| 1. |
- Axelsson, Olle, 1986, et al.
(författare)
-
Impact of Trapping Effects on the Recovery Time of GaN Based Low Noise
- 2016
-
Ingår i: IEEE Microwave and Wireless Components Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-1764 .- 1531-1309. ; 26:1, s. 31-33
-
Tidskriftsartikel (refereegranskat)abstract
- This study investigates recovery time of the gain of AlGaN/GaN HEMT based low noise amplifiers (LNA) after an input overdrive pulse. Three LNAs, fabricated in two commercial MMIC processes and a Chalmers in-house process, are evaluated. The Chalmers process has an unintentionally doped buffer instead of the intentional Fe doping of the buffer which is standard in commercial GaN HEMT technologies. It is shown that the LNAs from the two commercial processes experience a severe drop in gain after input overdrive pulses higher than 28 dBm, recovering over a duration of around 20 ms. In contrast the LNA fabricated in-house at Chalmers experienced no visible effects up to an input power of 33 dBm. These results have impact for radar and electronic warfare receivers, which need to be operational immediately after an overdrive pulse. The long time constants suggest that these effects are due to trapping in the transistors with the Fe doped buffer playing an important role.
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| 2. |
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| 3. |
- Westlund, Andreas, 1985, et al.
(författare)
-
Graphene self-switching diodes as zero-bias microwave detectors
- 2015
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 106:9, s. 093116-
-
Tidskriftsartikel (refereegranskat)abstract
- Self-switching diodes (SSDs) were fabricated on as-grown and hydrogen-intercalated epitaxial graphene on SiC. The SSDs were characterized as zero-bias detectors with on-wafer measurements from 1 to 67 GHz. The lowest noise-equivalent power (NEP) was observed in SSDs on the hydrogen-intercalated sample, where a flat NEP of 2.2 nW/Hz½ and responsivity of 3.9 V/W were measured across the band. The measured NEP demonstrates the potential of graphene SSDs as zero-bias microwave detectors.
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| 4. |
- Winters, Michael, 1986, et al.
(författare)
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Hysteresis modeling in graphene field effect transistors
- 2015
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 117:7, s. Art. no, 074501-
-
Tidskriftsartikel (refereegranskat)abstract
- Graphene field effect transistors with an Al2O3 gate dielectric are fabricated on H-intercalated bilayer graphene grown on semi-insulating 4H-SiC by chemical vapour deposition. DC measurements of the gate voltage nu(g) versus the drain current i(d) reveal a severe hysteresis of clockwise orientation. A capacitive model is used to derive the relationship between the applied gate voltage and the Fermi energy. The electron transport equations are then used to calculate the drain current for a given applied gate voltage. The hysteresis in measured data is then modeled via a modified Preisach kernel.
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| 5. |
- Axelsson, Olle, 1986, et al.
(författare)
-
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
-
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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| 6. |
- Axelsson, Olle, 1986, et al.
(författare)
-
The Effect of Forward Gate Bias Stress on the Noise Performance of Mesa Isolated GaN HEMTs
- 2015
-
Ingår i: IEEE Transactions on Device and Materials Reliability. - : Institute of Electrical and Electronics Engineers (IEEE). - 1530-4388 .- 1558-2574. ; 15:1, s. 40-46
-
Tidskriftsartikel (refereegranskat)abstract
- This study investigates degradation of gallium nitride (GaN) high-electron mobility transistor (HEMT) noise performance after both dc and RF stress with forward gate current. The results are used to facilitate optimization of the robustness of GaN low-noise amplifiers (LNAs). It is shown that forward biasing the gate of a GaN HEMT results in permanent degradation of noise performance and gate current leakage, without affecting S-parameters and drain current characteristics. The limit of safe operation of the 2 x 50 mu m devices in this study is found to be between 10 and 20 mW dissipated in the gate diode for both dc and RF stress. We propose that degradation could be caused by excessive leakage through the mesa sidewalls at the edges of each gate finger. Circuit simulations may be used together with device robustness rating to optimize LNAs for maximum input power tolerance. Using a resistance in the gate biasing network of 10 k Omega, it is estimated that an LNA utilizing a 2 x 50 mu m device could withstand input power levels up to 33 dBm without degradation in noise performance.
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| 7. |
- Bergsten, Johan, 1988, 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. - : 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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| 8. |
- Bergsten, Johan, 1988, et al.
(författare)
-
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
-
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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| 9. |
- Bergsten, Johan, 1988, et al.
(författare)
-
Low resistive Au-free, Ta-based, recessed ohmic contacts to InAlN/AlN/GaN heterostructures
- 2015
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 30:10, s. 105034-
-
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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| 10. |
- 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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| 11. |
- Bremer, Johan, 1991, et al.
(författare)
-
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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| 12. |
- Bremer, Johan, 1991, et al.
(författare)
-
Compensation of Performance Degradation Due to Thermal Effects in GaN LNA Using Dynamic Bias
- 2018
-
Ingår i: 2018 48th European Microwave Conference, EuMC 2018. - 9781538652855 ; , s. 1213-1216
-
Konferensbidrag (refereegranskat)abstract
- This paper investigates the possibilities of using a dynamic bias control scheme for a low noise amplifier to compensate for performance degradation due to thermal effects. The study was performed by characterization of bias voltage and temperature dependence between -25°C to 75°C of a GaN MMIC LNA. The performance, in terms of gain, linearity and noise, degraded, at elevated chip temperatures. Nonlinear behavioral models were developed and used to predict performance for different bias and temperature conditions. Bias conditions to achieve constant gain and noise figure versus temperature are determined. Enhanced RF performance, with improved gain and linearity is demonstrated and is shown to require increased power and involves a trade-off between improving noise figure and gain.
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| 13. |
- Chen, J. T., et al.
(författare)
-
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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| 14. |
- Chen, J. T., et al.
(författare)
-
Carbon-doped GaN on SiC materials for low-memory-effect devices
- 2016
-
Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-5862 .- 1938-6737. - 9781607685395 ; 75:12, s. 61-65, s. 61-65
-
Konferensbidrag (refereegranskat)abstract
- AlGaN/GaN on SiC HEMT structures suitable for high power, high frequency applications are demonstrated. The material manifests record breaking thermal management and electron mobility. Moreover, thanks to the fact that the buffer layer is doped with carbon, the memory effect of processed devices is very low making system design and manufacturing significantly easier and less expensive.
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| 15. |
- Chen, Jr-Tai, et al.
(författare)
-
Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface
- 2015
-
Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 428, s. 54-58
-
Tidskriftsartikel (refereegranskat)abstract
- The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200 °C in H2 ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240 °C, the crystalline quality of the 105 nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447 arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276 arcsec to 149 and 194 arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35 nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35 nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3×10-8 m2 K/W, measured at an elevated temperature of 160 °C, in a GaN-on-SiC transistor structure was achieved.
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| 16. |
- Desmaris, Vincent, 1977, et al.
(författare)
-
Characterization of GaN-based Low Noise Amplifiers at Cryogenic Temperatures
- 2019
-
Ingår i: ISSTT 2019 - 30th International Symposium on Space Terahertz Technology, Proceedings Book. ; , s. 67-68
-
Konferensbidrag (refereegranskat)abstract
- In this paper, we present the first characterization of GaN-based Low-Noise Amplifiers (LNAs) at cryogenic temperatures for prospective use in radio-astronomy receivers. Both commercial and prototype LNAs fabricated in-house demonstrate a nine-fold improvement of their room-temperature noise performance when cooled to about 10 K. Very promising noise temperatures of about 8 K were measured without any specific optimization of the LNA design for cryogenic operation.
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| 17. |
- Fager, Christian, 1974, et al.
(författare)
-
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
-
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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| 18. |
- Guerriero, E., et al.
(författare)
-
High-Gain Graphene Transistors with a Thin AlOx Top-Gate Oxide
- 2017
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 7:1
-
Tidskriftsartikel (refereegranskat)abstract
- The high-frequency performance of transistors is usually assessed by speed and gain figures of merit, such as the maximum oscillation frequency f(max), cutoff frequency f(T), ratio f(max)/f(T), forward transmission coefficient S-21, and open-circuit voltage gain A(v). All these figures of merit must be as large as possible for transistors to be useful in practical electronics applications. Here we demonstrate high-performance graphene field-effect transistors (GFETs) with a thin AlOx gate dielectric which outperform previous state-of-the-art GFETs: we obtained f(max)/f(T) > 3, A(v) > 30 dB, and S-21 = 12.5 dB (at 10 MHz and depending on the transistor geometry) from S-parameter measurements. A dc characterization of GFETs in ambient conditions reveals good current saturation and relatively large transconductance similar to 600 S/m. The realized GFETs offer the prospect of using graphene in a much wider range of electronic applications which require substantial gain.
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| 19. |
- Gustafsson, Sebastian, 1990, et al.
(författare)
-
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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| 20. |
- Habibpour, Omid, 1979, et al.
(författare)
-
A W-band MMIC Resistive Mixer Based on Epitaxial Graphene FET
- 2017
-
Ingår i: IEEE Microwave and Wireless Components Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-1764 .- 1531-1309. ; 27:2, s. 168-170
-
Tidskriftsartikel (refereegranskat)abstract
- This letter presents the design, fabrication and characterization of the first graphene based monolithic microwave integrated circuit (MMIC) in microstrip technology operating in W-band. The circuit is a resistive mixer in a 250 nm graphene field effect transistor (G-FET) technology on a SiC substrate. A conversion loss of 18 dB is achieved which is limited by the available local oscillator (LO) power. The mixer exhibits a flat response over radio frequency (RF) range of 90-100 GHz. The RF bandwidth is also limited by the measurement setup.
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| 21. |
- Habibpour, Omid, 1979, et al.
(författare)
-
Developing Graphene based MMICs on SiC substrate
- 2016
-
Ingår i: 2015 Asia-Pacific Microwave Conference (Apmc), Vols 1-3. ; 1
-
Konferensbidrag (refereegranskat)abstract
- This paper presents the fabrication process development of monolithic microwave integrated circuits (MMICs) based on graphene field effect transistors (G-FETs). In this process, the G-FETs are based on epitaxial graphene on SiC substrate. Base on this process, a wide-band amplifier (0-3GHz) with 6-7 dB gain is realized.
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| 22. |
- Habibpour, Omid, 1979, et al.
(författare)
-
Generation of multi-Gigabit/s OFDM signals at W-band with a graphene FET MMIC mixer
- 2017
-
Ingår i: IEEE MTT-S International Microwave Symposium Digest. - 0149-645X. - 9781509063604 ; , s. 1185-1187
-
Konferensbidrag (refereegranskat)abstract
- This paper presents multi-Gigabit/s Orthogonal Frequency Division Multiplexing (OFDM) signal generation by using a graphene field effect transistor based resistive mixer at w-band. The OFDM signals consist of 64 subcarriers each carrying a quadrature-phase-shift-keying (QPSK) symbols. The results show that a bit error rate of 10 -4 is achievable for 8 Gbps data rate.
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| 23. |
- Habibpour, Omid, 1979, et al.
(författare)
-
Generic Graphene Based Components and Circuits for Millimeter Wave High Data-rate Communication Systems
- 2017
-
Ingår i: MRS Advances. - : Springer Science and Business Media LLC. - 2059-8521. ; 2:58-59, s. 3559-3564
-
Tidskriftsartikel (refereegranskat)abstract
- We are developing millimeter wave (mm-wave) components and circuits based on hydrogen-intercalated graphene. The development covers epitaxial graphene growth, device fabrication, modelling, integrated circuit design and fabrication, and circuit characterizations. The focus of our work is to utilize the distinctive graphene properties and realize new components that can overcome some of the main challenges of existing mm-wave technologies in term of linearity.
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| 24. |
- Habibpour, Omid, 1979, et al.
(författare)
-
Graphene FET Gigabit On-Off Keying Demodulator at 96 GHz
- 2016
-
Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 37:3, s. 333-336
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the demodulation of a multi-Gb/s ON-OFF keying (OOK) signal on a 96 GHz carrier by utilizing a 250-nm graphene field-effect transistor as a zero bias power detector. From the eye diagram, we can conclude that the devices can demodulate the OOK signals up to 4 Gb/s.
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| 25. |
- Habibpour, Omid, 1979, et al.
(författare)
-
Wafer scale millimeter-wave integrated circuits based on epitaxial graphene in high data rate communication
- 2017
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 7, s. 41828-
-
Tidskriftsartikel (refereegranskat)abstract
- In recent years, the demand for high data rate wireless communications has increased dramatically, which requires larger bandwidth to sustain multi-user accessibility and quality of services. This can be achieved at millimeter wave frequencies. Graphene is a promising material for the development of millimeter-wave electronics because of its outstanding electron transport properties. Up to now, due to the lack of high quality material and process technology, the operating frequency of demonstrated circuits has been far below the potential of graphene. Here, we present monolithic integrated circuits based on epitaxial graphene operating at unprecedented high frequencies (80-100 GHz). The demonstrated circuits are capable of encoding/decoding of multi-gigabit-per-second information into/from the amplitude or phase of the carrier signal. The developed fabrication process is scalable to large wafer sizes.
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| 26. |
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| 27. |
- Huang, Tongde, 1985, et al.
(författare)
-
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
-
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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| 28. |
- Huang, Tongde, 1985, et al.
(författare)
-
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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| 29. |
- Huang, Tongde, 1985, et al.
(författare)
-
Fabrication and improved performance of AlGaN/GaN HEMTs with regrown ohmic contacts and passivation-first process
- 2016
-
Ingår i: 2016 Compound Semiconductor Week, CSW 2016; Toyama, Japan. - 9781509019649 ; , s. Article no. 7528722-
-
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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| 30. |
- Huang, Tongde, 1985, et al.
(författare)
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Influence on Noise Performance of GaN HEMTs With In Situ and Low-Pressure-Chemical-Vapor-Deposition SiNx Passivation
- 2016
-
Ingår i: IEEE Transactions on Electron Devices. - : Institute of Electrical and Electronics Engineers (IEEE). - 1557-9646 .- 0018-9383. ; 63:10, s. 3887-3892
-
Forskningsöversikt (refereegranskat)abstract
- High-frequency and low-frequency noise (LFN) performance of GaN high electron-mobility transistors (HEMTs), passivated with SiNx deposited by either in situ or low-pressure-chemical-vapor-deposition (LPCVD), are compared. From 8-26 GHz, the LPCVD sample has a lower minimum noise figure (1 dB at 8 GHz) because of lower power spectral density of noise sources and less transconductance (g(m)) dispersion. The LPCVD and the in situ SiNx passivated HEMTs exhibit similar LFN in the 1 Hz-100 kHz range (drain current noise spectra similar to 10(-17) A(2)/Hz at 100 kHz). Nevertheless, LPCVD should be a preferred choice for voltage-controlled oscillator (VCO) applications, since it is capable of suppressing current collapse more effectively, which results in a higher output power and, therefore, a lower phase noise. Furthermore, the low current collapse, low LFN, and minimum noise figure makes the LPCVD SiNx passivation a promising candidate for multifunctional monolithic microwave integrated circuits, including power amplifiers, low-noise amplifier, switches, mixers, and VCOs.
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| 31. |
- 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
-
Ingår i: 2015 Asia-Pacific Microwave Conference (Apmc), Vols 1-3. - 9781479987672 ; 3
-
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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| 32. |
- 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
-
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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| 33. |
- 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
-
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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| 34. |
- Khosa, Rabia Y., et al.
(författare)
-
Electrical characterization of amorphous Al2O3 dielectric films on n-type 4H-SiC
- 2018
-
Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226 .- 2158-3226. ; 8:2
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the electrical properties of Al 2 O 3 films grown on 4H-SiC by successive thermal oxidation of thin Al layers at low temperatures (200°C - 300°C). MOS capacitors made using these films contain lower density of interface traps, are more immune to electron injection and exhibit higher breakdown field (5MV/cm) than Al 2 O 3 films grown by atomic layer deposition (ALD) or rapid thermal processing (RTP). Furthermore, the interface state density is significantly lower than in MOS capacitors with nitrided thermal silicon dioxide, grown in N 2 O, serving as the gate dielectric. Deposition of an additional SiO 2 film on the top of the Al 2 O 3 layer increases the breakdown voltage of the MOS capacitors while maintaining low density of interface traps. We examine the origin of negative charges frequently encountered in Al 2 O 3 films grown on SiC and find that these charges consist of trapped electrons which can be released from the Al 2 O 3 layer by depletion bias stress and ultraviolet light exposure. This electron trapping needs to be reduced if Al 2 O 3 is to be used as a gate dielectric in SiC MOS technology.
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| 35. |
- Khosa, Rabia Y., et al.
(författare)
-
Electrical characterization of high k-dielectrics for 4H-SiC MIS devices
- 2019
-
Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 98, s. 55-58
-
Tidskriftsartikel (refereegranskat)abstract
- We report promising results regarding the possible use of AlN or Al 2 O 3 as a gate dielectric in 4H-SiC MISFETs. The crystalline AlN films are grown by hot wall metal organic chemical vapor deposition (MOCVD) at 1100 °C. The amorphous Al 2 O 3 films are grown by repeated deposition and subsequent low temperature (200 °C) oxidation of thin Al layers using a hot plate. Our investigation shows a very low density of interface traps at the AlN/4H-SiC and the Al 2 O 3 /4H-SiC interface estimated from capacitance-voltage (CV) analysis of MIS capacitors. Current-voltage (IV) analysis shows that the breakdown electric field across the AlN or Al 2 O 3 is ∼ 3 MV/cm or ∼ 5 MV/cm respectively. By depositing an additional SiO 2 layer by plasma enhanced chemical vapor deposition at 300 °C on top of the AlN or Al 2 O 3 layers, it is possible to increase the breakdown voltage of the MIS capacitors significantly without having pronounced impact on the quality of the AlN/SiC or Al 2 O 3 /SiC interfaces.
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| 36. |
- Khosa, Rabia Y., et al.
(författare)
-
Electrical characterization of MOCVD grown single crystalline ALN thin films on 4H-SiC
- 2019
-
Ingår i: Materials Science Forum. - : Trans Tech Publications Ltd. - 1662-9752 .- 0255-5476. ; 963 MSF, s. 460-464, s. 460-464
-
Konferensbidrag (refereegranskat)abstract
- We report on a very low density of interface traps at the AlN/4H-SiC interface estimated from capacitance-voltage (CV) analysis of metal-insulator-semiconductor (MIS) capacitors. Single crystalline aluminum nitride (AlN) films are grown by metal organic chemical vapor deposition (MOCVD). Current-voltage (IV) analysis shows that the breakdown electric field across the AlN dielectric is 3 MV/cm. By depositing an additional SiO2 layer on top of the AlN layer it is possible to increase the breakdown voltage of the MIS capacitors significantly without having pronounced impact on the quality of the AlN/SiC interface.
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| 37. |
- Khosa, Rabia Y., et al.
(författare)
-
Electrical properties of 4H-SiC MIS capacitors with AlN gate dielectric grown by MOCVD
- 2019
-
Ingår i: Solid-State Electronics. - : Elsevier BV. - 0038-1101 .- 1879-2405. ; 153, s. 52-58
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the electrical properties of the AlN/4H-SiC interface using capacitance- and conductance-voltage (CV and GV) analysis of AlN/SiC MIS capacitors. The crystalline AlN layers are made by hot wall MOCVD. CV analysis at room temperature reveals an order of magnitude lower density of interface traps at the AlN/SiC interface than at nitrided SiO2/SiC interfaces. Electron trapping in bulk traps within the AlN is significant when the MIS capacitors are biased into accumulation resulting in a large flatband voltage shift towards higher gate voltage. This process is reversible and the electrons are fully released from the AlN layer if depletion bias is applied at elevated temperatures. Current-voltage (IV) analysis reveals that the breakdown electric field intensity across the AlN dielectric is 3–4 MV/cm and is limited by trap assisted leakage. By depositing an additional SiO2 layer on top of the AlN layer, it is possible to increase the breakdown voltage of the MIS capacitors significantly without having much impact on the quality of the AlN/SiC interface.
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| 38. |
- Li, Xun, et al.
(författare)
-
Carbon doped GaN buffer layer using propane for high electron mobility transistor applications: Growth and device results
- 2015
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:26
-
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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| 39. |
- 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
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 33:9
-
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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| 40. |
- Malmros, Anna, 1977, et al.
(författare)
-
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
-
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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| 41. |
- Malmros, Anna, 1977, et al.
(författare)
-
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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| 42. |
- Malmros, Anna, 1977, et al.
(författare)
-
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
-
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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| 43. |
- Melios, C., et al.
(författare)
-
Tuning epitaxial graphene sensitivity to water by hydrogen intercalation
- 2017
-
Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 9:10, s. 3440-3448
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices.
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| 44. |
- Pooth, A., et al.
(författare)
-
Morphological and electrical comparison of Ti and Ta based ohmic contacts for AlGaN/GaN-on-SiC HFETs
- 2017
-
Ingår i: Microelectronics and Reliability. - : Elsevier BV. - 0026-2714. ; 68, s. 2-4
-
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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| 45. |
- Sanchez Perez, Cesar, 1981, et al.
(författare)
-
Design and Large-Signal Characterization of High-Power Varactor-Based Impedance Tuners
- 2018
-
Ingår i: IEEE Transactions on Microwave Theory and Techniques. - 0018-9480 .- 1557-9670. ; 66:4, s. 1744-1753
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a new systematic and efficient method for the design of high-power varactor-based impedance tuners. By incorporating the varactor losses and voltage handling capabilities, the method allows the impedance tuning range to be maximized and losses to be minimized under high-power operating conditions. A 2.2-GHz large-signal double-stub tuner has been designed using the proposed methodology and SiC varactor diodes. Using comprehensive large-signal measurements, a 25-W input power handling is demonstrated up to Γ max ∼ 0.8. Two-tone linearity measurements show an input third-order intercept point exceeding 50 dBm for most impedances. These results clearly demonstrate the feasibility of the proposed technique for the design of high-performance large-signal tuners.
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| 46. |
- Sanchez Perez, Cesar, 1981, et al.
(författare)
-
Optimized Design of a Dual-Band Power Amplifier With SiC Varactor-Based Dynamic Load Modulation
- 2015
-
Ingår i: IEEE Transactions on Microwave Theory and Techniques. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9480 .- 1557-9670. ; 63:8, s. 2579-2588
-
Tidskriftsartikel (refereegranskat)abstract
- A new methodology for the design of single/multi-band power amplifiers (PAs) with dynamic load modulation (DLM) is presented. First, the topology for the output matching network (OMN) including the control varactor is selected. A comprehensive optimization of the OMN parameters is then developed by which varactor and transistor losses are considered to ensure maximum efficiency enhancement at each frequency. To verify the method, a dual-band PA with DLM is realized. Drain efficiencies of 75% and 60% at 685 MHz and 1.84 GHz, respectively, are measured at peak output power. At 10-dB output power back-off efficiencies of 43.5% and 49.5%, respectively, are obtained. Linearized modulated measurements with a 6.5-dB peak-to-average power ratio WCDMA signal show average drain efficiencies of 56% and 54% at 685 MHz and 1.84 GHz, respectively, at an adjacent channel leakage ratio of -49 and -47.5 dBc, respectively. The proposed method shows the effectiveness of applying an optimization process for the design of single-or multi-band DLM PAs. The results demonstrate that near-optimum performance may be obtained in terms of efficiency enhancement for a given transistor and varactor-based OMN, thus making DLM competitive against other load modulation techniques.
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| 47. |
- Thanh, Thi Ngoc Do, 1984, et al.
(författare)
-
Effects of Surface Passivation and Deposition Methods on the 1/f Noise Performance of AlInN/AlN/GaN High Electron Mobility Transistors
- 2015
-
Ingår i: IEEE Electron Device Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 0741-3106 .- 1558-0563. ; 36:4, s. 315-317
-
Tidskriftsartikel (refereegranskat)abstract
- This letter reports on effects of Si3N4 and Al2O3 surface passivation as well as different deposition methods on the low-frequency noise (LFN) characteristics for AlInN/AlN/GaN high electron mobility transistors (HEMTs). Two samples are passivated with Al2O3, deposited by two different methods: 1) thermal atomic layer deposition (ALD) and 2) plasma-assisted ALD. The third sample is passivated with Si3N4 using plasma-enhanced chemical vapor deposition. The LFN of the three samples is measured under a bias condition relevant for amplifier and oscillator applications. It is found that the surface passivation has a major impact on the noise level. The best surface passivation, with respect to LFN, is the thermal ALD Al2O3 for which the noise current spectral density measured at 10 kHz is 1 x 10(-14) Hz(-1) for a bias of V-dd/I-dd = 10 V/80 mA. To the best of our knowledge, this result sets a standard as the best reported LFN of AlInN/GaN HEMTs. It is also in the same order as good commercial AlGaN/GaN HEMTs reported in literature and thus demonstrates that AlInN/GaN HEMTs, passivated with thermal ALD Al2O3, is a good candidate for millimeter-wave power generation.
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| 48. |
- ul-Hassan, Jawad, et al.
(författare)
-
Quasi-free-standing monolayer and bilayer graphene growth on homoepitaxial on-axis 4H-SiC(0001) layers
- 2015
-
Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 82:C, s. 12-23
-
Tidskriftsartikel (refereegranskat)abstract
- Quasi-free-standing monolayer and bilayer graphene is grown on homoepitaxial layers of 4H-SiC. The SiC epilayers themselves are grown on the Si-face of nominally on-axis semi-insulating substrates using a conventional SiC hot-wall chemical vapor deposition reactor. The epilayers were confirmed to consist entirely of the 4H polytype by low temperature photoluminescence. The doping of the SiC epilayers may be modified allowing for graphene to be grown on a conducing substrate. Graphene growth was performed via thermal decomposition of the surface of the SiC epilayers under Si background pressure in order to achieve control on thickness uniformity over large area. Monolayer and bilayer samples were prepared through the conversion of a carbon buffer layer and monolayer graphene respectively using hydrogen intercalation process. Micro-Raman and reflectance mappings confirmed predominantly quasi-free-standing monolayer and bilayer graphene on samples grown under optimized growth conditions. Measurements of the Hall properties of Van der Pauw structures fabricated on these layers show high charge carrier mobility (> 2000 cm(2)/Vs) and low carrier density (
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| 49. |
- Uz Zaman, Ashraf, 1975, et al.
(författare)
-
Novel Low-Loss Millimeter- Wave Transition From Waveguide-to-Microstrip Line Suitable for MMIC Integration and Packaging
- 2017
-
Ingår i: IEEE Microwave and Wireless Components Letters. - 1558-1764 .- 1531-1309. ; 27:12, s. 1098-1100
-
Tidskriftsartikel (refereegranskat)abstract
- This letter presents a unique low-loss transition from microstrip to full height rectangular waveguide at W-band. This microstrip transition can be made as a part of the mm-wave monolithic microwave integrated circuit (MMIC) of arbitrary size, and thus, avoid the use of bond wires at the high-frequency port of the MMIC circuit. As a result, the MMIC can be coupled directly to the waveguide. The working principle of the transition is based on electromagnetic coupling, where the coupling between the microstrip mode and the TE10 waveguide mode is achieved via a resonant cavity. A perfect magnetic conductor (PMC) surface is placed over the cavity to facilitate the smooth coupling of electromagnetic energy from the microstrip line to the cavity and then from the cavity to the waveguide. The PMC surface also suppresses the unwanted waveguide mode coupling to the oversized MMIC substrate. The measured back-to-back transition works over the frequency band of 80-114 GHz (relative BW of 35%) with minimum return loss of 13.5 dB. The total insertion loss of the manufactured prototype is found to be varying from 0.54 to 0.803 dB, which implies a single transition loss of less than 0.27-0.4015 dB in W-band.
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| 50. |
- Winters, Michael, 1986, et al.
(författare)
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Assessment of H-intercalated graphene for microwave FETs through material characterization and electron transport studies
- 2015
-
Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 81:1, s. 96-104
-
Tidskriftsartikel (refereegranskat)abstract
- Epitaxial graphene is grown on semi-insulating (SI) 4H-SiC in a hot wall CVD reactor by graphitization and in-situ intercalation with (H)ydrogen. A holistic material characterization is performed in order to ascertain the number of layers, layer uniformity, and electron transport properties of the epi-layers via electronic test structures and Raman spectroscopy. Bilayer graphene field effect transistors (GFETs) are fabricated using a full electron beam lithography (EBL) process which is optimized for low contact resistances of r(c)
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