Stefano Dalcanale

Bio: Stefano Dalcanale is an academic researcher from University of Bristol. The author has contributed to research in topics: High-electron-mobility transistor & Gallium nitride. The author has an hindex of 10, co-authored 21 publications receiving 403 citations. Previous affiliations of Stefano Dalcanale include University of Padua.

Time-Dependent Failure of GaN-on-Si Power HEMTs With p-GaN Gate

Abstract: This paper reports an experimental demonstration of the time-dependent failure of GaN-on-Si power high-electron-mobility transistors with p-GaN gate, submitted to a forward gate stress. By means of combined dc, optical analysis, and 2-D simulations, we demonstrate the following original results: 1) when submitted to a positive voltage stress (in the range of 7–9 V), the transistors show a time-dependent failure, which leads to a sudden increase in the gate current; 2) the time-to-failure (TTF) is exponentially dependent on the stress voltage and Weibull-distributed; 3) the TTF depends on the initial gate leakage current, i.e., on the initial defectiveness of the devices; 4) during/after stress, the devices show a localized luminescence signal (hot spots); the spectral investigation mainly reveals a peak corresponding to yellow luminescence and a broadband related to bremsstrahlung radiation; and 5) 2-D simulations were carried out to clarify the origin of the degradation process. The results support the hypothesis that the electric field in the AlGaN has a negligible impact on the device failure; on the contrary, the electric field in the SiN and in the p-GaN gate can play an important role in favoring the failure, which is possibly due to a defect generation/percolation process.

Temperature-Dependent Dynamic $R_{\mathrm {\mathrm{{\scriptstyle ON}}}}$ in GaN-Based MIS-HEMTs: Role of Surface Traps and Buffer Leakage

Abstract: This paper reports an investigation of the trapping mechanisms responsible for the temperature-dependent dynamic- $R_{\mathrm {\mathrm{{\scriptstyle ON}}}}$ of GaN-based metal–insulator–semiconductor (MIS) high electron mobility transistors (HEMTs). More specifically, we perform the following. First, we propose a novel testing approach, based on combined OFF-state bias, backgating investigation, and positive substrate operation, to separately investigate the buffer- and the surface-related trapping processes. Then, we demonstrate that the dynamic $R_{\mathrm {\mathrm{{\scriptstyle ON}}}}$ of GaN-based MIS-HEMTs significantly increases when the devices are operated at high temperature levels. We explain this effect by demonstrating that it is due to the increased injection of electrons from the substrate to the buffer (under backgating conditions) and from the gate to the surface (under positive substrate operation). Finally, we demonstrate that by optimizing the buffer and by reducing the vertical leakage, substrate-related trapping effects can be completely suppressed. The results described within this paper provide general guidelines for the evaluation of the origin of dynamic $R_{\mathrm {\mathrm{{\scriptstyle ON}}}}$ in GaN power HEMTs and point out the important role of the buffer leakage in favouring the trapping processes.

Evidence of Hot-Electron Effects During Hard Switching of AlGaN/GaN HEMTs

Abstract: This paper reports on the impact of soft- and hard-switching conditions on the dynamic ON-resistance of AlGaN/GaN high-electron mobility transistors. For this study, we used a special double pulse setup, which controls the overlapping of the drain and gate waveforms (thus inducing soft and hard switching), while measuring the corresponding impact on the ON-resistance, drain current, and electroluminescence (EL). The results demonstrate that the analyzed devices do not suffer from dynamic ${R}_{ {\mathrm{\scriptscriptstyle ON}}}$ increase when they are submitted to soft switching up to ${V}_{{\text {DS}}}= 600$ V. On the contrary, hard-switching conditions lead to a measurable increase in the dynamic ON-resistance (dynamic- ${R}_{ \mathrm{\scriptscriptstyle ON}})$ . The increase in dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ induced by hard switching is ascribed to hot-electrons effects: during each switching event, the electrons in the channel are accelerated by the high electric field and subsequently trapped in the AlGaN/GaN heterostructure or at the surface. This hypothesis is supported by the following results: 1) the increase in ${R}_{ \mathrm{\scriptscriptstyle ON}}$ is correlated with the EL signal measured under hard-switching conditions and 2) the impact of hard switching on dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ becomes weaker at high-temperature levels, as the average energy of hot electrons decreases due to the increase scattering with the lattice.

Pulsed Large Signal RF Performance of Field-Plated Ga 2 O 3 MOSFETs

Abstract: Comparison between pulsed and CW large signal RF performance of field-plated $\beta $ -Ga2O3 MOSFETs has been reported. Reduced self-heating when pulse resulted in a power added efficiency of 12%, drain efficiency of 22.4%, output power density of 0.13 W/mm, and maximum gain up to 4.8 dB at 1 GHz for a 2- $\mu \text{m}$ gate length device. Increased power dissipation for higher ${V} _{\textsf {DS}}$ and ${I} _{\textsf {DS}}$ resulted in a degradation in performance, which, thermal simulation showed, could be entirely explained by self-heating. Buffer and surface trapping contributions have been evaluated using gate and drain lag measurements, showing minimal impact on device performance. These results suggest that $\beta $ -Ga2O3 is a good candidate for future RF applications.

Raman Thermography of Peak Channel Temperature in $\beta$ -Ga 2 O 3 MOSFETs

Abstract: $\beta $ -Ga2O3 is an attractive material for high-voltage applications and has the potential for monolithically integrated RF devices. A combination of Raman nano-particle thermometry measurement and thermal simulation has been used to measure the peak channel temperature due to self-heating in $\beta $ -Ga2O3 MOSFETs. The peak channel thermal resistance measured at the gate surface in the device center was $88~mm\,\! \cdot \, K/W$ . This value is higher than what has been previously reported using electrical methods, which determine an average temperature over the whole device area. Experimentally validated thermal simulations have been used to propose possible thermal management mitigation approaches.

The 2018 GaN power electronics roadmap

Abstract: Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

Perspective: Ga2O3 for ultra-high power rectifiers and MOSFETS

Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) power devices offer potential savings in both energy and cost. However, converters powered by WBG devices require innovation at all levels, entailing changes to system design, circuit architecture, qualification metrics, and even market models. The performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors benefits from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. Reverse breakdown voltages of over 2 kV for β-Ga2O3 have been reported, either with or without edge termination and over 3 kV for a lateral field-plated Ga2O3 Schottky diode on sapphire. The metal-oxide-semiconductor field-effect transistors fabricated on Ga2O3 to date have predominantly been depletion (d-mode) devices, with a few demonstrations of enhancement (e-mode) operation. While these results are promising, what are the limitations of this technology and what needs to occur for it to play a role alongside the more mature SiC and GaN power device technologies? The low thermal conductivity might be mitigated by transferring devices to another substrate or thinning down the substrate and using a heatsink as well as top-side heat extraction. We give a perspective on the materials’ properties and physics of transport, thermal conduction, doping capabilities, and device design that summarizes the current limitations and future areas of development. A key requirement is continued interest from military electronics development agencies. The history of the power electronics device field has shown that new technologies appear roughly every 10-12 years, with a cycle of performance evolution and optimization. The older technologies, however, survive long into the marketplace, for various reasons. Ga2O3 may supplement SiC and GaN, but is not expected to replace them.

Review—Ionizing Radiation Damage Effects on GaN Devices

Abstract: GalliumNitridebasedhighelectronmobilitytransistors(HEMTs)areattractiveforuseinhighpowerandhighfrequencyapplications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaNbased blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20‐30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN‐based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60 Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. In InGaN/GaN blue LEDs irradiated with protons at fluences near 10 14 cm −2 or electrons at fluences near 10 16 cm −2 , both current-voltage and light output-current characteristics are degraded with increasing proton dose. The optical performance of the LEDs is more sensitive to the proton or electron irradiation than that of the corresponding electrical performances. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any

Maximizing the Performance of 650-V p-GaN Gate HEMTs: Dynamic RON Characterization and Circuit Design Considerations

Abstract: The systematic characterization of a 650-V/13-A enhancement-mode GaN power transistor with p-GaN gate is presented. Critical device parameters such as ON-resistance $R_{{\rm{ON}}}$ and threshold voltage $V_{{\rm{TH}}}$ are evaluated under both static and dynamic (i.e., switching) operating conditions. The dynamic R ON is found to exhibit different dependence on the gate drive voltage $V_{{\rm{GS}}}$ from the static $R_{{\rm{ON}}}$ . While reasonably suppressed at higher $V_{{\rm{GS}}}$ of 5 and 6 V, the degradation in dynamic R ON is significantly larger at lower $V_{{\rm{GS}}}$ of 3–4 V, which is attributed to the positive shift in $V_{{\rm{TH}}}$ under switching operations. In addition to characterization of discrete devices, a custom-designed double-pulse test circuit with 400-V, 10-A test capability is built to evaluate the transient switching performance of the p-GaN gate power transistors. Optimal gate drive conditions are proposed to: 1) provide sufficient gate over-drive to minimize the impact of the $V_{{\rm{TH}}}$ shift on the dynamic $R_{{\rm{ON}}}$ ; and 2) leave enough headroom to save the device from excessive gate stresses. Moreover, gate drive circuit design and board layout considerations are also discussed by taking into account the fast switching characteristics of GaN devices.

Deep traps in GaN-based structures as affecting the performance of GaN devices

Abstract: New developments in theoretical studies of defects and impurities in III-Nitrides as pertinent to compensation and recombination in these materials are discussed. New results on experimental studies on defect states of Si, O, Mg, C, Fe in GaN, InGaN, and AlGaN are surveyed. Deep electron and hole traps data reported for GaN and AlGaN are critically assessed. The role of deep defects in trapping in AlGaN/GaN, InAlN/GaN structures and transistors and in degradation of transistor parameters during electrical stress tests and after irradiation is discussed. The recent data on deep traps influence on luminescent efficiency and degradation of characteristics of III-Nitride light emitting devices and laser diodes are reviewed.