27. |
- Bergsten, Eva L., 1969-, et al.
(author)
-
Evaluation of an ergonomic intervention in Swedish flight baggage handlers
- 2016
-
Conference paper (peer-reviewed)abstract
- Background: Flight baggage handling is a worldwide occupation where baggage and cargo is sorted, loaded and unloaded on and off aircrafts. With the ultimate purpose of reducing and preventing musculoskeletal disorders among flight baggage handlers in Sweden, the Vocational Training and Working Environment Council (TYA) - a council formed by employers and unions in the Swedish transportation sector – initiated and implemented a project (2010-2012). This project revealed that ergonomics equipment was not used adequately, and this was considered a major factor of concern. Therefore, a training program was initiated 2014 in one handling company, aiming to improve ergonomics, behavior and attitudes. We evaluated the implementation process with regard to process items, intermediate outcomes, barriers and facilitators; for the purpose of gaining knowledge that could facilitate successful implementation in other handling companies. Methods: A mixed methods design was applied, based on qualitative and quantitative data. We evaluated six process items, recruitment, context, reach, dose delivered, dose received and satisfaction; intermediate outcomes of the intervention; skills, confidence and behaviour in the workforce; barriers and facilitators for successful implementation. Data was retrieved using company data, course evaluations, web questionnaires, and telephone interviews with company ‘observers’ and key persons. Preliminary results: The implementation process was judged to be feasible with regard to some of the process items. According to the informants, work place behaviour related to use of equipment had, however, not changed after the training period. Reported barriers were, 1) insufficient time and leader support for practicing new procedures during and after the training, 2) simultaneous reorganization of teams and work tasks, 3) lack of follow-up of the training, which would have supported good performance according to the informants. Conclusion: The implementation process was hampered by barriers, some of which could be addressed in future ergonomics training programs in other baggage handling companies.
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| 28. |
- Bergsten, Eva L., 1969-, et al.
(author)
-
Implementation of an ergonomics intervention in a Swedish flight baggage handling company : a process evaluation
- 2018
-
In: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 13:3
-
Journal article (peer-reviewed)abstract
- Objective: To conduct a process evaluation of the implementation of an ergonomics training program aimed at increasing the use of loading assist devices in flight baggage handling.Methods: Feasibility (recruitment, reach, context, dose delivered, dose received, satisfaction); intermediate outcomes (skills, confidence and behaviors); and barriers and facilitators of the training intervention were assessed by qualitative and quantitative methods.Results: Implementation proved feasible regarding dose delivered, dose received and satisfaction. Confidence among participants in the training program in using and talking about devices, observed use of devices among colleagues, and internal feedback on work behavior increased significantly (p<0.01). Main facilitators were self-efficacy, motivation, and perceived utility of training among the trainees. Barriers included lack of peer support, opportunities to observe and practice behaviors, and follow-up activities; as well as staff reduction and job insecurity.Conclusions: In identifying important barriers and facilitators for a successful outcome, our study can help supporting the effectiveness of future interventions. Our results show that barriers caused by organizational changes may likely be alleviated by recruiting motivated trainees and securing strong organizational support for the implementation.
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| 29. |
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| 30. |
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| 31. |
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| 32. |
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| 33. |
- Bergsten, Johan, 1988
(author)
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Advanced Heterostructure Designs and Recessed Ohmic Contacts for III-Nitride-Based HEMTs
- 2015
-
Licentiate thesis (other academic/artistic)abstract
- Modern III-Nitride (III-N) heterostructures offer high mobility, high electron density, large breakdown voltages and good thermal capabilities. High electron mobility transistors (HEMT) based on III-Ns are therefore ideal for high frequency, high power amplification. The intended applications are within radar and mobile communication. Commercial devices are available although some issues remain unsolved. This thesis deals with two of these issues: the lack of a reliable, low resistive ohmic contact and problems with dispersion, which can severely impede output power.Recessed ohmic contacts were developed to an Indium Aluminium Nitride / Aluminium Nitride / Gallium Nitride (InAlN/AlN/GaN) heterostructure. The lowest contact resistance (0.14 Ωmm) was found after annealing at 550 °C for samples where the recess was almost through the barrier. Recessing through the whole barrier also gave low resistive contacts but required slightly higher anneal temperatures. The results indicate the viability of a reliable recessed ohmic contact process.Furthermore, two different aspects of heterostructure development has been investigated; buffer doping and the AlGaN/GaN interface. An optimized carbon doping profile in the buffer has been evaluated by fabricating HEMTs and characterizing isolation and dispersion. The optimized buffer showed to minimize dispersive effects while providing good isolation. Low values of drain induced barrier lowering was measured (1.2 mV/V).A common way to increase electron mobility in AlGaN/GaN heterostructures is to include an AlN-exclusion layer. Unfortunately this approach makes it more difficult to form ohmic contacts. An optimized AlGaN/GaN heterostructure, with a sharp transition from AlGaN to GaN, has been investigated. HEMTs on the optimized sample showed less electron penetration into the barrier layer as compared to an un-optimized sample resulting in an increased mobility (1800 compared to 1700 cm2/Vs). HEMTs with a gate-length of 200 nm exhibited transconductance of 400 mS/mm and fT/fmax of 40/156 GHz using the optimized interface, compared to 390 mS/mm and 37/146 GHz for the standard interface.
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| 34. |
- Bergsten, Johan, 1988, et al.
(author)
-
AlGaN/GaN high electron mobility transistors with intentionally doped GaN buffer using propane as carbon precursor
- 2016
-
In: Japanese Journal of Applied Physics. - : IOP Publishing. - 1347-4065 .- 0021-4922. ; 55:5
-
Journal article (peer-reviewed)abstract
- AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on a heterostructure grown by metalorganic chemical vapor deposition using an alternative method of carbon (C) doping the buffer are characterized. C-doping is achieved by using propane as precursor, as compared to tuning the growth process parameters to control C-incorporation from the gallium precursor. This approach allows for optimization of the GaN growth conditions without compromising material quality to achieve semi-insulating properties. The HEMTs are evaluated in terms of isolation and dispersion. Good isolation with OFF-state currents of 2 x 10(-6)A/mu m, breakdown fields of 70V/mu m, and low drain induced barrier lowering of 0.13mV/V are found. Dispersive effects are examined using pulsed current-voltage measurements. Current collapse and knee walkout effects limit the maximum output power to 1.3W/mm. With further optimization of the C-doping profile and GaN material quality this method should offer a versatile approach to decrease dispersive effects in GaN HEMTs. (C) 2016 The Japan Society of Applied Physics
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| 35. |
- Bergsten, Johan, et al.
(author)
-
AlGaN/GaN high electron mobility transistors with intentionally doped GaN buffer using propane as carbon precursor
- 2016
-
In: Japanese Journal of Applied Physics. - : Institute of Physics Publishing (IOPP). - 0021-4922 .- 1347-4065. ; 55, s. 05FK02-1-05FK02-4
-
Journal article (peer-reviewed)abstract
- AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on a heterostructure grown by metalorganic chemical vapor deposition using analternative method of carbon (C) doping the buffer are characterized. C-doping is achieved by using propane as precursor, as compared to tuningthe growth process parameters to control C-incorporation from the gallium precursor. This approach allows for optimization of the GaN growthconditions without compromising material quality to achieve semi-insulating properties. The HEMTs are evaluated in terms of isolation anddispersion. Good isolation with OFF-state currents of 2 ' 10%6A/mm, breakdown fields of 70V/μm, and low drain induced barrier lowering of0.13mV/V are found. Dispersive effects are examined using pulsed current–voltage measurements. Current collapse and knee walkout effectslimit the maximum output power to 1.3W/mm. With further optimization of the C-doping profile and GaN material quality this method should offer aversatile approach to decrease dispersive effects in GaN HEMTs.
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| 36. |
- Bergsten, Johan
(author)
-
Buffer Related Dispersive Effects in Microwave GaN HEMTs
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- In applications such as mobile communication and radar, microwave power generation at high frequency is of utmost importance. The GaN HEMT offers a unique set of properties that makes it suitable for high power amplification at high frequencies. However, their performance is limited by trap states, leading to reduced output power and time variant effects. Furthermore, for good high frequency performance a high efficiency it is essential to limit the access resistances in the transistor. The GaN HEMT technology has long lacked a good ohmic contact with good reproducibility. In this thesis, three buffer designs are considered; C-doped GaN, AlGaN back barriers and a thin GaN structure. The three designs are evaluated in terms of trapping effects using the drain current transient technique. For the C-doped GaN buffer, trapping at dislocations covered with C-clusters is believed to be the main factor limiting output power. Dislocations are presumed to play a major role for the trapping behavior of AlGaN back barriers and the thin structure as well. The maximum output powers for C-doped GaN, AlGaN back barriers and the thin structure are 3.3, 2.7, and 3.9 W/mm at 30 GHz. The output power is found to be limited by trapping effects for all buffer designs. Moreover, a Ta-based, recessed ohmic contact enables a contact resistance of down to 0.14 Ωmm. The results also indicate that a highly reproducible process might be possible for deeply recessed contacts. An optimized AlGaN/GaN interface shows high mobility \textgreater2000 cm2/Vs without the use of an AlN-exclusion layer. The improved interface also decreases trapping effects and the gate-source capacitance at large electric fields compared to an unoptimized interface.
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| 37. |
- Bergsten, Johan, 1988, et al.
(author)
-
Electron Trapping in Extended Defects in Microwave AlGaN/GaN HEMTs with Carbon-Doped Buffers
- 2018
-
In: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1557-9646 .- 0018-9383. ; 65:6, s. 2446-2453
-
Journal article (peer-reviewed)abstract
- This paper investigates AlGaN/GaN high-electron mobility transistors (HEMTs) fabricated on epistructures with carbon (C)-doped buffers. Metalorganic chemical vapor deposition is used to grow two C-doped structures with different doping profiles, using growth parameters to change the C incorporation. The C concentration is low enough to result in n-type GaN. Reference devices are also fabricated on a structure using iron (Fe) as dopant, to exclude any process related variations and provide a relevant benchmark. All devices exhibit similar dc performance. However, pulsed I - V$ measurements show extensive dispersion in the C-doped devices, with values of dynamic R-mathrm-scriptscriptstyle ON 3 -4 times larger than in the dc case. Due to the extensive trapping, the devices with C-doped buffers can only supply about half the output power of the Fe-doped sample, 2.5 W/mm compared to 4.8 W/mm at 10 GHz. In drain current transient measurements, the trap filling time is varied, finding large prevalence of trapping at dislocations for the C-doped samples. Clusters of C around the dislocations are suggested to be the main cause for the increased dispersion.
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| 38. |
- Bergsten, Johan, et al.
(author)
-
Impact of AlGaN/GaN interface sharpness on HEMT performance
-
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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| 39. |
- Bergsten, Johan, 1988, et al.
(author)
-
Low resistive Au-free, Ta-based, recessed ohmic contacts to InAlN/AlN/GaN heterostructures
- 2015
-
In: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 30:10, s. 105034-
-
Journal article (peer-reviewed)abstract
- The formation of recess etched Au-free ohmic contacts to an InAlN/AlN/GaN heterostructure is investigated. A Ta/Al/Ta metal stack is used to produce contacts with contact resistance (R-c) as low as 0.14 Omega mm. It is found that R-c decreases with increasing recess depth until the InAlN barrier is completely removed. For even deeper recesses R-c remains low but requires annealing at higher temperatures for contact formation. The lowest R-c is found for contacts where the recess etch has stopped just above the 2D electron gas channel. At this depth the contacts are also found to be less sensitive to other process parameters, such as anneal duration and temperature. An optimum bottom Ta layer thickness of 5-10 nm is found. Two reliability experiments preliminary confirm the stability of the recessed contacts.
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| 40. |
- Bergsten, Johan, et al.
(author)
-
Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process
- 2016
-
In: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0018-9383 .- 1557-9646. ; 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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| 41. |
- 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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| 42. |
- Bergsten, U, et al.
(author)
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Batch measurements of wood density on intact or prepared drill cores using x-ray microdensitometry
- 2001
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In: Wood Science and Technology. - 0043-7719 .- 1432-5225. ; 35, s. 435-452
-
Journal article (peer-reviewed)abstract
- The performance of a batch scanning x-ray densitometer for measuring wood density without sample preparation, i.e., on intact drill cores, or on rectangular samples prepared from drill cores, was analysed. Effects of x-ray intensity, sample thickness and fiber direction, as well as extractives content, were evaluated for young (mainly sapwood) and old (mainly heartwood) wood from Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.). The x-ray power level used as standard (1.4 kW; 40 kV and 35 mA) seemed appropriate for the tested species and specimen thickness. The density of intact drill cores could be determined with a mean standard deviation of 1.6% for each sample, with a single machine run, if the cores were mounted with a fixed fiber direction and calibrations were made for each wood type. The corresponding precision for rectangular samples was 1.0%. Further improvements are attainable by using standard reference samples in each machine run and batch-wise analysis. For the chosen wood types and measurement technique, a sample thickness of 5 mm should give the best precision. However, for species with very narrow rings, thinner samples would improve the spatial resolution when ring boundaries are angled or curved. Extractives should be removed, especially for pine, but possibly also for spruce, if high precision in density determination is required.
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| 43. |
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| 44. |
- Blicharska, Malgorzata, et al.
(author)
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Effects of management intensity, function and vegetation on the biodiversity in urban ponds
- 2016
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In: Urban Forestry & Urban Greening. - : Elsevier BV. - 1618-8667 .- 1610-8167. ; 20, s. 103-112
-
Journal article (peer-reviewed)abstract
- Ponds are important elements of green areas in cities that help counteract the negative consequences of urbanization, by providing important habitats for biodiversity in cities and being essential nodes in the overall landscape-scale habitat network. However, there is relatively little knowledge about the impacts of pond management intensity, function and environmental variables on urban pond biodiversity. In this study we addressed this gap by investigating which factors were correlated with the level of biodiversity in urban ponds, indicated by species richness of aquatic insects, in Stockholm, Sweden. Our study did not confirm any direct link between the perceived intensity of management or function of ponds and overall biodiversity. However, it seems that management can influence particular groups of species indirectly, since we found that Trichoptera richness (Caddisflies) was highest at intermediate management intensity. We suggest that this is caused by management of vegetation, as the amount of floating and emergent vegetation was significantly correlated with both the overall species richness and the richness of Trichoptera (Caddisflies). This relationship was non-linear, since ponds with an intermediate coverage of vegetation had the highest richness. Interestingly, the amount of vegetation in the pond was significantly affected by pond function and pond management. The overall species richness and richness of Trichoptera were also positively correlated with pond size. Since we found that the pattern of relations between species richness and environmental variables differed between the insect groups we suggest that it will be difficult to provide overall design and management recommendations for ponds in urban green areas. Therefore, it is recommended that to provide high aquatic diversity of species in urban areas one should aim at promoting high diversity of different types of ponds with differing management and environmental factors that shape them.
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| 45. |
- Blicharska, Malgorzata, 1979-, et al.
(author)
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Is there a relationship between socio-economic factors and biodiversity in urban ponds? : A study in the city of Stockholm
- 2017
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In: Urban Ecosystems. - : Springer Science and Business Media LLC. - 1083-8155 .- 1573-1642. ; 20:6, s. 1209-1220
-
Journal article (peer-reviewed)abstract
- Urban small water bodies, such as ponds, are essential elements of human socio-economic landscapes. Ponds also provide important habitats for species that would otherwise not survive in the urban environment. Knowledge on the biodiversity of urban ponds and the relationship between their ecological value and factors linked to urbanization and socio-economic status is crucial for decisions on where and how to establish and manage ponds in cities to deliver maximum biodiversity benefits. Our study investigates if the pattern of urban-pond biodiversity can be related to different socio-economic factors, such as level of wealth, education or percentage of buildings of different types. Because of lack of previous studies investigating that, our study is of exploratory character and many different variables are used. We found that the biodiversity of aquatic insects was significantly negatively associated with urbanisation variables such as amount of buildings and number of residents living around ponds. This relationship did not differ depending on the spatial scale of our investigation. In contrast, we did not find a significant relationship with variables representing socio-economic status, such as education level and wealth of people. This latter result suggests that the socio-economic status of residents does not lead to any particular effect in terms of the management and function of ponds that would affect biodiversity. However, there is a need for a finer-scale investigation of the different potential mechanism in which residents in areas with differing socio-economic status could indirectly influence ponds.
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| 46. |
- Cederbladh, Johan, et al.
(author)
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Barriers for Adopting FMI-Based Co-Simulation in Industrial MBSE Processes
- 2023
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In: Proceedings - 2023 ACM/IEEE International Conference on Model Driven Engineering Languages and Systems Companion, MODELS-C 2023. - : Institute of Electrical and Electronics Engineers Inc.. - 9798350324983 ; , s. 510-519
-
Conference paper (peer-reviewed)abstract
- Model-Based Systems Engineering (MBSE) is a growing paradigm for system development where models are the primary considered artefacts. However, MBSE often relies on semi-formal modelling languages and methods, limiting analytical capabilities. Co-Simulation is argued in the literature to be a promising technology in the simulation domain for integrating heterogeneous models in unified simulations. The most commonly used standard for Co-Simulation is currently the Functional-Mockup-Interface (FMI), supported by many tools in the industry. Recently there has been increasing interest in utilizing co-simulation in MBSE processes to enable simulation capabilities earlier in development, mainly via instantiating simulations using the FMI standard from system architecture views. This paper briefly argues the case for co-simulation for industrial MBSE and presents several barriers to integration from a holistic point of view. The paper highlights the need for further research and progress to improve the maturity of the industrial adoption for MBSE workflows while discussing the current outlook for FMI-based co-simulation orchestrated from architecture models.
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| 47. |
- Chen, J. T., et al.
(author)
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A GaN-SiC hybrid material for high-frequency and power electronics
- 2018
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 113:4
-
Journal article (peer-reviewed)abstract
- We demonstrate that 3.5% in-plane lattice mismatch between GaN (0001) epitaxial layers and SiC (0001) substrates can be accommodated without triggering extended defects over large areas using a grain-boundary-free AlN nucleation layer (NL). Defect formation in the initial epitaxial growth phase is thus significantly alleviated, confirmed by various characterization techniques. As a result, a high-quality 0.2-μm thin GaN layer can be grown on the AlN NL and directly serve as a channel layer in power devices, like high electron mobility transistors (HEMTs). The channel electrons exhibit a state-of-the-art mobility of >2000 cm2/V-s, in the AlGaN/GaN heterostructures without a conventional thick C- or Fe-doped buffer layer. The highly scaled transistor processed on the heterostructure with a nearly perfect GaN-SiC interface shows excellent DC and microwave performances. A peak RF power density of 5.8 W/mm was obtained at VDSQ= 40 V and a fundamental frequency of 30 GHz. Moreover, an unpassivated 0.2-μm GaN/AlN/SiC stack shows lateral and vertical breakdowns at 1.5 kV. Perfecting the GaN-SiC interface enables a GaN-SiC hybrid material that combines the high-electron-velocity thin GaN with the high-breakdown bulk SiC, which promises further advances in a wide spectrum of high-frequency and power electronics.
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| 48. |
- Chiavari, C, et al.
(author)
-
Atmospheric corrosion of historical organ pipes: The influence of environment and materials
- 2008
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In: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 50:9, s. 2444-2455
-
Journal article (peer-reviewed)abstract
- The corrosion of lead-rich pipes in historical organs in different parts of Europe has been investigated. The influence of the environment and the composition and microstructure of the pipe metal was studied. Pipe Corrosion was documented by visual inspection (boroscope). The corrosion attack and the composition and microstructure of the metal were characterized by OM, SEM, XRD, IC and FAAS. It is shown that the degree of corrosion of the pipes is correlated to the concentration of gaseous acetic and formic acid in the organ. The organic acids are emitted by the wood from which the wind system is built. It is also shown that pipe corrosion decreases with increasing tin content in the range 0-4% (wt). Possible conservation strategies are discussed. (c) 2008 Elsevier Ltd. All rights reserved.
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| 49. |
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| 50. |
- Dahlkvist, Eva, et al.
(author)
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First-Line Managers’ Leadership Behavior Profiles and Use of Gardens in Residential Care Facilities: An Interview Study
- 2023
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In: Journal of Aging and Environment. - : Taylor & Francis. - 2689-2618 .- 2689-2626. ; 37:1, s. 65-84
-
Journal article (peer-reviewed)abstract
- This study explored first-line managers’ leadership behavior profiles regarding their goals for utilizing the garden at residential care facilities for older people. Semi-structured interviews were conducted with a convenience sample of first-line managers (n = 12) in Sweden. Data were analyzed using deductive content analysis theoretically guided by the Three-dimensional Leadership Model. The results showed that the main leadership behavior profiles were related to the dimensions structure, relation and change. The managers emphasized workplace regulations and goals. They allowed staff to make decisions and encouraged them to see problems and opportunities.
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