Technology and reliability of normally-off GaN HEMTs with p-type gate

GaN-based transistors with p-GaN gate are commonly accepted as promising devices for application in power converters, thanks to the positive and stable threshold voltage, the low on-resistance and the high breakdown field. This paper reviews the most recent results on the technology and reliability of these devices by presenting original data. The first part of the paper describes the technological issues related to the development of a p-GaN gate, and the most promising solutions for minimizing the gate leakage current. In the second part of the paper, we describe the most relevant mechanisms that limit the dynamic performance and the reliability of GaN-based normally-off transistors. More specifically, we discuss the following aspects: (i) the trapping effects specific for the p-GaN gate; (ii) the time-dependent breakdown of the p-GaN gate during positive gate stress and the related physics of failure; (iii) the stability of the electrical parameters during operation at high drain voltages. The results presented within this paper provide information on the current status of the performance and reliability of GaN-based E-mode transistors, and on the related technological issues

analysis and reliability study of luminescent materials for white lighting

In this work, we report on the characterization and reliability/stability study of phosphorescent materials for lighting applications. More specifically, we investigated (a) phosphors directly deposited over light-emitting diodes (LED) chip, (b) remote phosphor (RP) solutions encapsulated in plastic medium for LED lighting, and (c) phosphors without binder for extreme high-intensity laser diode white lighting. The optical and thermal properties of phosphors were studied to develop a sample based on a mix of phosphor compounds in order to achieve different correlated color temperatures (CCT) and high color rendering index (CRI) LEDs. Thermal properties of cerium-doped YAG (Yttrium Aluminum Garnet) phosphor materials were evaluated in order to study thermal quenching. A maximum phosphor operating temperature of 190–200 °C was found to cause a sensible efficiency degeneration. Reduced efficiency and Stokes shift also caused a localized temperature increase in the photoluminescent materials. In the case of remote phosphors, heat did not find a low thermal resistance path to the heatsink (as occurred through the GaN LED chip for direct phosphor-converted devices) and thermal analysis indicated that material temperature might therefore increase to values in excess of 60 °C when a radiation of 435 mW/cm2 hit the sample template. Reliability was also investigated for both plastic-encapsulated materials and binder-free depositions. Pure thermal reliability study indicated that phosphors encapsulated in polycarbonate material were stable up to temperature of approximately 100 °C, while binder-free phosphor did not show any sensible degradation up to temperatures of 525 °C

Trapping phenomena and degradation mechanisms in GaN-based power HEMTs

Abstract This paper reports an overview of the most relevant trapping and degradation mechanisms that limit the performance and lifetime of GaN-based transistors for application in power electronics. Results obtained on state-of-the-art devices are described and discussed throughout the paper, with the aim of providing a clear description of the topic. The first part of the paper deals with the issue of dynamic-Ron: after describing a robust test strategy for the analysis of the pulsed characteristics of the devices, we discuss the voltage- and temperature-dependent pulsed I-V characteristics of 650 V-rated transistors, and the physical origin of dynamic Ron in these devices. The results demonstrate that through proper buffer optimization it is possible to reach negligible trapping at high voltage. The properties of the traps responsible for dynamic-Ron are also discussed in detail in the paper, based on drain-current transient data. A specific discussion is devoted to hot-electron trapping processes, that – under hard switching conditions – may lead to significant modifications in the resistance of the 2DEG. The second part of the paper deals with device degradation: based on a wide set of experimental results, we describe the physical mechanisms responsible for the worsening of the properties of the devices. More specifically, we demonstrate that stress in off-state conditions may result in measurable changes in the pinch-off voltage, mostly consisting in a negative-threshold instability (NBTI). The origin of this shift is discussed in detail; we also demonstrate that in a real-life cascode configuration (where a low, subthreshold leakage current flows through the device in the off-state), NBTI effects are mitigated. Finally, we discuss the stability of the gate-stack, induced by the exposure to positive gate bias

Electric Field and Self-Heating Effects on the Emission Time of Iron Traps in GaN HEMTs

In this paper we separately investigate the role of electric field and device self-heating (SHE) in enhancing the charge emission process from Fe-related buffer traps (0.52 eV from Ec) in AlGaN/GaN High Electron Mobility Transistors (HEMTs). The experimental analysis was performed by means of Drain Current Transient (DCT) measurements for either i) different dissipated power (PD,steady) at constant drain-to-source bias (VDS,steady) or ii) constant PD,steady at different VDS,steady. We found that i) an increase in PD,steady yields an acceleration in the thermally activated emission process, consistently with the temperature rise induced by SHE. On the other hand, ii) the field effect turned out to be negligible within the investigated voltage range, indicating the absence of Poole-Frenkel effect (PFE). A qualitative analysis based on the electric field values obtained by numerical simulations is then presented to support the interpretation and conclusions

Gate stability of GaN-Based HEMTs with P-Type Gate

status: publishe

Fast System to measure the dynamic on‐resistance of on‐wafer 600 V normally off GaN HEMTs in hard‐switching application conditions

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Study and development of a fluorescence based sensor system for monitoring oxygen in wine production: The WOW project

The importance of oxygen in the winemaking process is widely known, as it affects the chemical aspects and therefore the organoleptic characteristics of the final product. Hence, it is evident the usefulness of a continuous and real-time measurements of the levels of oxygen in the various stages of the winemaking process, both for monitoring and for control. The WOW project (Deployment of WSAN technology for monitoring Oxygen in Wine products) has focused on the design and the development of an innovative device for monitoring the oxygen levels in wine. This system is based on the use of an optical fiber to measure the luminescent lifetime variation of a reference metal/porphyrin complex, which decays in presence of oxygen. The developed technology results in a high sensitivity and low cost sensor head that can be employed for measuring the dissolved oxygen levels at several points inside a wine fermentation or aging tank. This system can be complemented with dynamic modeling techniques to provide predictive behavior of the nutrient evolution in space and time given few sampled measuring points, for both process monitoring and control purposes. The experimental validation of the technology has been first performed in a controlled laboratory setup to attain calibration and study sensitivity with respect to different photo-luminescent compounds and alcoholic or non-alcoholic solutions, and then in an actual case study during a measurement campaign at a renown Italian winery

Use of Bilayer gate insulator in GaN-on-Si Vertical Trench MOSFETs : impact on performance and reliability

We propose to use a bilayer insulator (2.5 nm Al2O3 + 35 nm SiO2) as an alternative to a conventional uni-layer Al2O3 (35 nm), for improving the performance and the reliability of GaN-on-Si semi vertical trench MOSFETs. This analysis has been performed on a test vehicle structure for module development, which has a limited OFF-state performance. We demonstrate that devices with the bilayer dielectric present superior reliability characteristics than those with the uni-layer, including: (i) gate leakage two-orders of magnitude lower; (ii) 11 V higher off-state drain breakdown voltage; and (iii) 18 V higher gate-source breakdown voltage. From Weibull slope extractions, the uni-layer shows an extrinsic failure, while the bilayer presents a wear-out mechanism. Extended reliability tests investigate the degradation process, and hot-spots are identified through electroluminescence microscopy. TCAD simulations, in good agreement with measurements, reflect electric field distribution near breakdown for gate and drain stresses, demonstrating a higher electric field during positive gate stress. Furthermore, DC capability of the bilayer and unilayer insulators are found to be comparable for same bias points. Finally, comparison of trapping processes through double pulsed and V-th transient methods confirms that the V-th shifts are similar, despite the additional interface present in the bilayer devices

Opportunities from Doping of Non-Critical Metal Oxides in Last Generation Light-Conversion Devices

The need to develop sustainable energy solutions is an urgent requirement for society, with the additional requirement to limit dependence on critical raw materials, within a virtuous circular economy model. In this framework, it is essential to identify new avenues for light-conversion into clean energy and fuels exploiting largely available materials and green production methods. Metal oxide semiconductors (MOSs) emerge among other species for their remarkable environmental stability, chemical tunability, and optoelectronic properties. MOSs are often key constituents in next generation energy devices, mainly in the role of charge selective layers. Their use as light harvesters is hitherto rather limited, but progressively emerging. One of the key strategies to boost their properties involves doping, that can improve charge mobility, light absorption and tune band structures to maximize charge separation at heterojunctions. In this review, effective methods to dope MOSs and to exploit the derived benefits in relation to performance enhancement in different types of devices are identified and critically compared. The work is focused specifically on the best opportunities coming from the use of non-critical raw materials, so as to contribute in defining an economically feasible roadmap for light conversion technologies based on these highly stable and widely available compounds