| 1. |
- Houshuai, Wang, et al.
(författare)
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Molecular phylogeny of Lymantriinae (Lepidoptera, Noctuoidea, Erebidae) inferred from eight gene regions
- 2015
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Ingår i: Cladistics. - : Wiley. - 0748-3007 .- 1096-0031. ; 31:6, s. 579-592
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Tidskriftsartikel (refereegranskat)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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| 2. |
- Bergsten, Johan, 1988, et al.
(författare)
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AlGaN/GaN high electron mobility transistors with intentionally doped GaN buffer using propane as carbon precursor
- 2016
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Ingår i: Japanese Journal of Applied Physics. - : IOP Publishing. - 1347-4065 .- 0021-4922. ; 55:5
-
Tidskriftsartikel (refereegranskat)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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| 3. |
- Bergsten, Johan, et al.
(författare)
-
AlGaN/GaN high electron mobility transistors with intentionally doped GaN buffer using propane as carbon precursor
- 2016
-
Ingår i: Japanese Journal of Applied Physics. - : Institute of Physics Publishing (IOPP). - 0021-4922 .- 1347-4065. ; 55, s. 05FK02-1-05FK02-4
-
Tidskriftsartikel (refereegranskat)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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| 4. |
- Bergsten, Johannes, 1975-, et al.
(författare)
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Arbetet med donationer av insektsamlingar vid Naturhistoriska Riksmuseet
- 2014
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Ingår i: Entomologisk Tidskrift. - 0013-886X. ; 134, s. 153-162
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- We describe the work with donated insect collections at the Swedish Museum of NaturalHistory (NRM) in Stockholm, Sweden. The museum receives donations yearly fromamateur entomologists, and they are an important contribution to the enrichment of thecollections. For the collector it is satisfying that a public institution takes on the long termresponsibility of safeguarding the scientific value in a collection, curating and making itavailable for study. Significant donations in the last years include that of Lars Huggert(Hymenoptera, Coleoptera), Hans Bartsch (Diptera) and Anders N. Nilsson (aquatic Coleoptera)to name a few. The curatorial and digitizing workload at the Entomology collectionare unfortunately not matched by staff funding, and as at other European museumsvolunteer work constitute vital and invaluable help. We acknowledge especially some ofthe volunteer work in the Coleoptera and Hymenoptera collections. Recently we have engagedwith amateur entomologists by organizing taxon-specific workshops at the museumwhich has stimulated exchange and collaboration. The Hymenoptera-day was visited by 30participants, and the Diptera-meeting by 49. As an example of what happens with a donationonce it reaches the museum, we describe the work with a recent Coleoptera collectiondonation by Jan Olsson, Vallentuna. A few highlights from the unidentified material,including the Archostematan beetle Priacma serrata (Cupedidae) and the false jewelbeetleSchizopus laetus (Schizopodidae), are presented as they were new to the NRM collections.We also bring attention to two new websites: www.naturarv.se is the webportal presentingdigitized material in Swedish natural history collections. Both metadata on specimens andphotos are made searchable here. We also launch a new webpage at www.nrm.se/insektsdonationerwhere we write about new donations to the Entomology collections, with JanOlsson’s Coleoptera collection first out.
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| 5. |
- Bergsten, Johan, 1988, et al.
(författare)
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Electron Trapping in Extended Defects in Microwave AlGaN/GaN HEMTs with Carbon-Doped Buffers
- 2018
-
Ingår i: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1557-9646 .- 0018-9383. ; 65:6, s. 2446-2453
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Tidskriftsartikel (refereegranskat)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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| 6. |
- Bergsten, Johan, et al.
(författare)
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Impact of AlGaN/GaN interface sharpness on HEMT performance
-
Annan publikation (övrigt vetenskapligt/konstnärligt)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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| 7. |
- Bergsten, Johan, 1988, et al.
(författare)
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Low resistive Au-free, Ta-based, recessed ohmic contacts to InAlN/AlN/GaN heterostructures
- 2015
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Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 30:10, s. 105034-
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Bergsten, Johan, et al.
(författare)
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Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process
- 2016
-
Ingår i: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0018-9383 .- 1557-9646. ; 63:1, s. 333-338
-
Tidskriftsartikel (refereegranskat)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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| 9. |
- Bergsten, Johan, 1988, et al.
(författare)
-
Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process
- 2016
-
Ingår i: IEEE Transactions on Electron Devices. - : Institute of Electrical and Electronics Engineers (IEEE). - 1557-9646 .- 0018-9383. ; 63:1, s. 333-338
-
Tidskriftsartikel (refereegranskat)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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| 10. |
- Bremer, Johan, 1991, et al.
(författare)
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Analysis of Lateral Thermal Coupling for GaN MMIC Technologies
- 2018
-
Ingår i: IEEE Transactions on Microwave Theory and Techniques. - 0018-9480 .- 1557-9670. ; 66:10, s. 4430-4438
-
Tidskriftsartikel (refereegranskat)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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| 11. |
- Chen, J. T., et al.
(författare)
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A GaN-SiC hybrid material for high-frequency and power electronics
- 2018
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 113:4
-
Tidskriftsartikel (refereegranskat)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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| 12. |
- Fager, Christian, 1974, et al.
(författare)
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Analysis of Thermal Effects in Integrated Radio Transmitters
- 2018
-
Ingår i: 2018 INTERNATIONAL SYMPOSIUM ON VLSI TECHNOLOGY, SYSTEMS AND APPLICATION (VLSI-TSA). - 1930-8868. - 9781538648254 - 9781538648254
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Konferensbidrag (refereegranskat)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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| 13. |
- Gustafsson, Sebastian, et al.
(författare)
-
Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer
- 2015
-
Ingår i: IEEE Transactions on Electron Devices. - : IEEE Press. - 0018-9383 .- 1557-9646. ; 62:7, s. 2162-2169
-
Tidskriftsartikel (refereegranskat)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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| 14. |
- Gustafsson, Sebastian, 1990, et al.
(författare)
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Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer
- 2015
-
Ingår i: IEEE Transactions on Electron Devices. - : Institute of Electrical and Electronics Engineers (IEEE). - 1557-9646 .- 0018-9383. ; 62:7, s. 2162-2169
-
Tidskriftsartikel (refereegranskat)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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| 15. |
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| 16. |
- Huang, Tongde, 1985, et al.
(författare)
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Achieving Low-Recovery Time in AlGaN/GaN HEMTs With AlN Interlayer Under Low-Noise Amplifiers Operation
- 2017
-
Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 38:7, s. 926-928
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Tidskriftsartikel (refereegranskat)abstract
- Three transistors with different AlGaN/GaN interface designs (sharp interface, standard interface, and an extra AlN interlayer) were studied in-depth under conditions mimicking low-noise amplifiers (LNAs) operation. A new measurement setup, analog to LNAs operation condition, is established to measure recovery time on device level. For the first time, a direct relationship between the recovery time and the design of AlGaN/GaN interface is revealed in devices with Carbon doping buffer in this letter. An extremely low-recovery time is demonstrated in the transistor with an AlN interlayer. Both transistors without an AlN interlayer exhibit severe gain and drain current degradation after pulsed input stress. The transistor with a sharp interface shows a recovery time around 10 ms, whereas the transistor with a standard interface shows even much longer recovery time. These results imply that AlN interlayer, which can effectively block the injection of hot electrons to AlGaN bulk or surface traps, is highly preferred in systems where LNAs need to function promptly after an input overdrive.
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| 17. |
- Huang, Tongde, 1985, et al.
(författare)
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Enhanced gate stack stability in GaN transistors with gate dielectric of bilayer SiNx by low pressure chemical vapor deposition
- 2018
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 113:23
-
Tidskriftsartikel (refereegranskat)abstract
- We report enhanced gate stack stability in GaN metal insulator semiconductor high electron mobility transistors (MISHEMTs) by using a bilayer SiNx as the gate dielectric. To obtain the bilayer gate dielectric scheme, a thin Si-rich SiNx interlayer was deposited before a high-resistivity SiNx layer by low pressure chemical vapor deposition. The Si-rich SiNx can effectively suppress the trapping phenomenon at the interface of the dielectric/AlGaN barrier. The upper high-resistivity SiNx layer can greatly block the gate leakage current to enable a large gate swing. Compared with the MISHEMTs using a single Si-rich or high-resistivity SiNx layer, the MISHEMTs with a bilayer gate dielectric take the advantages of both, realizing a gate stack with a stable threshold voltage and low leakage current. These results thus present great potential for developing high-performance GaN MISHEMTs using the bilayer SiNx gate dielectric scheme for highly efficient power applications. Published by AIP Publishing.
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| 18. |
- Huang, Tongde, 1985, et al.
(författare)
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Fabrication and improved performance of AlGaN/GaN HEMTs with regrown ohmic contacts and passivation-first process
- 2016
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Ingår i: 2016 Compound Semiconductor Week, CSW 2016; Toyama, Japan. - 9781509019649 ; , s. Article no. 7528722-
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Konferensbidrag (refereegranskat)abstract
- This work reports the fabrication of AlGaN/GaN HEMTs with regrown ohmic contacts using either a passivation-last or a passivation-first process, where the order of surface passivation and gate metallization processes is different. An improved performance is demonstrated using the passivation-first process, achieving a maximum current/power gain cutoff frequency (fT/fmax) around 60/127 GHz with an 80-nm gate length. The ohmic contacts were regrown with highly doped n-GaN, resulting in a contact resistance of ~0.2 Ω·mm. The RF performance can be further enhanced by reducing the extrinsic gate capacitance and short channel effects.
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| 19. |
- Huang, Tongde, 1985, et al.
(författare)
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Impact of AlGaN/GaN Interface and Passivation on the Robustness of Low-Noise Amplifiers
- 2020
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Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 67:6, s. 2297-2303
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Tidskriftsartikel (refereegranskat)abstract
- Poststress dc characteristics of AlGaN/GaN HEMTs can be used to study the effect of high-power stress on the noise figure (NF) and gain of low-noise amplifiers (LNAs) subjected to large input overdrives. This enables a shift from circuit- to transistor-level measurements to investigate the impact of variations in HEMT design parameters on the robustness (including both recovery time and survivability) by mimicking LNA operation. Using this method, a tradeoff between survivability and recovery time is demonstrated for different AlGaN/GaN interface profiles (sharp interface, standard interface, and AlN interlayer). Furthermore, the impact of different surface passivation schemes (Si-rich, Si-poor, and bilayer SiNx) on robustness is investigated. The bilayer passivation, which features low leakage current and small gain compression under overdrive stress, exhibits relatively weak survivability. The mechanisms influencing the robustness are analyzed based on transistor physics. The short recovery time is mainly due to impeding the injection of hot electrons into surface traps and high reverse current, whereas the survivability is dependent on the local or global peak electrical fields around the gate under high power stress.
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| 20. |
- Huang, Tongde, 1985, et al.
(författare)
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Low-Pressure-Chemical-Vapor-Deposition SiNx Passivated AlGaN/GaN HEMTs for Power Amplifier Application
- 2015
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Ingår i: 2015 Asia-Pacific Microwave Conference (Apmc), Vols 1-3. - 9781479987672 ; 3
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Konferensbidrag (refereegranskat)abstract
- A Low-Pressure-Chemical-Vapor-Deposition (LPCVD) bilayer SiNx passivation scheme has been investigated and developed, which effectively suppress the current collapse in AlGaN/GaN HEMTs. Low current slump is very helpful for microwave power amplifier application. Compared to in-situ SiNx passivations by metal-organic-chemical-vapor-deposition (MOCVD) and ex-situ SiNx passivations by plasma-enhanced-chemical-vapor-deposition (PECVD), the LPCVD SiNx exhibits the quickest recovery time from double-sweep IV curves. From pulsed IV and load-pull measurements, LPCVD SiNx is also confirmed to deliver superior large signal performance. The bilayer LPCVD SiNx passivated device shows negligible current slump (
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| 21. |
- Huang, Tongde, 1985, et al.
(författare)
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Small- and Large-Signal Analyses of Different Low-Pressure-Chemical-Vapor-Deposition SiNx Passivations for Microwave GaN HEMTs
- 2018
-
Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 65:3, s. 908-914
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Tidskriftsartikel (refereegranskat)abstract
- Three types of SiN x passivation for microwave AlGaN/GaN HEMTs were deposited with low-pressure chemical vapor deposition under different deposition conditions, resulting in different silicon contents. The performance of the HEMTs is comprehensively investigated and compared. Both small- and large-signal analyses, such as generation-recombination (G-R) trap analysis, low-frequency noise characterization, and load-pull measurement, are indispensable to evaluate the effectiveness of a surface passivation. A Si-rich SiN x passivation shows excess G-R centers, whereas a Si-poor SiN x passivation exhibits significant current slump (30%). A bilayer SiN x passivation successfully shows not only a small current slump (9.7%) but also a suppressed G-R trapping/detrapping process. Moreover, the bilayer passivation demonstrates almost 2 orders of magnitude lower gate current noise spectra compared with the single-layer Si-rich SiN x passivation. The capacitance-voltage measurements reveal that the Si-rich SiN x layer removes the deep-level traps at the AlGaN/SiN x interface. Considering both small- and large-signal operations, it is concluded that the bilayer SiN x passivation is a suitable and versatile candidate for microwave GaN devices.
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| 22. |
- Huang, Tongde, 1985, et al.
(författare)
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Suppression of Dispersive Effects in AlGaN/GaN High-Electron-Mobility Transistors Using Bilayer SiNx Grown by Low Pressure Chemical Vapor Deposition
- 2015
-
Ingår i: IEEE Electron Device Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 0741-3106 .- 1558-0563. ; 36:6, s. 537-539
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Tidskriftsartikel (refereegranskat)abstract
- A bilayer SiNx passivation scheme has been developed using low pressure chemical vapor deposition (LPCVD), which effectively suppresses the dispersive effects in AlGaN/GaN high-electron-mobility transistors (HEMTs) for microwave power operation. The bilayer LPCVD passivation is compared with in-situ SiNx passivations by metal-organic chemical vapor deposition (MOCVD) and ex-situ SiNx passivations by plasma-enhanced chemical vapor deposition (PECVD). The HEMTs were fabricated and characterized in terms of pulsed IV, transient drain current, and load pull. The devices passivated with in-situ MOCVD SiNx or PECVD SiNx exhibit significant current slump (similar to 40%) and knee-voltage walkout, while the bilayer LPCVD SiNx passivated device shows negligible current slump (similar to 6%) and knee-voltage walkout. These characteristics are directly reflected in the large signal operation, where HEMTs with bilayer LPCVD SiNx have the lowest dynamic ON-state resistance and highest output power (5.4 W/mm at 3 GHz).
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| 23. |
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| 24. |
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| 25. |
- Li, Junjie, 1995, et al.
(författare)
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Influence of Spacer Thickness on the Noise Performance in InP HEMTs for Cryogenic LNAs
- 2022
-
Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 43:7, s. 1029-1032
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Tidskriftsartikel (refereegranskat)abstract
- InP high electron mobility transistors (InP HEMTs) with different spacer thickness 1 to 7 nm in the InAlAs-InGaAs heterostructure have been fabricated and characterized at 5 K with respect to electrical dc and rf properties. The InP HEMT noise performance was extracted from gain and noise measurements of a hybrid low-noise amplifier (LNA) at 5 K equipped with discrete transistors. When biased for optimal noise operation, the LNA using 5 nm spacer thickness InP HEMTs achieved the lowest average noise temperature of 1.4 K at 4-8 GHz. The InP HEMT channel noise was estimated from the drain noise temperature which confirmed the minimum in noise temperature for the 5 nm spacer thickness InP HEMT. It is suggested that the spacer thickness acts to control the degree of real-space transfer of electrons from the channel to the barrier responsible for the observed noise variation in the cryogenic InP HEMTs.
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| 26. |
- Li, Junjie, 1995, et al.
(författare)
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On the relation between rf noise and subthreshold swing in InP HEMTs for cryogenic LNAs
- 2022
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Ingår i: Asia-Pacific Microwave Conference Proceedings, APMC. ; 2022-November, s. 10-12
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Konferensbidrag (refereegranskat)abstract
- 4 - 8 GHz low-noise amplifiers (LNAs) based on InP high electron mobility transistors (InP HEMTs) with different spacer thickness in the InAlAs-InGaAs heterostructure were fabricated and characterized at 5 K. A variation in the lowest average noise temperature of the LNA was observed with spacer thickness. We here report that the subthreshold swing (SS) at 5 K for the HEMT exhibited similar dependence with spacer thickness as the lowest average noise temperature of the LNA. This suggests that low-temperature characterization of SS for the HEMT can be used as a rapid assessment of anticipated noise performance in the cryogenic HEMT LNA.
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| 27. |
- Li, Junjie, 1995, et al.
(författare)
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Optimization of InGaAs Channel for Cryogenic InP HEMT Low-Noise Amplifiers
- 2023
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The noise temperature was measured for InP HEMTs in cryogenic low-noise amplifiers (LNAs) with channel indium content varied from 53% to 70%. The Hall measurements for all the epitaxial structures and dc characteristics of InP HEMTs were characterized. The 4–8 GHz LNA with lattice-matched channel InP HEMTs has the highest noise temperature at 5 K. The LNAs with 60% and 70% channel exhibited similar noise of 1.4 K at the optimum noise bias at 5 K. The 60% channel InP HEMT showed the lowest drain noise temperature with 10.5 mW power consumption and the lowest LNA noise at low power with 3.3 K for 100 μW. The results indicate that there is an optimum indium channel composition for InP HEMTs in cryogenic LNAs.
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| 28. |
- Li, Junjie, 1995, et al.
(författare)
-
Reduction of Noise Temperature in Cryogenic InP HEMT Low Noise Amplifiers with Increased Spacer Thickness in InAlAs-InGaAs-InP Heterostructures
- 2021
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The impact of InP HEMT spacer thickness on cryogenic performance in low noise amplifiers (LNAs) has been investigated. 100 nm gate-length InP HEMTs based on InAlAs-InGaAs-InP heterostructures with different spacer thickness (1 nm, 3 nm and 5 nm) were fabricated. The Hall measurements, simulated band structures and dc characteristics of InP HEMTs were compared for all the three different epitaxial structures at 5 K. The noise performance of the InP HEMT was studied using a three-stage 4–8 GHz hybrid LNA at 5 K. All LNAs yielded an average gain above 30 dB across the whole band. When biased for optimal low noise operation, the LNA with 5 nm spacer thickness InP HEMTs achieved an average noise temperature of 1.3 K. The LNAs with spacer thickness of 1 nm and 3 nm InP HEMTs exhibited a higher average noise temperature of 1.9 K and 1.7 K, respectively. The reduction in LNA noise temperature with increased spacer thickness was observed to correlate with a strongly enhanced electron mobility in the InP HEMT structure at 5 K.
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| 29. |
- Li, Xun, et al.
(författare)
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Carbon doped GaN buffer layer using propane for high electron mobility transistor applications: Growth and device results
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:26
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Tidskriftsartikel (refereegranskat)abstract
- The creation of a semi insulating (SI) buffer layer in AlGaN/GaN High Electron Mobility Transistor (HEMT) devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). In this investigation, we evaluate the use of a gaseous carbon gas precursor, propane, for creating a SI GaN buffer layer in a HEMT structure. The carbon doped profile, using propane gas, is a two stepped profile with a high carbon doping (1.5 x 10(18) cm(-3)) epitaxial layer closest to the substrate and a lower doped layer (3 x 10(16) cm(-3)) closest to the 2DEG channel. Secondary Ion Mass Spectrometry measurement shows a uniform incorporation versus depth, and no memory effect from carbon doping can be seen. The high carbon doping (1.5 x 10(18) cm(-3)) does not influence the surface morphology, and a roughness root-mean-square value of 0.43 nm is obtained from Atomic Force Microscopy. High resolution X-ray diffraction measurements show very sharp peaks and no structural degradation can be seen related to the heavy carbon doped layer. HEMTs are fabricated and show an extremely low drain induced barrier lowering value of 0.1 mV/V, demonstrating an excellent buffer isolation. The carbon doped GaN buffer layer using propane gas is compared to samples using carbon from the trimethylgallium molecule, showing equally low leakage currents, demonstrating the capability of growing highly resistive buffer layers using a gaseous carbon source.
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| 30. |
- Li, Xun, et al.
(författare)
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Intentionally carbon doped GaN buffer layer for HEMT application: growth and device results
- 2015
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The creation of a semi-insulating (SI) buffer layer in AlGaN/GaN HEMT devices is crucial for preventing a current path beneath the two-dimensional electron gas (2DEG). Here we evaluate the use of a carbon precursor, propane, for creating a SI GaN buffer layer. The carbon doping profile obtained from SIMS measurement shows a very uniform incorporation versus depth and no significant memory effect from carbon doping is seen, allowing for the creation of a very abrupt profile. The high carbon doping (1.5×1018 cm-3) does not influence the surface morphology. HRXRD ω rocking curve showed a FWHM of 200 arcsec of the (0002) and 261 arcsec for (10-12) reflection of the GaN, respectively. HEMT devices were processed on the epitaxial layers. An extremely low drain induced barrier lowering value of 0.1 mV/V was measured for a HEMT with a gate length of 0.2 ?m. This demonstrates the capability of growing a highly resistive buffer layer using intentional carbon doping.
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| 31. |
- Lin, Yen-Ku, et al.
(författare)
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A versatile low-resistance ohmic contact process with ohmic recess and low-temperature annealing for GaN HEMTs
- 2018
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Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 33:9
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Tidskriftsartikel (refereegranskat)abstract
- Deeply recessed ohmic contacts for GaN-based high electron mobility transistors (HEMTs) are demonstrated. It is shown that low-resistance ohmic contacts can be achieved with recessing beyond the AlGaN Schottky barrier where the ohmic contacts are formed on the sidewall of the recess. This makes the process versatile and relatively insensitive to the exact recess depth. The ohmic contact is based on a gold-free metallization scheme consisting of a Ta/Al/Ta metal stack requiring a low-temperature annealing. Important parameters for this type of ohmic contact process include the metal coverage, slope of the etched sidewall, bottom Ta-layer thickness, as well as annealing temperature and duration. The optimized contact resistance is as low as 0.24 Omega mm after annealing at 575 degrees C. Moreover, this sidewall contact approach was successfully implemented on different epitaxial heterostructures with different AlGaN barrier thickness as well as with and without AlN exclusion layer. All the samples exhibited excellent contact resistances in a wide range of recess depths. The Ta-based, sidewall ohmic contact process is a promising method for forming an ohmic contact on a wide range of GaN HEMT epitaxial designs.
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| 32. |
- Pooth, A., et al.
(författare)
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Morphological and electrical comparison of Ti and Ta based ohmic contacts for AlGaN/GaN-on-SiC HFETs
- 2017
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Ingår i: Microelectronics and Reliability. - : Elsevier BV. - 0026-2714. ; 68, s. 2-4
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Tidskriftsartikel (refereegranskat)abstract
- The morphology and impact on leakage currents of two different ohmic metal stacks for GaN based transistor devices is investigated in this work. The results have implications for the performance and reliability of a GaN transistor device. A low temperature Ta based and a higher temperature anneal Ti based metallization are compared. The low temperature process shows a smoother metal semiconductor interface together with several orders of magnitude lower vertical and lateral leakage compared to the conventional higher temperature process. In addition to the leakage tests, back bias ramping experiments are performed unveiling potential advantages of the conventional approach in mitigating current collapse. However the low leakage will enable higher voltage operation making the low temperature process the preferable choice for high power RF applications, if simultaneously current collapse can be controlled.
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