| 1. |
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| 2. |
- Rahman, Syed, 1981, et al.
(författare)
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Hydrodynamic Simulations of Unitraveling-Carrier Photodiodes
- 2007
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Ingår i: IEEE Journal of Quantum Electronics. - 0018-9197 .- 1558-1713. ; 43:11, s. 1088-1094
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Tidskriftsartikel (refereegranskat)abstract
- We present simulated results of a unitraveling-carrier photodiode (UTC-PD) using the hydrodynamic carrier transportation model. A maximum responsivity of 0.25 A/W and a small-signal 3-dB bandwidth of 52 GHz were obtained for a 220-nm-thick InGaAs absorption layer. The physical properties of the UTC-PD have been investigated at different optical injection levels. Modulation of the energy-band profile due to the space charge effect has been observed at high injection level, and an electron velocity overshoot of 3e7 cm/s has been found to effectively delay the onset of space charge effects. Comparisons with reported simulated results using the drift–diffusion model as well as reported experimental results are presented. The results suggest the necessity of using the hydrodynamic transport equations to accurately model the UTC-PD. In addition, it has been corroborated that the photoresponse of the UTC-PD could be improved by incorporating a graded doping profile in the absorption layer.
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| 3. |
- Andersson, Kristoffer, 1976, et al.
(författare)
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Fabrication and characterization of field-plated buried-gate SiC MESFETs
- 2006
-
Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 27:7, s. 573-575
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Tidskriftsartikel (refereegranskat)abstract
- Silicon carbide (SiC) MESFETs were fabricated using a standard SiC MESFET structure with the application of the "buried-channel" and field-plate (FP) techniques in the process. FPs combined with a buried-gate are shown to be favorable concerning output power density and power-added efficiency (PAE), due to higher breakdown voltage and decreased output conductance. A very high power density of 7.8 W/mm was measured on-wafer at 3 GHz for a two-finger 400-/spl mu/m gate periphery SiC MESFET. The PAE for this device was 70% at class AB bias. Two-tone measurements at 3 GHz /spl plusmn/ 100 kHz indicate an optimum FP length for high linearity operation.
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| 4. |
- Axelsson, Olle, 1986, et al.
(författare)
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Application Relevant Evaluation of Trapping Effects in AlGaN/GaN HEMTs With Fe-Doped Buffer
- 2016
-
Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 63:1, s. 326-332
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the impact of different iron (Fe) buffer doping profiles on trapping effects in microwave AlGaN/gallium nitride (GaN) high electron mobility transistors (HEMTs). We characterize not only the current collapse due to trapping in the buffer, but also the recovery process, which is important in the analysis of suitable linearization schemes for amplitude modulated signals. It is shown that the simple pulsed dc measurements of current transients can be used to investigate transient effects in the RF power. Specifically, it is revealed that the design of the Fe-doping profile in the buffer greatly influences the recovery time, with the samples with lower Fe concentration showing slower recovery. In contrast, traditional indicators, such as S-parameters and dc as well as pulsed $I$-$V$ characteristics, show very small differences. An analysis of the recovery shows that this effect is due to the presence of two different detrapping processes with the same activation energy (0.6 eV) but different time constants. For highly doped buffers, the faster process dominates, whereas the slower process is enhanced for less doped buffers.
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| 5. |
- Banik, Biddut Kumar, 1978, et al.
(författare)
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High Power Photonic MW/THz Generation Using UTC-PD
- 2008
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Ingår i: GigaHertz SympoSium 2008. ; , s. 45-
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Konferensbidrag (refereegranskat)abstract
- The ongoing research work concentrates on extending the previously accomplished UTC-PD fabrication and modelling techniques to 340 GHz and above. We have fabricated and characterized UTC-PDs intended for high power MW/THz generation. Several integrated antenna-detector circuits have been designed and characterised.
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| 6. |
- Banik, Biddut Kumar, 1978, et al.
(författare)
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Optimization of the UTC-PD Epitaxy for Photomixing at 340 GHz
- 2008
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Ingår i: International Journal of Infrared and Millimeter Waves. - : Springer Science and Business Media LLC. - 0195-9271 .- 1572-9559. ; 29:10, s. 914-923
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Tidskriftsartikel (refereegranskat)abstract
- We present a method to optimize the epitaxial layer structure of an InGaAs/InP uni-traveling-carrier photo-diode (UTC-PD) for continuous THz-wave generation. The design approach used is general in that it can be applied for any target frequency while this study focuses on 340 GHz. The photodiode epitaxy is modeled and optimized using a TCAD software implementing the hydrodynamic semiconductor equations. This physical device model was found to be in good agreement with reported experimental results. It is shown that the UTC-PD can generate ~1 mW at 340 GHz by choosing the optimum absorption layer and collection layer thicknesses.
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| 7. |
- Banik, Biddut Kumar, 1978, et al.
(författare)
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UTC-PD Integration for Submillimetre-wave Generation
- 2008
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Ingår i: 19th International Symposium on Space Terahertz Technology, ISSTT 2008; Groningen; Netherlands; 28 April 2008 through 30 April 2008. ; , s. P7-1, 135
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Konferensbidrag (refereegranskat)abstract
- Because of the inherent difficulty to generate power in the frequency range 0.l-10 THz, the term 'THzgap' has been coined. Among a number of MW/THz generation techniques, the photomixer based sources hold high potential offering wide tunability and decent amounts of output power. The photomixing technique relies on the nonlinear mixing of two closely spaced laser wavelengths generating a beat oscillation at the difference frequency. In recent years, there has been an increasing interest in the Uni-Travelling-Carrier PhotoDiode (UTCPD) [1] for photomixing, photo receivers, MW/THz-wave generation, fibre-optic communication systems, and wireless communications. UTC-PDs have become very promising by demonstrating output powers of 20 mW at 100 GHz [1] and 25 μW at 0.9 THz [2].
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| 8. |
- Chen, Ding-Yuan, et al.
(författare)
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Impact of the Channel Thickness on Electron Confinement in MOCVD-Grown High Breakdown Buffer-Free AlGaN/GaN Heterostructures
- 2023
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Ingår i: Physica Status Solidi (A) Applications and Materials Science. - : Wiley. - 1862-6319 .- 1862-6300. ; 220:16
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Tidskriftsartikel (refereegranskat)abstract
- The 2D electron gas (2DEG) confinement on high electron mobility transistor (HEMT) heterostructures with a thin undoped GaN channel layer on the top of a grain-boundary-free AlN nucleation layer is studied. This is the first time demonstration of a buffer-free epi-structure grown with metal-organic chemical vapor deposition with thin GaN channel thicknesses, ranging from 250 to 150 nm, without any degradation of the structural quality and 2DEG properties. The HEMTs with a gate length of 70 nm exhibit good DC characteristics with peak transconductances of 500 mS mm(-1) and maximum saturated drain currents above 1 A mm(-1). A thinner GaN channel layer improves 2DEG confinement because of the enhanced effectiveness of the AlN nucleation layer acting as a back-barrier. An excellent drain-induced barrier lowering of only 20 mV V-1 at a V-DS of 25 V and an outstanding critical electric field of 0.95 MV cm(-1) are demonstrated. Good large-signal performance at 28 GHz with output power levels of 2.0 and 3.2 W mm(-1) and associated power-added efficiencies of 56% and 40% are obtained at a V-DS of 15 and 25 V, respectively. These results demonstrate the potential of sub-100 nm gate length HEMTs on a buffer-free GaN-on-SiC heterostructure.
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| 9. |
- Chen, Ding Yuan, et al.
(författare)
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Microwave Performance of 'Buffer-Free' GaN-on-SiC High Electron Mobility Transistors
- 2020
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Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 41:6, s. 828-831
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Tidskriftsartikel (refereegranskat)abstract
- High performance microwave GaN-on-SiC HEMTs are demonstrated on a heterostructure without a conventional thick doped buffer. The HEMT is fabricated on a high-quality 0.25~\boldsymbol {\mu }\text{m} unintentional doped GaN layer grown directly on a transmorphic epitaxially grown AlN nucleation layer. This approach allows the AlN-nucleation layer to act as a back-barrier, limiting short channel effects and removing buffer leakage. The devices with the 'buffer-free' heterostructure show competitive DC and RF characteristics, as benchmarked against the devices made on a commercial Fe-doped epi-wafer. Peak transconductances of 500 mS/mm and a maximum saturated drain current of 1 A/mm are obtained. An extrinsic \text{f}-{\sf T} of 70 GHz and \text{f}-{\sf max} of 130 GHz are achieved for transistors with a gate length of 100 nm. Pulsed-IV measurements reveal a lower current slump and a smaller knee walkout. The dynamic IV performance translates to an output power of 4.1 W/mm, as measured with active load-pull at 3 GHz. These devices suggest that the 'buffer-free' concept may offer an alternative route for high frequency GaN HEMTs with less electron trapping effects.
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| 10. |
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| 11. |
- Galt, Sheila, 1956, et al.
(författare)
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Students’ own collective criteria - influence on peer feedback and lab report quality
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- SHORT SUMMARY What happens when you let the students collectively decide together what’s meant by “good quality” for example in a lab report? Will this adequately guide their own learning as shown by their written lab reports and peer feedback comments? Or is it better to just tell them what you expect? ABSTRACT Intended audience: This presentation will likely mostly interest teachers who use or want to start using peer review in their courses (at bachelor or master level). Teachers of courses with lab report assignments may also be interested in my example of use of peer feedback in this learning context. Problem statement: Many students, at the onset of advanced level studies at Chalmers, lack the ability to produce a good quality lab report. Such generic skills are expected to be in place before students leave Chalmers with their master’s degree. Experience has shown that “just” telling the students how to write a good report is not sufficient. Might they listen more to each other than to the teacher? Suggested solution: If students are collectively given the chance to spell out what they mean by a “good quality” lab report, agreeing upon a list of criteria in class, this list can be used to guide their learning through applying their own criteria. This list of criteria can support their individual first draft writing process, as well as their individual feedback given to each other in a structured peer review process. Finally, the feedback given by the teacher can reference the students’ own list of criteria when making the final assessment of the revised report. Chosen learning scenarios: This method has been applied for two consecutive years in an introductory course within the MPWPS program. An introductory lecture before labs start includes a collective exercise where students provide their take on quality criteria for lab reports. The teacher takes the role of secretary, creating their collective list on the whiteboard, and making the list available in PingPong after the lecture. (See Ref. 1.) If key aspects are being forgotten by the students collectively during the process, the teacher can drop gentle hints in order to have the final quality criteria list quality assured. Seven obligatory short labs are performed in pairs, but only one of these labs is individually and randomly assigned to each student for writing a formal lab report. Students are then assigned peer review roles, and apply these quality criteria to a report on a different lab by another student. Revised lab reports are then submitted, along with a short text on how the peer feedback was incorporated into the final version. With peer review of first drafts, the reviewer as well as the author of the report will be learning during the process, while carefully applying the quality criteria. During the two most recent years, when the above system was in place in this course, the teaching differed in one significant way: the most recent year included a lecture dedicated to academic honesty and the avoidance of plagiarism. (See Ref. 2.) In previous years, quality criteria were provided by the teacher, not by the collective group of students. (See Ref. 3.) Student achievement measures: We analyze the student achievements (lab report quality) as a measure of their learning in three different aspects: core subject content learning, generic written communication skills, and academic honesty. The latter analysis is reported in a separate presentation (Ref. 2.) The generic written communication skills will be the focus of my presentation, with an attempt at measuring the degree to which the student’s own quality criteria list was actually successfully applied in their lab report and peer review writing. It will however not be possible to make a “fair and scientific” comparison to previous years’ student lab reports, since other factors were also changed at the same time. Comparison of resulting achievement for different scenarios: Student learning connected to similar quality criteria will be compared for the “student collectively generated” criteria (the latest two years of the course) and the “teacher generated” criteria (provided previously). The two different years this student generated criteria scenario was used had slightly different criteria lists – which may or may not be evident in the outcome of the students’ writing. This course has just finished, and the analysis will be performed during the coming study period before the KUL conference, so results are pending. Alternative solutions: In a more traditional scenario, the teacher informs the students of the quality criteria, set by the teacher, and the teacher applies the criteria when grading the assignment. Both comments and grades are usually provided by the teacher to individual students, who sometimes resubmit assignments after taking into account the teacher-provided feedback. However, in many cases, the feedback comes without any further requirement posed, and therefore without strong incentives for further learning. Much research has been done on criteria based assessment including negotiating criteria with students, (see e.g. Ref. 4.) Here, however, I have just given my suggestion of one way of assessing, which I have found to work well References: 1. ”Lab report quality criteria”, available on PingPong at the following link: https://pingpong.chalmers.se/courseId/7038/content.do?id=3366624 2. KUL 2017, Undervisa och examinera akademisk hederlighet, submitted ”short presentation” 3. KUL 2012, Systematisk feedback och progression som stöd för studenters lärande inom generella kompetenser. 4. Biggs, J. ”Teaching for quality Learning at University”, 2003, chapters 8 and 9.
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| 12. |
- Gudjonsson, Gudjon, 1973, et al.
(författare)
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Design and Fabrication of 4H-SiC RF MOSFETs
- 2007
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Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 54:12, s. 3138-3145
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Tidskriftsartikel (refereegranskat)abstract
- We present simulations, fabrication and analysis of 4H-SiC RF power MOSFETs. We obtain an extrinsic transition frequency of 11.2 GHz and an f max = 11.9 GHz, a breakdown voltage above 200 V and an output power of 1.9 W/mm at 3 GHz. The measured devices are double fingered, source-gate-draingate-source with 2 × 0.4 mm total gate width and the nominal channel length of the devices is 0.5 μm. To the authors knowledge, this is the highest transition frequency and output power density ever reported for SiC RF MOSFETs. The antipunchthrough is introduced as a way to take advantage of the SiC's material properties. A detailed description of the device processing is also given. © 2007 IEEE.
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| 13. |
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| 14. |
- Gudjonsson, Gudjon, 1973, et al.
(författare)
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High Frequency 4H-SiC MOSFETs
- 2007
-
Ingår i: Materials Science Forum. - 1662-9752 .- 0255-5476. ; 556-557, s. 795-798
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Konferensbidrag (refereegranskat)
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| 15. |
- Gudjonsson, Gudjon, 1973, et al.
(författare)
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High Power Density 4H-SiC RF MOSFETs
- 2006
-
Ingår i: IEEE Electron Device Letters. ; 527-529, s. 1277-1280
-
Tidskriftsartikel (refereegranskat)
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| 16. |
- Hjelmgren, Hans, 1960, et al.
(författare)
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Electro-thermal simulations of a microwave 4H-SiC MESFET on high purity semi-insulating substrate
- 2007
-
Ingår i: Solid-State Electronics. - : Elsevier BV. - 0038-1101. ; 51:8, s. 1144-1152
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Tidskriftsartikel (refereegranskat)abstract
- DC and small-signal electro-thermal simulations of a 4H-SiC microwave power MESFET on a high purity semi-insulating substrate are compared to measurements. The focus is on the electron transport, substrate traps, and thermal heating. The doping concentrations are described by measured SIMS profiles, and the material parameters are in accordance with published results. Although the simulated MESFET has a p-buffer and a high purity substrate, the simulations show that the density of shallow traps affects the device characteristics.The very good agreement between simulated and measured DC and small-signal characteristics indicates that the models for electron mobility, substrate traps, and heating are the most important to achieve good agreement with measured data.
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| 17. |
- Hjelmgren, Hans, 1960, et al.
(författare)
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Extraction of an Electrothermal Mobility Model for AlGaN/GaN Heterostructures
- 2012
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Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 59:12, s. 3344-3349
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Tidskriftsartikel (refereegranskat)abstract
- An electron mobility model for AlGaN/GaN heterostructures taking both the lattice temperature and electric field into account is proposed. Numerical device simulation of an ungated Al0.25Ga0.75N/GaN HEMT structure on 4H-SiC substrate is compared to measured electrical characteristics. Mobility model parameters are extracted by comparing iso-thermal numerical simulations with microwave (6 GHz) large signal measurements. The extracted model was used in static simulations, showing good agreement with measurements.
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| 18. |
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| 19. |
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| 20. |
- Hjelmgren, Hans, 1960
(författare)
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Numerical modelling of hot electron transport in Schottky-diodes and heterojunction structures
- 1991
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A one-dimensional drift-diffusion model, including energy balance equations, is used to model Schottky-diodes and heterojunction structures. The set of equations are solved simultaneously with a finite-difference iterative scheme. The flux of energy across the metal-semiconductor interface is set equal to the energy transport caused by the net-flow of charged carriers arising from the electric current. This boundary condition allows the carrier temperature in the semiconductor at the Schottky contact to differ from the lattice temperature.The model has been used to investigate dc-properties (IV, CV) of strongly forward biased Schottky-diodes on n-GaAs. The modelled IV of four different diodes are compared with measured IV curves. The inclusion of hot electron transport is essential to obtain good agreement between modelled and measured IV characteristics for diodes with short epitaxial layers. The effects of image force lowering and thermionic-field emission are modelled with a field-dependent barrier height. The differential capacitance is determined from the change of electric charge in the semiconductor for an incremental voltage change. The numerically simulated capacitance increases with forward voltage to a finite value, and then it starts to decrease.The computer program is extended to model heterojunctions by including position dependent material parameters. The simulated IV characteristic of a forward biased graded heterojunction diode is in good agreement with published Monte Carlo results of a similar device.
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| 21. |
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| 22. |
- Hjelmgren, Hans, 1960, et al.
(författare)
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Transient Simulation of Microwave SiC MESFETs With Improved Trap Models
- 2010
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Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 57:3, s. 729-732
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Tidskriftsartikel (refereegranskat)abstract
- Measured and simulated transient characteristics ofa SiC metal–semiconductor field-effect transistor are compared. Self-heating, gate tunneling, substrate, and surface traps are taken into account in the simulations. By explicitly filling surface traps at the vicinity of the gate during pinchoff, close correspondence between simulated and measured gate lags is achieved.
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| 23. |
- Hjelmgren, Hans, 1960, et al.
(författare)
-
Undervisa och examinera akademisk hederlighet
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- KORT SAMMANFATTNING Vi har lagt in en obligatorisk föreläsning om Akademisk hederlighet, plagiering och otillåtet samarbete i en av de inledande obligatoriska kurserna i mastersprogrammet MPWPS. Färdigheterna testas dels direkt med en quiz, men framför allt genom en individuell skriftlig laborationsrapport. ABSTRACT Antalet anmälda fall av misstänkt plagiering vid högskolorna ökar i en betydligt högre takt än andra former av anmälda disciplinärenden (UKÄ, 2014). Vad som avses med plagiering och hur man undviker det är dock inte självklart. För att kopiering ska bli otillåten i dessa sammanhang måste den ha utförts med avsikt att vilseleda under examination (Carroll och Zetterling, 2009). Våra erfarenheter från tidigare år visar att vi behöver arbeta mer aktivt med begrepp som akademisk hederlighet och plagiering. Det har inte varit tillräckligt att dela ut Chalmers policy kring akademisk hederlighet (Wennberg 2009) och en kort genomgång vid mottagning av mastersstudenterna. I en av de obligatoriska kurserna i första läsperioden på mastersprogrammet Trådlös Teknik, Fotonik och Rymdteknik har vi därför kombinerat en obligatorisk föreläsning kring akademisk hederlighet med inlämning av en individuell laborationsrapport. Under första halvan av föreläsningen definierar vi plagiering, ser vad som står i högskoleförordningen, hur man skriver utifrån källor, samt informerar om den hjälp Chalmers kan erbjuda vid rapportskrivning, t.ex. CHOCS och engelska-kurser. Under andra halvan av föreläsningen besvarar studenterna m.h.a. sina mobiler en quiz med åtta olika påståenden kring plagiering och otillåtet samarbete. Frågorna är hämtade från Henriksson (2008). Vi avslutar med att gå igenom de olika påståendena tillsammans och tittar på fördelningen av korrekta och felaktiga svar. Inför rapportskrivandet tilldelas varje student slumpmässigt att skriva en mer utförlig laborationsrapport kring en av de sju obligatoriska laborationsuppgifter som ingår i kursen. De skickar in ett utkast av rapporten till Ping Pong via Urkund. Studenterna ger sedan återkoppling på varandras utkast, s.k. peer review. Studenterna tar med rekommendationerna som de fått från peer review och lämnar sedan in en slutlig rapport till lärarna för granskning. Kursen har precis avslutats, men vi kommer utvärdera effekten m.h.a. en tillagd fråga i kursenkäten, och en jämförelse av förekomsten av plagiat i årets och föregående års rapporter. Använd litteratur Carroll, J.J., Zetterling C.M. (2009). Hjälp studenterna att undvika plagiering. Stockholm: Kungliga tekniska högskolan. Henriksson, A.-S. (2008) Att förebygga plagiat i studentarbeten – en pedagogisk utvecklingsmöjlighet. Uppsala: Uppsala universitet. UKÄ rapport 2014:3, Disciplinärenden 2013 vid universitet och högskolor. Stockholm, Universitetskansler-ämbetet. Wennberg. B. (2009) Akademisk hederlighet på Chalmers – Vilka är våra spelregler? Göteborg: Chalmers tekniska högskola.
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| 24. |
- Malmros, Anna, 1977, et al.
(författare)
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Enhanced Mobility in InAlN/AlN/GaN HEMTs Using a GaN Interlayer
- 2019
-
Ingår i: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1557-9646 .- 0018-9383. ; 66:7, s. 2910-2915
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Tidskriftsartikel (refereegranskat)abstract
- © 1963-2012 IEEE. An enhancement of the electron mobility ( \mu ) in InAlN/AlN/GaN heterostructures is demonstrated by the incorporation of a thin GaN interlayer (IL) between the InAlN and AlN. The introduction of a GaN IL increases \mu at room temperature (RT) from 1600 to 1930 cm2/Vs. The effect is further enhanced at cryogenic temperature (5 K), where the GaN IL sample exhibits a \mu of 16000 cm2/Vs, compared to 6900 cm2/Vs without IL. The results indicate the reduction of one or more scattering mechanisms normally present in InAlN/AlN/GaN heterostructures. We propose that the improvement in \mu is either due to the suppression of fluctuations in the quantum well subband energies or to reduced Coulomb scattering, both related to compositional variations in the InAlN. HEMTs fabricated on the GaN IL sample demonstrate larger improvement in dc-and high-frequency performance at 5 K; {f}-{\text {max}} increases by 25 GHz to 153 GHz, compared to an increase of 6 GHz to 133 GHz without IL. The difference in improvement was associated mainly with the drop in the access resistances.
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| 25. |
- Malmros, Anna, 1977, et al.
(författare)
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Evaluation of Thermal Versus Plasma-Assisted ALD Al2O3 as Passivation for InAlN/AlN/GaN HEMTs
- 2015
-
Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 36:3, s. 235-237
-
Tidskriftsartikel (refereegranskat)abstract
- Al2O3 films deposited by thermal and plasma-assisted atomic layer deposition (ALD) were evaluated as passivation layers for InAlN/AlN/GaN HEMTs. As a reference, a comparison was made with the more conventional plasma enhanced chemical vapor deposition deposited SiNx passivation. The difference in sheet charge density, threshold voltage, f(T) and f(max) was moderate for the three samples. The gate leakage current differed by several orders of magnitude, in favor of Al2O3 passivation, regardless of the deposition method. Severe current slump was measured for the HEMT passivated by thermal ALD, whereas near-dispersion free operation was observed for the HEMT passivated by plasma-assisted ALD. This had a direct impact on the microwave output power. Large-signal measurements at 3 GHz revealed that HEMTs with Al2O3 passivation exhibited 77% higher output power using plasma-assisted ALD compared with thermal ALD.
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| 26. |
- Malmros, Anna, 1977, et al.
(författare)
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Impact of Channel Thickness on the Large-Signal Performance in InAlGaN/AlN/GaN HEMTs with an AlGaN Back Barrier
- 2019
-
Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 66:1, s. 364-371
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Tidskriftsartikel (refereegranskat)abstract
- The impact of varying the GaN channel layer thickness (t ch ) in InAlGaN/AlN/GaN HEMTs with C-doped AlGaN back barriers is investigated. t ch was 50, 100, and 150 nm, and the gate length of the fabricated HEMTs ranged from 50 to 200 nm. It is found that short-channel effects (SCEs) are significantly mitigated with a small t ch . For HEMTs with a gate length of 50 nm, the drain-induced barrier lowering changes from 40 to 93 mV/V as t ch is increased from 50 to 150 nm. On the other hand, it is shown that dispersive effects are more severe for a smaller t ch , as demonstrated by a sixfold increase in the dynamic ON-resistance for t ch = 50 nm compared to t ch = 150 nm. The tradeoff between dispersion and SCEs is reflected in large-signal measurements at 30 GHz. The 50-nm channel, mainly limited by dispersion, exhibits an output power of 3.5 W/mm. The thicker channels reach a maximum of around 5 W/mm, but for different gate lengths due to the difference in severity of the SCEs. This paper elucidates the interplay between SCEs and dispersion related to t ch , its consequences for the large-signal performance and for the limitation in downscaling of the gate length.
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| 27. |
- Nilsson, Per-Åke, 1964, et al.
(författare)
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Influence of Field Plates and Surface Traps on Microwave Silicon Carbide MESFETs
- 2008
-
Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 55:8, s. 1875-1879
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Tidskriftsartikel (refereegranskat)abstract
- The influence of field plates and surface traps on silicon carbide MESFETs for microwave operation was investigated. By increasing the length of gate-connected field plates from 50 to 800 nm, it was possible to increase the gate–drain breakdown voltage of the devices from 125 to 170 V. At the same time, the current slump effect of traps in the passivation oxide was reduced. By using a combination of field plates and a passivation oxide with low interface trap density, it was possible to reach an output power density of 8 W/mm at 3 GHz.
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| 28. |
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| 29. |
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| 30. |
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| 31. |
- Papamichail, A., et al.
(författare)
-
Compositionally graded channel HEMTs towards improved linearity for low-noise RF amplifiers
- 2022
-
Ingår i: 2022 Compound Semiconductor Week, CSW 2022.
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Konferensbidrag (refereegranskat)abstract
- Although AlGaN/GaN HEMTs with high power and current gain have been demonstrated in RF device applications, at high signal operation they show an inherent non-linear behavior which leads to gain compression and signal distortion. Polarization-doped AlGaN/GaN HEMTs, with a compositionally graded channel enables a linear response improvement through formation of a 3-D electron gas. In this work, we develop the growth process for graded channel HEMTs in a hot-wall MOCVD reactor. Control of the grading profile is established through growth parameter tuning. Afterwards, analysis by EDS allows for precise determination of Al composition across the channel. Conventional and graded channel HEMT structures were fabricated and characterized. Furthermore, the sheet resistance, carrier density and mobility in HEMT structures with different grading profiles are compared and discussed. The conventional (non-graded) structure revealed the highest electron mobility of ~2350 cm2/V.s, which is among the highest values reported.
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| 32. |
- Persson, P. O.Å., et al.
(författare)
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Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers
- 2023
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 122:15
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Tidskriftsartikel (refereegranskat)abstract
- Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼9 × 10 12 cm-2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity.
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| 33. |
- Sudow, Mattias, 1980, et al.
(författare)
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An SiC MESFET-based MMIC process
- 2006
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Ingår i: IEEE Transactions on Microwave Theory and Techniques. ; 54:12, Part 1, s. 4072-4078
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Tidskriftsartikel (refereegranskat)abstract
- A monolithic microwave integrated circuit (MMIC)process based on an in-house SiC MESFET technology has been developed. The process uses microstrip technology, and a complete set of passive components, including MIMcapacitors, spiral inductors,thin-film resistors, and via-holes, has been developed. The potential of the process is demonstrated by an 8-W power amplifierat 3 GHz, a high-linearity -band mixer showing a third-order intercept point of 38 dBm, and a high-power limiter.
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| 34. |
- Thorsell, Mattias, 1982, et al.
(författare)
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Characterization of Electro-Thermal Effects in GaN Based HEMTs
- 2010
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Ingår i: 5th Space Agency - MOD Round Table Workshop on GaN Component Technologies.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents a new method for characterizing electro-thermal effects in GaN based HEMTs. The proposed method characterizes the thermal and bias dependence of the current through the 2-DEG individually. The temperature dependence is characterized by TLM measurements versus ambient temperature, showing an increasing sheet resistivity with temperature. The thermal impedance is determined from low frequency impedance measurements, indicating thermal response up to at least 100 MHz. The isothermal bias dependence is measured above the thermal cut-off with a LSNA, showing a nonlinear current versus voltage characteristic.
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| 35. |
- Thorsell, Mattias, 1982, et al.
(författare)
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Electrothermal Access Resistance Model for GaN-Based HEMTs
- 2011
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Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 58:2, s. 466 - 472
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Tidskriftsartikel (refereegranskat)abstract
- The high-power density in GaN-based high-electron-mobility transistors (HEMTs) increases demands on the accurate extraction and modeling of electrothermal effects such as self-heating. This paper presents a new electrothermal model of the access resistances in GaN HEMTs, taking into account both self heating and bias dependence. A coplanar ungated transfer length method (TLM) structure has been used to extract the resistance versus temperature, bias, and RF power. The temperature dependence is extracted from dc measurements at ambient temperatures between 293 and 443 K. Small-signal measurements are used to extract the time constants in the thermal impedance. The bias dependence of the current is characterized by isothermal large-signal RF measurements between 1 and 6 GHz. A new method for extracting the thermal resistance from the large-signal measurements together with temperature-dependent dc measurements is also presented.
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