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AlGaN/GaN high electron mobility transistors on QST engineered substrates are grown with different GaN/AlGaN buffer layer thickness. The as‐grown heterostructures are evaluated for their structural quality via atomic force microscopy, high‐resolution X‐ray diffraction, Raman spectroscopy, and steady‐state thermoreflectance. Transistor devices are fabricated and evaluated via DC and pulsed electrical techniques, as well as thermoreflectance imaging. It is reported that buffer layer thickness of at least 10 μm can result in lateral high electron mobility transistors (HEMTs) with simultaneously high GaN quality, low stress, good DC electrical performance, low current collapse, and low thermal resistance. [ABSTRACT FROM AUTHOR]

Herein, the electrical and thermal performance of lateral AlGaN/GaN high electron mobility transistors (HEMTs) and metal‐insulator‐semiconductor field effect transistors (MISFETs) fabricated with 11 μm thick GaN buffer layers on 200 mm diameter Qromis Substrate Technology (QST) substrates are investigated. The QST substrate has a polycrystalline core engineered to be coefficient of thermal expansion (CTE)‐matched to GaN to minimize wafer bow and residual stress in the GaN film as a result of epitaxial growth. Raman spectroscopy is used to determine the biaxial residual stress in the GaN buffer of the as‐fabricated devices. Electrical characterization is demonstrated on the HEMTs including DC and pulsed output characteristics, DC transfer characteristics, Hall mobility, carrier concentration, sheet resistance, median transition frequency, and maximum stable gain. Finally, the thermal performance of the AlGaN/GaN MISFET is assessed via thermoreflectance thermal imaging at DC power densities up to 19 W mm−1. The thermal resistance of the MISFET, calculated using the peak temperature rises on the gate electrode for DC power densities <10 W mm−1, is measured to be 15.4 mm K W−1, which is comparable with state‐of‐the‐art GaN‐on‐Si lateral transistors. [ABSTRACT FROM AUTHOR]

Selective area doping via ion implantation is crucial to the implementation of most modern devices and the provision of reasonable device design latitude for optimization. Herein, we report highly effective silicon ion implant activation in GaN via Symmetrical Multicycle Rapid Thermal Annealing (SMRTA) at peak temperatures of 1450 to 1530 °C, producing a mobility of up to 137 cm2/Vs at 300K with a 57% activation efficiency for a 300 nm thick 1 × 1019 cm−3 box implant profile. Doping activation efficiency and mobility improved alongside peak annealing temperature, while the deleterious degradation of the as-grown material electrical properties was only evident at the highest temperatures. This demonstrates efficient dopant activation while simultaneously maintaining low levels of unintentional doping and thus a high blocking voltage potential of the drift layers for high-voltage, high-power devices. Furthermore, efficient activation with high mobility has been achieved with GaN on sapphire, which is known for having relatively high defect densities but also for offering significant commercial potential due to the availability of cheap, large-area, and robust substrates for devices. [ABSTRACT FROM AUTHOR]

Fundamental research and development of ultra-wide bandgap (UWBG) semiconductor devices are under way to realize next-generation power conversion and wireless communication systems. Devices based on aluminum gallium nitride (AlxGa1−xN, x is the Al composition), β-phase gallium oxide (β-Ga2O3), and diamond give promise to the development of power switching devices and radio frequency power amplifiers with higher performance and efficiency than commercial wide bandgap semiconductor devices based on gallium nitride (GaN) and silicon carbide (SiC). However, one of the most critical challenges for the successful deployment of UWBG device technologies is to overcome adverse thermal effects that impact the device performance and reliability. Overheating of UWBG devices originates from the projected high power density operation and poor intrinsic thermal properties of AlxGa1−xN and β-Ga2O3. This Perspective delineates the need and process for the "electro-thermal co-design" of laterally configured UWBG electronic devices and provides a comprehensive review of current state-of-the-art thermal characterization methods, device thermal modeling practices, and both device- and package-level thermal management solutions. [ABSTRACT FROM AUTHOR]

This letter demonstrates vertical Ga2O3 Schottky barrier diodes (SBDs) with a novel edge termination, the small-angle beveled field plate (SABFP), fabricated on thinned $\text{G}_{{2}}\text{O}_{{3}}$ substrates. Non-punch-though design is used for the drift region with a donor concentration of ${3}\sim {3.5} \times {10} ^{{16}}$ cm−3, rendering a device differential ON-resistance of $\sim 2~\text{m}\Omega ~\cdot $ cm2. A new wet-etch technique is developed by using a bi-layer mask, which consists of spin-on-glass (SOG) and plasma-enhanced chemical vapor deposited (PECVD) SiO2, to fabricate a very small bevel angle (∼ 1°) in the mesa and field plates. This SABFP structure facilitates the electric field spreading at device edges, rendering a breakdown voltage of 1100 V, a peak electric field of 3.5 MV/cm in Ga2O3 at the Schottky contact edge, and an averaged electric field over 3.4 MV/cm underneath the contact. Our device demonstrates a Baliga’s figure of merit of 0.6 GW/cm2, which is among the highest in all reported Ga2O3 power devices and comparable to the state-of-the-art GaN SBDs. These results show the great potential of Ga2O3 SBDs for future power applications. [ABSTRACT FROM AUTHOR]

Record DC power has been demonstrated in AlGaN/GaN high electron mobility transistors fabricated using a substrate replacement process in which a thick diamond substrate is grown by chemical vapor deposition following removal of the original Si substrate. Crucial to the process is a ~30 nm thick SiN interlayer that has been optimized for thermal resistance. The reductions obtained in self-heating have been quantified by transient thermoreflectance imaging and interpreted using 3D numerical simulation. With a DC power dissipation level of 56 W/mm, the measured average and maximum temperatures in the gate-drain access region were 176 °C and 205 °C, respectively. [ABSTRACT FROM AUTHOR]

The coefficients of thermal expansion (CTEs) of single crystalline, monoclinic β-Ga2O3 were determined by employing high-resolution X-ray diffraction measurements. This work reports the CTE measurements on a single crystalline β-Ga2O3 substrate. The CTE values along the "a," "b," and "c" axes are 3.77 × 10−6 °C−1, 7.80 × 10−6 °C−1, and 6.34 × 10−6 °C−1, respectively, and the CTE of the angle β (the angle between the "a" and "c" axes) is determined to be 1.31 × 10−4 ° K−1. All CTE values reported here are linear under the temperature regime between room temperature and 1000 °C. All measurements were performed in a controlled nitrogen gas environment, and no surface degradation was observed after these measurements. Thermal strain calculations with different material combinations involving β-Ga2O3 are also presented relevant to both epitaxial and wafer bonding applications for Si, InP, 3C–SiC, 6H–SiC, GaN, and sapphire. [ABSTRACT FROM AUTHOR]

This letter demonstrates vertical GaN junction barrier Schottky (JBS) rectifiers fabricated with novel ion implantation techniques. We used two different methods to form the lateral p-n grids below the Schottky contact: 1) Mg implantation into n-GaN to form p-wells and 2) Si implantation into p-GaN to form n-wells. Specific differential ON-resistances ( R \mathrm{\scriptscriptstyle ON} ) of 1.5–2.5 \textm\Omega ~\cdot cm2 and 7–9 \textm\Omega ~\cdot cm2 were obtained in the Mg-implanted and Si-implanted JBS rectifiers, respectively. A breakdown voltage of 500–600 V was achieved in both devices, with a leakage current at high reverse biases at least 100-fold lower than conventional vertical GaN Schottky barrier diodes. The impact of n-well and p-well widths on the R \mathrm{\scriptscriptstyle ON} and BV was investigated. Fast switching capability was also demonstrated. This letter shows the feasibility of forming patterned p-n junctions by novel ion implantation techniques, to enable high-performance vertical GaN power devices. [ABSTRACT FROM AUTHOR]

Radiation tolerance of AlGaN/GaN high-electron mobility transistors (HEMTs) is studied with 2-MeV protons, up to a fluence of 6 \times 10^14 H+/cm2 (about 200 times of typical Si MOSFET rating). The increase in dynamic ON-resistance ( $R_{{\textit{ONDYN}}})$ after radiation is observed to be much more severe than that of static ON-resistance. Radiation-induced donorlike traps located near the two-dimensional electron gas trap electrons, which is responsible for the phenomenon. Compared with the devices passivated by conventional plasma-enhanced chemical vapor deposition (PECVD) SiN, GaN HEMTs with 10 nm of in situ SiN before the PECVD SiN step demonstrate much less increase in $R_{{\textit{ONDYN}}}$ from 2300% to only 300%. The in situ SiN is believed to reduce the process damage by PECVD, improving radiation tolerance. [ABSTRACT FROM PUBLISHER]

This article presents the use of flexible carbon substrates for the growth of III-nitride nanowire light emitters. A dense packing of gallium nitride nanowires were grown on a carbon paper substrate. The nanowires grew predominantly along the a-plane direction, normal to the local surface of the carbon paper. Strong photo- and electro-luminescence was observed from InGaN quantum well light emitting diode nanowires. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A finite-element model coupling 2-D electron gas (2-DEG) density, piezoelectric polarization charge Q{\bf P}, and intrinsic stress induced by a nanocrystalline diamond capping layer, was developed for AlGaN/GaN high electron mobility transistors. Assuming the surface potential is unchanged by an additional stress from diamond capping, tensile stress from the diamond cap leads to an additional tensile stress in the heterostructure and, thus an increase in the 2-DEG under the gate. As a result, additional compressive stress near the gate edges would develop and lead to decreased 2-DEG in the regions between the source and drain contacts (SDCs). Increased saturation drain current will be due to the reduced total resistance between SDC. Integration of the 2-DEG density from SDC revealed a redistribution of sheet density with total sheet charge concentration remaining unchanged. The modeling results were compared with the experimental data from Raman spectroscopy and I-V characterization, and good agreements were obtained. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high-electron mobility transistor (HEMT) structures were grown by metal–organic chemical vapor deposition on SiC, hydride vapor phase epitaxy (HVPE) GaN, and ammonothermal GaN substrates to achieve HEMTs with over five orders of magnitude variation in extended defect density. This enables a direct comparison of the effect of extended defects on device performance to achieve the best possible reliability. As-grown material was characterized by atomic force microscopy, electron channeling contrast imaging, and Raman spectroscopy. Devices were characterized by Hall, dc $I$ – $V$ , and pulsed $I$ – $V$ behavior. Reduced threading dislocation density provides an increased 2-D electron gas mobility, but inhibits ohmic contact formation resulting in high contact resistance. Transistor characteristics were nominally identical, with higher OFF-state leakage in the HEMTs on ammonothermal GaN. The pulsed $I$ – $V$ response indicated significantly reduced current collapse in the HEMT on HVPE GaN due to reduced buffer trapping. [ABSTRACT FROM PUBLISHER]

AlGaN/GaN high-electron mobility transistors (HEMTs) were exposed to 2-MeV protons irradiation, at room temperature, up to a fluence of 6 \times 10^\mathrm \mathbf 14 H+/ \mathrmcm^\mathrm \mathbf 2 . Aside from degradation resulting from radiation-induced charge trapping, transmission electron microscopy and electrical measurements reveal a radiation-induced defect located at the edges of the Ni/Au Schottky gate in the proton-irradiated devices. At the edges of the Ni/Au gate, the Ni of the Ni/Au gate diffused up into the Au layer and migrated into the AlGaN barrier, leaving voids in the Ni layer at the gate edges after irradiation. These radiation-induced voids are caused by diffusion of Ni through vacancy exchange, known as the Kirkendall effect, resulting in reduced gate area and degrading the HEMT performance. [ABSTRACT FROM AUTHOR]

In this split-wafer study, we have compared the dc, pulsed, small and large signal RF electrical performance of nanocrystalline diamond (NCD) coated AlGaN/GaN high electron mobility transistors (HEMTs) to reference devices with silicon nitride passivation only. The NCD-coated HEMTs were observed to outperform reference devices in transconductance, large-signal gain, output power density, and power-added efficiency at 4 GHz. The measured improvements were suspected to be related to reduced dispersion and lower source access resistance afforded by the NCD film. [ABSTRACT FROM AUTHOR]

Boron-doped p^+ nanocrystalline diamond (NCD) films are implemented as heat spreading gate contacts to AlGaN/GaN high-electron-mobility transistors. This device demonstrates a reduced ON-resistance, reduced gate leakage, and significantly increased ON-state current density compared with the reference Ni/Au-gated devices from the same wafer. The NCD gate electrode is thermally stable, chemically stable, optically transparent, and places a heat spreading film in direct contact with the gate edge, which is the hottest part of the device. [ABSTRACT FROM AUTHOR]

Enhancements in AlGaN/GaN high-electron-mobility transistor (HEMT) performance have been realized through ultrathin (4 nm) AlN passivation layers, formed by atomic layer epitaxy (ALE). A combination of ex situ and in situ surface cleans prepare the surface for deposition of ALE AlN. HEMTs passivated by high crystallinity AlN, grown at 500^\circC, show improvements in 2-D electron gas sheet carrier density, gate leakage current, off-state drain leakage current, subthreshold slope, and breakdown voltage. In addition, degradation of dynamic on resistance during pulsed off-state voltage switching stress is suppressed by \sim50\% compared with HEMTs passivated by conventional plasma enhanced chemical vapor deposition SiNx. [ABSTRACT FROM PUBLISHER]

Nanocrystalline diamond (NCD) thin films are deposited as a heat-spreading capping layer on AlGaN/GaN HEMT devices. Compared to a control sample, the NCD-capped HEMTs exhibited approximately 20% lower device temperature from 0.5 to 9 W/mm dc power device operation. Temperature measurements were performed by Raman thermography and verified by solving the 2-D heat equation within the device structure. NCD-capped HEMTs exhibited 1) improved carrier density NS, sheet resistance RSH; 2) stable Hall mobility \muH and threshold voltage VT; and 3) degraded on-state resistance RON, contact resistance RC, transconductance Gm, and breakdown voltage VBR. [ABSTRACT FROM PUBLISHER]