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1.	Divinyi, Andreas, et al. (författare) Transition Time of GaN HEMT Switches and its Dependence on Device Geometry 2023 Ingår i: 2023 18th European Microwave Integrated Circuits Conference, EuMIC 2023. ; , s. 46-49 Konferensbidrag (refereegranskat)abstract This paper presents the impact on the slow transition time of GaN HEMT switch transistor due to size and geometry. Measurements in the time domain are used to characterize the transition time of the switch from off to on. This is done for several switch transistors with variations on total gate width and number of fingers. In order to enable the comparison of transition times a figure of merit is established. This is achieved by using a commonly used model for trapping effects to quantify the amplitude of the slow transient with respect to the gate voltage. The resulting analysis indicates that transition time is sensitive to transistor size and dependent on the geometry of the device as increasing the width of the gate fingers is more advantageous compared to increasing the number of fingers.
2.	Angelov, Iltcho, 1943, et al. (författare) On the delay implementation in FET Large Signal Models 2020 Ingår i: 2020 International Workshop on Integrated Nonlinear Microwave and Millimetre-Wave Circuits, INMMiC 2020 - Proceedings. - 9781728126456 Konferensbidrag (refereegranskat)abstract Several options for implementing FET channel delay in user defined Large Signal models (LS) are discussed. The high importance of precise definition of reference planes for MMIC operating in mm waves is discussed as well. The delay implementation and a way to improve the reference planes accuracy was tested in a special two stage W-band test amplifier circuit. The LS model was used later to design several practical MMIC circuits.
3.	Bremer, Johan, 1991, et al. (författare) Electric-Based Thermal Characterization of GaN Technologies Affected by Trapping Effects 2020 Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 67:5, s. 1952-1958 Tidskriftsartikel (refereegranskat)abstract This article presents an electric-based methodology for thermal characterization of semiconductor technologies. It is shown that for technologies such as gallium nitride (GaN) high electron mobility transistors, which exhibit several field induced electron trapping effects, the thermal characterization has to be performed under specific conditions. The electric field is limited to low levels to avoid activation of trap states. At the same time, the dissipated power needs to be high enough to change the operating temperature of the device. The method is demonstrated on a test structure implemented as a GaN resistor with large contact separation. It is used to evaluate the thermal properties of samples with different silicon carbide suppliers and buffer thickness.
4.	Bremer, Johan, 1991, et al. (författare) Method for Suppressing Trap-Related Memory Effects in IV Characterizations of GaN HEMTs 2024 Ingår i: IEEE International Conference on Microelectronic Test Structures. - 1071-9032 .- 2158-1029. Konferensbidrag (refereegranskat)abstract A method to suppress trapping-related effects when performing IV characterizations of field effect transistors is presented. Standard IV measurements usually utilize voltage sweeps with fixed start, stop, and step values. At high electric fields in these sweeps, the charging of electron traps with long time constants may occur. The trapped electrons cause different memory effects such as hysteresis and the kink effect. The proposed method suppresses these effects, by reordering the bias points, so to prevent charging due to high preceding electric fields. The method provides more rudimentary IV measurements, useful for e.g. technology evaluation and modeling purposes.
5.	Chen, Ding-Yuan, et al. (författare) Impact of the Channel Thickness on Electron Confinement in MOCVD-Grown High Breakdown Buffer-Free AlGaN/GaN Heterostructures 2023 Ingår i: Physica Status Solidi (A) Applications and Materials Science. - : Wiley. - 1862-6319 .- 1862-6300. ; 220:16 Tidskriftsartikel (refereegranskat)abstract The 2D electron gas (2DEG) confinement on high electron mobility transistor (HEMT) heterostructures with a thin undoped GaN channel layer on the top of a grain-boundary-free AlN nucleation layer is studied. This is the first time demonstration of a buffer-free epi-structure grown with metal-organic chemical vapor deposition with thin GaN channel thicknesses, ranging from 250 to 150 nm, without any degradation of the structural quality and 2DEG properties. The HEMTs with a gate length of 70 nm exhibit good DC characteristics with peak transconductances of 500 mS mm(-1) and maximum saturated drain currents above 1 A mm(-1). A thinner GaN channel layer improves 2DEG confinement because of the enhanced effectiveness of the AlN nucleation layer acting as a back-barrier. An excellent drain-induced barrier lowering of only 20 mV V-1 at a V-DS of 25 V and an outstanding critical electric field of 0.95 MV cm(-1) are demonstrated. Good large-signal performance at 28 GHz with output power levels of 2.0 and 3.2 W mm(-1) and associated power-added efficiencies of 56% and 40% are obtained at a V-DS of 15 and 25 V, respectively. These results demonstrate the potential of sub-100 nm gate length HEMTs on a buffer-free GaN-on-SiC heterostructure.
6.	Chen, Ding Yuan, et al. (författare) Microwave Performance of 'Buffer-Free' GaN-on-SiC High Electron Mobility Transistors 2020 Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 41:6, s. 828-831 Tidskriftsartikel (refereegranskat)abstract High performance microwave GaN-on-SiC HEMTs are demonstrated on a heterostructure without a conventional thick doped buffer. The HEMT is fabricated on a high-quality 0.25~\boldsymbol {\mu }\text{m} unintentional doped GaN layer grown directly on a transmorphic epitaxially grown AlN nucleation layer. This approach allows the AlN-nucleation layer to act as a back-barrier, limiting short channel effects and removing buffer leakage. The devices with the 'buffer-free' heterostructure show competitive DC and RF characteristics, as benchmarked against the devices made on a commercial Fe-doped epi-wafer. Peak transconductances of 500 mS/mm and a maximum saturated drain current of 1 A/mm are obtained. An extrinsic \text{f}-{\sf T} of 70 GHz and \text{f}-{\sf max} of 130 GHz are achieved for transistors with a gate length of 100 nm. Pulsed-IV measurements reveal a lower current slump and a smaller knee walkout. The dynamic IV performance translates to an output power of 4.1 W/mm, as measured with active load-pull at 3 GHz. These devices suggest that the 'buffer-free' concept may offer an alternative route for high frequency GaN HEMTs with less electron trapping effects.
7.	Chen, Ding-Yuan, 1991, et al. (författare) Structural investigation of ultra-low resistance deeply recessed sidewall ohmic contacts for AlGaN/GaN HEMTs based on Ti/Al/Ti-metallization 2023 Ingår i: Semiconductor Science and Technology. - : IOP Publishing Ltd. - 1361-6641 .- 0268-1242. ; 38:10 Tidskriftsartikel (refereegranskat)abstract This study presents a novel approach to forming low-resistance ohmic contacts for AlGaN/GaN HEMTs. The optimized contacts exhibit an outstanding contact resistance of approximately 0.15 & omega;& BULL;mm. This is achieved by firstly recessing the barrier of the heterostructure to a depth beyond the channel. In this way, the channel region is exposed on the sidewall of the recess. The coverage of the Ti/Al/Ti ohmic metalization on the sidewall is ensured through tilting of the sample during evaporation. The annealing process is performed at a low temperature of 550 & DEG;C. The approach does not require precise control of the recess etching. Furthermore, the method is directly applicable to most barrier designs in terms of thickness and Al-concentration. The impact of recessed sidewall angle, thickness and ratio of Ti and Al layers, and the annealing procedure are investigated. Structural and chemical analyses of the interface between the ohmic contacts and epi-structure indicate the formation of ohmic contacts by the extraction of nitrogen from the epi-structure. The approach is demonstrated on HEMT-structures with two different barrier designs in terms of Al-concentration and barrier thickness. The study demonstrate large process window in regard to recess depth and duration of the annealing as well as high uniformity of the contact resistance across the samples, rendering the approach highly suitable for industrial production processes.
8.	Ding Yuan, Chen, 1991, et al. (författare) Impact of in situ NH3 pre-treatment of LPCVD SiN passivation on GaN HEMT performance 2022 Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 37:3 Tidskriftsartikel (refereegranskat)abstract The impact on the performance of GaN high electron mobility transistors (HEMTs) of in situ ammonia (NH3) pre-treatment prior to the deposition of silicon nitride (SiN) passivation with low-pressure chemical vapor deposition (LPCVD ) is investigated. Three different NH3 pre-treatment durations (0, 3, and 10 min) were compared in terms of interface properties and device performance. A reduction of oxygen (O) at the interface between SiN and epi-structure is detected by scanning transmission electron microscopy (STEM )-electron energy loss spectroscopy (EELS) measurements in the sample subjected to 10 min of pre-treatment. The samples subjected to NH3 pre-treatment show a reduced surface-related current dispersion of 9% (compared to 16% for the untreated sample), which is attributed to the reduction of O at the SiN/epi interface. Furthermore, NH3 pre-treatment for 10 min significantly improves the current dispersion uniformity from 14.5% to 1.9%. The reduced trapping effects result in a high output power of 3.4 W mm(-1) at 3 GHz (compared to 2.6 W mm(-1) for the untreated sample). These results demonstrate that the in situ NH3 pre-treatment before LPCVD of SiN passivation is critical and can effectively improves the large-signal microwave performance of GaN HEMTs.
9.	Ding Yuan, Chen, 1991, et al. (författare) Thin Al 0.5 Ga 0.5 N/GaN HEMTs on QuanFINE ® Structure 2021 Ingår i: CS MANTECH 2021 - 2021 International Conference on Compound Semiconductor Manufacturing Technology, Digest of Papers. ; , s. 153-155 Konferensbidrag (refereegranskat)abstract The performance of HEMTs fabricated on a thin Al0.5Ga0.5N/GaN heterostructure with a total barrier thickness of 6.5 nm is presented and benchmarked to the epi-structure with a 13 nm thick Al0.3Ga0.7N barrier on an identical QuanFINE® structure. DC transfer characteristics on both samples with a gate length of 100 nm demonstrate a high current above 1 A/mm. A higher extrinsic gm of 550 mS/mm is measured on the sample with a thinner high Al content barrier. Moreover, low trapping effects with a 12-14 % buffer-related dispersion at a VDSQ of 25 V are characterized for both samples, which indicate the advantage of the iron-free QuanFINE® heterostructure.
10.	Divinyi, Andreas, et al. (författare) On-Chip Sensors for Temperature Monitoring of Packaged GaN MMICs 2024 Ingår i: IEEE Transactions on Components, Packaging and Manufacturing Technology. - 2156-3985 .- 2156-3950. ; 14:5, s. 891-896 Tidskriftsartikel (refereegranskat)abstract A novel approach to on-chip temperature sensors for non-invasive thermal characterization and monitoring of packaged GaN MMICs is presented. The proposed sensor is fully compatible with commercial GaN foundry processes and enables improved reliability estimation of highly integrated systems. A dedicated test structure is developed to demonstrate the capabilities of the sensor, and an accurate calibration method of its temperature response is proposed. This combination allows for continuous temperature monitoring during operation with electrical acquisition of temperature transients. The method also enables the thermal characterization of the device and package.
11.	Ferrand-Drake Del Castillo, Ragnar, 1993, et al. (författare) Characterization of Trapping Effects Related to Carbon Doping Level in AlGaN Back-Barriers for AlGaN/GaN HEMTs 2024 Ingår i: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1557-9646 .- 0018-9383. ; 71:6, s. 3596-3602 Tidskriftsartikel (refereegranskat)abstract The impact of different carbon concentrations in the Al Ga N graded back-barrier and GaN buffer of high electron mobility transistors (HEMTs) is investigated. Four epi-wafers with different carbon concentrations, ranging from 1 10 to 5 10 cm , were grown by metal organic chemical vapor deposition (MOCVD). HEMTs with 100 and 200 nm gate lengths were fabricated and characterized with dc, Pulsed-IV, drain current transient spectroscopy (DCTS), and large-signal measurements at 30 GHz. It is shown that the back-barrier effectively prevents buffer-related electron trapping. The highest C-doping provides the best 2DEG confinement, while lower carbon doping levels are beneficial for a high output power and efficiency. A C-doping of 1 10 cm offers the highest output power at maximum power added efficiency (PAE) (1.8 W/mm), whereas 3 10 cm doping provides the highest PAE ( 40%). The C-profiles acquired by using secondary ion mass spectroscopy (SIMS), in combination with DCTS, is used to explain the electron trapping effects. Traps associated with the C-doping in the back-barrier are identified and the bias ranges for the trap activation are discussed. The study shows the importance of considering the C-doping level in the back-barrier of microwave GaN HEMTs for power amplification and generation.
12.	Ferrand-Drake Del Castillo, Ragnar, 1993, et al. (författare) Considerations in the development of a gate process module for ultra-scaled GaN HEMTs 2022 Ingår i: 2022 Compound Semiconductor Week, CSW 2022. Konferensbidrag (refereegranskat)abstract With the overarching goal of attaining mm-wave GaN High Electron Mobility Transistors (HEMTs), vertical and lateral downscaling is of essence. Utilizing Passivation first technology (coupled with mini-FP T-gates), Schottky Gate (SG) is formed by Fluorine plasma etching, where the plasma etching allows highly defined nanoscale gate-length (Lg) features. However, it damages the crystalline structure of the top barrier layer and leads to Fluorine implantation with ramifications on the sheet carrier density(ns), mobility (μ) and threshold-voltage (VTH) shift towards enhancement mode. In this study, CF4 or NF3 etching with varying over etch times are implemented, with high temperature annealing (600-800°C) post gate recess etching to repair crystal structure damages caused by the etch process.
13.	Harrysson Rodrigues, Isabel, 1993, et al. (författare) Mobility and quasi-ballistic charge carrier transport in graphene field-effect transistors 2022 Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 132, s. 244303-1-244303-9 Tidskriftsartikel (refereegranskat)abstract The optimization of graphene field-effect transistors (GFETs) for high-frequency applications requires further understanding of the physical mechanisms concerning charge carrier transport at short channel lengths. Here, we study the charge carrier transport in GFETs with gate lengths ranging from 2 μm down to 0.2 μm by applying a quasi-ballistic transport model. It is found that the carrier mobility, evaluated via the drain–source resistance model, including the geometrical magnetoresistance effect, is more than halved with decreasing the gate length in the studied range. This decrease in mobility is explained by the impact of ballistic charge carrier transport. The analysis allows for evaluation of the characteristic length, a parameter of the order of the mean-free path, which is found to be in the range of 359–374 nm. The mobility term associated with scattering mechanisms is found to be up to 4456 cm2/Vs. Transmission formalism treating the electrons as purely classical particles allows for the estimation of the probability of charge carrier transport without scattering events. It is shown that at the gate length of 2 μm, approximately 20% of the charge carriers are moving without scattering, while at the gate length of 0.2 μm, this number increases to above 60%.
14.	Huang, Tongde, 1985, et al. (författare) Impact of AlGaN/GaN Interface and Passivation on the Robustness of Low-Noise Amplifiers 2020 Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 67:6, s. 2297-2303 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.
15.	Hult, Björn, 1993, et al. (författare) AlGaN/GaN/AlN 'Buffer-Free' High Voltage MISHEMTs with Si-rich and Stoichiometric SiN x First Passivation 2022 Ingår i: 2022 Compound Semiconductor Week, CSW 2022. Konferensbidrag (refereegranskat)abstract Buffer-free' AlGaN/GaN/AlN high electron mobility transistors (HEMTs) with a thin GaN channel layer and a thin AlN nucleation layer grown on a semi-insulating SiC substrate are presented. Si-rich and a stoichiometric low-pressure chemical vapor deposition (LPCVD) SiNx first passivation were employed to study the impact of stoichiometry on off-state leakage currents in GaN-based metal-insulator-semiconductor (MIS)HEMTs. Nitrogen implantation isolation, SiOx second passivation, gate and source field plates were utilized. Off-state drain leakage current was reduced 2-3 orders of magnitude by depositing a stoichiometric instead of a Si-rich SiNx passivation. The gate leakage current was suppressed below 10nA/mm until breakdown. A destructive breakdown voltage of 1742V and 1532V was measured for the MISHEMTs with Si-rich and stoichiometric SiNx passivation, respectively. This demonstrates how high voltage, low leakage MISHEMTs can be achieved using a 'buffer-free' heterostructure by optimizing the first passivation stoichiometry.
16.	Hult, Björn, 1993, et al. (författare) High Voltage and Low Leakage GaN-on-SiC MISHEMTs on a ‘Buffer-Free’ Heterostructure 2022 Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 43:5, s. 781-784 Tidskriftsartikel (refereegranskat)abstract The performance of a novel ‘buffer-free’ Al-GaN/GaN-on-SiC MISHEMTs for power switching applications is demonstrated in this letter. High voltage operation with exceptionally low gate and drain leakage currents is shown. A specific on-resistance of 3.61 mΩ∙cm2 and an abrupt breakdown voltage of 1622 V at a drain current of 22 nA/mm is achieved. Using two-terminal breakdown measurements, nitrogen-implanted GaN display breakdown fields of 0.96 MV/cm. The semi-insulating SiC substrate is capable of suppressing vertical leakage currents, ensuring that off-state operation is limited by lateral breakdown. The impact of electron trapping effects on dynamic on-resistance is small up to a drain quiescent voltage of at least 240 V. Drain current transient characteristics display a 14% increase in dynamic on-resistance with respect to quiescent drain bias, and a negligible change in resistance up to 100 ms. These types of ‘buffer-free’ heterostructures are of interest for power electronic applications above 1000 V and with potential for co-integration of power and RF-electronics.
17.	Hult, Björn, 1993, et al. (författare) Investigation of Isolation Approaches and the Stoichiometry of SiN x Passivation Layers in “Buffer-Free” AlGaN/GaN Metal–Insulator–Semiconductor High-Electron-Mobility Transistors 2023 Ingår i: Physica Status Solidi (A) Applications and Materials Science. - : Wiley. - 1862-6319 .- 1862-6300. ; 220:8 Tidskriftsartikel (refereegranskat)abstract Critical process modules for the fabrication of metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) based on a novel ‘buffer-free’ AlGaN/GaN heterostructure grown with metal–organic chemical vapor deposition (MOCVD) are presented. The methods of isolation and passivation for this type of heterostructure are investigated. Utilizing nitrogen implantation, it is possible to achieve off-state destructive breakdown voltages (BVs) of 2496 V for gate–drain distances up to 25 μm, whereas mesa isolation techniques limit the BV below 1284 V. The stoichiometry of the SiNx passivation layer displays a small impact on the static and dynamic on-resistance. However, MISHEMTs with Si-rich passivation show off-state gate currents in the range of 1–100 μA mm−1 at voltages above 1000 V, which is reduced below 10 nA mm−1 using a stoichiometric SiNx passivation layer. Destructive BVs of 1532 and 1742 V can be achieved using gate-integrated and source-connected field plates for MIHEMTs with stoichiometric and Si–rich passivation layers, respectively. By decreasing the field plate lengths, it is possible to achieve BVs of 2200 V. This demonstrates the implementation of MISHEMTs with high-voltage operation and low leakage currents on a novel “buffer-free” heterostructure by optimizing the SiNx stoichiometry.
18.	Jian, Jhang-Jie, et al. (författare) Investigation of Multiple-Mesa-Nanochannel Array GaN-Based MOSHEMTs with Al2O3 Gate Dielectric Layer 2021 Ingår i: ECS Journal of Solid State Science and Technology. - : The Electrochemical Society. - 2162-8777 .- 2162-8769. ; 10:5 Tidskriftsartikel (refereegranskat)abstract In this work, an atomic layer deposition system was used to deposit Al2O3 high-k dielectric film as the gate insulator of GaN-based metal-oxide-semiconductor high-electron mobility transistors (MOSHEMTs). By using the Al2O3 gate dielectric layer, compared to planar channel structure, the direct current, high frequency, and flicker noise performances were improved in the GaN-based MOSHEMTs with fin-nanochannel array. For the GaN-based 80-nm-wide fin-nanochannel array MOSHEMTs, they exhibited superior performances of maximum extrinsic transconductance of 239 mS mm(-1), threshold voltage of -0.4 V, unit gain cutoff frequency of 7.3 GHz, maximum oscillation frequency of 14.1 GHz, normalized noise power of 2.5 x 10(-14) Hz(-1), and Hooge's coefficient of 1.4 x 10(-6). The enhanced performances were attributed to the features of fin-nanochannel array of better gate control capability, enhanced pinch-off effect, and better heat dissipation driven by lateral heat flow within the space between fin-channels.
19.	Lee, Hsin Ying, et al. (författare) Fabrication and Characterization of GaN-Based Fin-Channel Array Metal-Oxide-Semiconductor High-Electron Mobility Transistors with Recessed-Gate and Ga2O3 Gate Insulator Layer 2021 Ingår i: IEEE Journal of the Electron Devices Society. - 2168-6734. ; 9, s. 393-399 Tidskriftsartikel (refereegranskat)abstract In this work, the properties of gallium oxide (Ga2O3) and its excellent interface properties to GaN-based materials are explored as a gate insulator layer for GaN-based metal-oxide-semiconductor high-electron mobility transistors (MOSHEMTs). A novel vapor cooling condensation system was used to deposit the high quality Ga2O3 films with high insulation and low defect suitable for gate insulator layer. The characteristics of the Ga2O3 films were further explored by implementing GaN-based fin-channel array MOSHEMTs with recessed-gates and different channel widths. Compared to planar channel structure, the direct current, high frequency, and flicker noise performances were enhanced in the fin-channel MOSHEMTs with Ga2O3 gate insulator layer. For the GaN-based fin-channel array MOSHEMTs with 300-nm-wide channel, the devices exhibited superior performances of maximum extrinsic transconductance of 194.2 mS/mm, threshold voltage of.1.4 V, extrinsic unit gain cutoff frequency of 6.4 GHz, maximum oscillation frequency of 14.8 GHz, and normalized noise power of 8.45 × 10.15 Hz.1. It was also demonstrated that the associated performances were improved by reducing the width of fin-channel array.
20.	Mebarki, Mohamed Aniss, 1993, et al. (författare) Comparison of the low noise performance of GaN HEMTs and MIS-HEMTs at cryogenic temperatures 2023 Ingår i: 2023 18th European Microwave Integrated Circuits Conference, EuMIC 2023. ; , s. 29-32 Konferensbidrag (refereegranskat)abstract This work presents the comparison of the noise performance of AlGaN/GaN MIS-HEMTs and HEMTs at cryogenic temperatures. Wideband noise measurements at a physical temperature of 4K were performed in order to extract the noise characteristics of the devices, within the range of frequencies of 3-7 GHz. A DC and RF characterization of the devices are also presented to further assess their cryogenic performances. Over the measured frequency band, the results indicate that both technologies are able to present an average best noise temperature as low as 8 K. The MIS-HEMT presents a slight advantage at low bias condition, mainly due to its reduced gate capacitance. The presented results are the first report on the microwave low-noise performance of cryogenic GaN MIS-HEMT, and constitute their current state-of the art.
21.	Mebarki, Mohamed Aniss, 1993, et al. (författare) GaN HEMT with superconducting Nb gates for low noise cryogenic applications 2022 Ingår i: 2022 Compound Semiconductor Week, CSW 2022. Konferensbidrag (refereegranskat)abstract We report on the successful integration of superconducting Nb gate electrodes to AlGaN/GaN heterostructures and HEMTs for low noise cryogenic applications. First, a specific Nb-gate process was developed and implemented on stand-alone gate test structures. The latter were tested at cryogenic temperatures down to 4 K, using DC end-to-end measurements. The results show a clear transition to a superconducting state at Tc ~ 9.2 K. The superconducting nature of the Nb gates further verified on actual HEMTs, featuring 2 fingers design with gate length of 0.2 μm, through their S-parameters measurements at T
22.	Mebarki, Mohamed Aniss, 1993, et al. (författare) GaN High-Electron-Mobility Transistors with Superconducting Nb Gates for Low-Noise Cryogenic Applications 2023 Ingår i: Physica Status Solidi (A) Applications and Materials Science. - : Wiley. - 1862-6319 .- 1862-6300. ; 220:8 Tidskriftsartikel (refereegranskat)abstract The successful integration of superconducting niobium (Nb) gate electrodes into cryogenic gallium nitride (GaN)-based high-electron-mobility transistors (HEMTs) is reported. This is achieved through a specifically developed microfabrication process. The device's DC, microwave, and noise performances at cryogenic temperatures, down to 4 K, are studied and presented. The superconductivity of the gate is tested using DC end-to-end measurements. A clear superconducting state transition at a critical temperature, Tc, of ≈9.2 K is shown. This is further verified with GaN HEMTs with two gate fingers and a gate length of 0.2 μm, through the extraction and validation of a small-signal model at T < Tc. Additionally, the superconductivity of the gate is verified for several gate widths and lengths, showing a significant reduction of the gate resistance independently of its dimensions. Finally, a comparative study of the cryogenic microwave noise performances of the GaN HEMTs with gold (Au) and Nb gates is presented. The Au-gated device presents a competitive optimum noise temperature, Tmin-opt, of ≈8 K at 5 GHz, demonstrating the potential of this technology for cryogenic low-noise applications. The Nb-gated device presents a 5 K higher Tmin-opt, which is found to be related to the suppression of the superconductivity of the Nb gate at the optimum-noise bias.
23.	Mebarki, Mohamed Aniss, 1993, et al. (författare) Noise Characterization and Modeling of GaN-HEMTs at Cryogenic Temperatures 2023 Ingår i: IEEE Transactions on Microwave Theory and Techniques. - 0018-9480 .- 1557-9670. ; 71:5, s. 1923-1931 Tidskriftsartikel (refereegranskat)abstract We report on the noise characterization and modeling of AlGaN/GaN high-electron-mobility transistors (HEMTs) at a cryogenic temperature (CT) of ∼ 10 K within the frequency range of 4.5–6.5 GHz. This work is the first model in the literature describing the high-frequency noise behavior of GaN-based HEMTs at CTs using a two-parameter-noise concept. The suggested model, which is based on measured noise figures and scattering parameters, provides the frequency and the bias dependence of the cryogenic noise properties of AlGaN/GaN HEMTs. The noise contributions from the intrinsic device, the parasitic network, and the gate leakage are separately extracted. The contribution of the access network is found in the order of 1 K and increases with the frequency, while the gate leakage has an impact of the order of 0.1 K and increases at low frequency. The model provides a basis for the future design and implementation of GaN-based cryogenic low-noise amplifiers.
24.	Papamichail, A., et al. (författare) Compositionally graded channel HEMTs towards improved linearity for low-noise RF amplifiers 2022 Ingår i: 2022 Compound Semiconductor Week, CSW 2022. Konferensbidrag (refereegranskat)abstract Although AlGaN/GaN HEMTs with high power and current gain have been demonstrated in RF device applications, at high signal operation they show an inherent non-linear behavior which leads to gain compression and signal distortion. Polarization-doped AlGaN/GaN HEMTs, with a compositionally graded channel enables a linear response improvement through formation of a 3-D electron gas. In this work, we develop the growth process for graded channel HEMTs in a hot-wall MOCVD reactor. Control of the grading profile is established through growth parameter tuning. Afterwards, analysis by EDS allows for precise determination of Al composition across the channel. Conventional and graded channel HEMT structures were fabricated and characterized. Furthermore, the sheet resistance, carrier density and mobility in HEMT structures with different grading profiles are compared and discussed. The conventional (non-graded) structure revealed the highest electron mobility of ~2350 cm2/V.s, which is among the highest values reported.
25.	Papamichail, Alexis, et al. (författare) Mg-doping and free-hole properties of hot-wall MOCVD GaN 2022 Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 131:18 Tidskriftsartikel (refereegranskat)abstract The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range (2.45 × 10 18 cm-3 up to 1.10 × 10 20 cm-3) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance-voltage (C-V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 ω cm and 8.4 × 10 17 cm - 3, respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.
26.	Persson, P. O.Å., et al. (författare) Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers 2023 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 122:15 Tidskriftsartikel (refereegranskat)abstract Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼9 × 10 12 cm-2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity.
27.	Stanishev, Vallery, et al. (författare) Low Al-content n-type AlxGa1−xN layers with a high-electron-mobility grown by hot-wall metalorganic chemical vapor deposition 2023 Ingår i: Vacuum. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0042-207X .- 1879-2715. ; 217 Tidskriftsartikel (refereegranskat)abstract In this work, we demonstrate the capability of the hot-wall metalorganic chemical vapor deposition to deliver high-quality n-AlxGa1−xN (x = 0 – 0.12, [Si] = 1×1017 cm−3) epitaxial layers on 4H-SiC(0001). All layers are crack-free, with a very small root mean square roughness (0.13 – 0.25 nm), homogeneous distribution of Al over film thickness and a very low unintentional incorporation of oxygen at the detection limit of 5×1015 cm−3 and carbon of 2×1016 cm−3. Edge type dislocations in the layers gradually increase with increasing Al content while screw dislocations only raise for x above 0.077. The room temperature electron mobility of the n-AlxGa1−xN remain in the range of 400 – 470 cm2/(V.s) for Al contents between 0.05 and 0.077 resulting in comparable or higher Baliga figure of merit with respect to GaN, and hence demonstrating their suitability for implementation as drift layers in power device applications. Further increase in Al content is found to result in significant deterioration of the electrical properties.
28.	Vidarsson, Arnar M., et al. (författare) Observations of very fast electron traps at SiC/high-κ dielectric interfaces 2023 Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 11:11 Tidskriftsartikel (refereegranskat)abstract Very fast interface traps have recently been suggested to be the main cause behind poor channel-carrier mobility in SiC metal-oxide-semiconductor field effect transistors. It has been hypothesized that the NI traps are defects located inside the SiO2 dielectric with energy levels close to the SiC conduction band edge and the observed conductance spectroscopy signal is a result of electron tunneling to and from these defects. Using aluminum nitride and aluminum oxide as gate dielectrics instead of SiO2, we detect NI traps at these SiC/dielectric interfaces as well. A detailed investigation of the NI trap density and behavior as a function of temperature is presented and discussed. Advanced scanning transmission electron microscopy in combination with electron energy loss spectroscopy reveals no SiO2 at the interfaces. This strongly suggests that the NI traps are related to the surface region of the SiC rather than being a property of the gate dielectric.

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