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-Bias Induced RON Instability in p-GaN Power HEMTs

In this letter, we investigate the on-resistance ( RON ) instability in p-GaN power HEMTs induced by a positive or negative gate bias ( VGB ), following the application of a quasi-static initialization voltage ( VGP ) of opposite sign. The transient behavior of this instability was characterized at different temperatures in the 90–135 °C range. By monitoring the resulting drain current transients, the activation energy as well as time constants of the processes are characterized. Not trivially, both RON increase/decrease were found to be thermally activated and with same activation energy. We attribute the thermal activation of both RON increase/decrease to the charging/discharging of hole traps present in the AlGaN barrier in the region below the gate

The crater lake of Ilamatepec (Santa Ana) volcano, El Salvador: insights into lake gas composition and implications for monitoring

We here present the first chemical characterization of the volcanic gas plume issuing from the Santa Ana crater lake, a hyper-acidic crater lake (pH of -0.2 to 2.5) in north-western El Salvador. Our results, obtained during regular surveys in 2017 and 2018 using a Multi-GAS instrument, demonstrate an hydrous gas composition (H2O/SO2 ratios from 32 to 205), and SO2 as the main sulphur species (H2S/SO2 = 0.03-0.1). We also find that gas composition evolved during our investigated period, with the CO2/SO2 decreasing by one order of magnitude from March 2017 (37.2 ± 9.7) to 2018 (<3). This compositional evolution toward more magmatic (SO2-rich) compositions is interpreted in the context of the long-term evolution of the volcano following its 2005 and 2007 eruptions. We find that, in spite of reduced (background-level) seismicity, the magmatic gas supply into the lake was one order of magnitude higher in March 2017 (Total Volatile Flux: 20,200-30,200 t/day; the total volatile flux is the sum of H2O+CO2+SO2+H2 fluxes in our specific case) than in the following periods (Total Volatile Flux: 615-4112 t/day). We propose that the elevated magmatic/hydrothermal transport in March 2017, combined with a 15% reduction in precipitation, caused the volume of the lake to decrease, ultimately reducing its sulfur absorbing and scrubbing capacity, and hence causing the gas plume CO2/SO2 ratio to decrease. The recently observed increases in temperature, acidity and salinity of the lake are consistent with this hypothesis. The small volume of Santa Ana lake, compared to other crater lakes such as the pre-2017 Poás (Costa Rica), Yugama (Japan), Ruapehu (New Zealand) and Kawah Ijen (Indonesia), makes it sensitive to variations in the underlying magmatic-hydrothermal system. We conclude that the installation of a continuous, fully-automated Multi-GAS is highly desirable to monitor any future change in lake plume chemistry, and hence state of volcanic activity.Publishedid 664V. Processi pre-eruttiviJCR Journa

GaN-based power devices: Physics, reliability, and perspectives

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the fabrication of power devices. Among the semicon- ductors for which power devices are already available in the market, GaN has the widest energy gap, the largest critical field, and the highest saturation velocity, thus representing an excellent material for the fabrication of high-speed/high-voltage components. The presence of spon- taneous and piezoelectric polarization allows us to create a two-dimensional electron gas, with high mobility and large channel density, in the absence of any doping, thanks to the use of AlGaN/GaN heterostructures. This contributes to minimize resistive losses; at the same time, for GaN transistors, switching losses are very low, thanks to the small parasitic capacitances and switching charges. Device scaling and monolithic integration enable a high-frequency operation, with consequent advantages in terms of miniaturization. For high power/high- voltage operation, vertical device architectures are being proposed and investigated, and three-dimensional structures—fin-shaped, trench- structured, nanowire-based—are demonstrating great potential. Contrary to Si, GaN is a relatively young material: trapping and degradation processes must be understood and described in detail, with the aim of optimizing device stability and reliability. This Tutorial describes the physics, technology, and reliability of GaN-based power devices: in the first part of the article, starting from a discussion of the main proper- ties of the material, the characteristics of lateral and vertical GaN transistors are discussed in detail to provide guidance in this complex and interesting field. The second part of the paper focuses on trapping and reliability aspects: the physical origin of traps in GaN and the main degradation mechanisms are discussed in detail. The wide set of referenced papers and the insight into the most relevant aspects gives the reader a comprehensive overview on the present and next-generation GaN electronics

Quantitative Microbial Risk Assessment as support for bathing waters profiling

Profiling bathing waters supported by Quantitative Microbial Risk Assessment (QMRA) is key to the WHO's recommendations for the 2020/2021 revision of the European Bathing Water Directive. We developed an areaspecific QMRA model on four pathogens, using fecal indicator concentrations (E. coil, enterococci) for calculating pathogen loads. The predominance of illness was found to be attributable to Human Adenovirus, followed by Salmonella, Vibrio, and Norovirus. Overall, the cumulative illness risk showed a median of around 1 case/10000 exposures. The risk estimates were strongly influenced by the indicators that were used, suggesting the need for a more detailed investigation of the different sources of fecal contamination. Area-specific threshold values for fecal indicators were estimated on a risk-basis by modelling the cumulative risk against E. coll. and enterococci concentrations. To improve bathing waters assessment, we suggest considering source apportionment locally estimating of pathogen/indicator ratios, and calculating site-specific indicators thresholds based on risk assessment

Guidelines on the diagnosis, treatment and management of visceral and renal arteries aneurysms: a joint assessment by the Italian Societies of Vascular and Endovascular Surgery (SICVE) and Medical and Interventional Radiology (SIRM)

: The objective of these Guidelines is to provide recommendations for the classification, indication, treatment and management of patients suffering from aneurysmal pathology of the visceral and renal arteries. The methodology applied was the GRADE-SIGN version, and followed the instructions of the AGREE quality of reporting checklist. Clinical questions, structured according to the PICO (Population, Intervention, Comparator, Outcome) model, were formulated, and systematic literature reviews were carried out according to them. Selected articles were evaluated through specific methodological checklists. Considered Judgments were compiled for each clinical question in which the characteristics of the body of available evidence were evaluated in order to establish recommendations. Overall, 79 clinical practice recommendations were proposed. Indications for treatment and therapeutic options were discussed for each arterial district, as well as follow-up and medical management, in both candidate patients for conservative therapy and patients who underwent treatment. The recommendations provided by these guidelines simplify and improve decision-making processes and diagnostic-therapeutic pathways of patients with visceral and renal arteries aneurysms. Their widespread use is recommended

Caratterizzazione di Dispositivi di Potenza a Semiconduttore con Largo Band-Gap

In questa tesi viene discusso il tema della caratterizzazione di dispositivi basati su semiconduttori ad elevato gap energetico. Tali dispositivi sono di particolare interesse per applicazioni ad elevata potenza ed alta frequenza e ad oggi vi sono già in commercio dispositivi basati su Carburo di Silicio (SiC) e Nitruro di Gallio (GaN) che hanno il potenziale per fornire prestazioni ben al di sopra rispetto alla convenzionale tecnologia in Silicio (Si). Nonostante ciò, la presenza di fenomeni di trapping e de-trapping che si verificano all’interno di dispositivi in SiC e GaN, limitano le prestazioni di tali dispositivi ben al di sotto rispetto alle aspettative. Per tale ragione, questa dissertazione si concentra sulla caratterizzazione dei fenomeni di trapping e de-trapping che avvengono nei dispositivi in questione, con lo scopo di studiare i meccanismi fisici alla loro base. Per fare ciò, vengono utilizzate diverse tecniche di caratterizzazione come misure impulsate, transienti di corrente e caratterizzazione on-the-fly. In particolare, viene presentato un nuovo sistema per la caratterizzazione on-the-fly dei drift di resistenza di on-state (RON) e tensione di soglia (VTH) che avvengono durante le operazioni in commutazione. Tale sistema è stato impiegato per la caratterizzazione di dispositivi SiC e GaN, mettendo in luce le instabilità della tensione di soglia nel caso SiC e della RON nel caso GaN. Nel primo caso, le trappole all’interfaccia SiC/SiO2 vengono identificate come la principale causa d’instabilità, mentre nel secondo caso, le trappole di tipo accettore all’interno del buffer sono considerate come principale causa dell’aumento di resistenza dovuta alla tensione di Drain applicata in off-state. Ciò è dovuto al drogaggio Carbon che viene normalmente utilizzato per rendere il buffer semi-isolante e ridurre il leakage nei dispositivi GaN utilizzati per applicazioni di potenza. Nel caso di applicazioni RF, tale drogaggio può essere sostituito da una diversa specie drogante come il ferro. Anche tale drogaggio, però, introduce stati trappola nel GaN-buffer che sono a loro volta responsabili per il collasso della corrente di Drain. Per studiare questo tipo di stati trappola, è possibile utilizzare sia misure impulsate che i transienti di corrente. La prima tecnica è stata applicata per mettere in luce la presenza dei fenomeni di intrappolamento ed investigare l’effetto della dinamica di tali trappole sulla stabilità della tensione di breakdown dei dispositivi testati. La seconda tecnica, invece, è stata utilizzata per studiare l’effetto del campo elettrico e dell’auto-riscaldamento sulla dinamica di emissione da tali trappole. In particolare, viene mostrato come un aumento di temperatura nel dispositivo, dovuto all’auto-riscaldamento, porta ad un’accelerazione dell’emissione da tali stati trappola. La correlazione osservata tra temperatura e dinamica di tali stati trappola, ha permesso di implementare una nuova tecnica di stima della temperatura all’interno di dispositivi in GaN, la cui accuratezza è risultata in linea con le tecniche allo stato dell’arte.This thesis discusses the characterization of devices based on wide-bandgap semiconductors. Such devices are of particular interest for high-power and high-frequency applications, and to date there are already commercially available devices based on Silicon Carbide (SiC) and Gallium Nitride (GaN) that have the potential to provide performance well above conventional Silicon (Si) technology. Nevertheless, the presence of trapping and de-trapping phenomena occurring within SiC and GaN materials limit the performance of such devices well below expectations. For this reason, this dissertation focuses on the characterization of trapping and de-trapping phenomena occurring in these devices, with the aim of studying the physical underlying mechanisms. To do so, various characterization techniques such as pulsed measurements, current transients and on-the-fly characterization are used. Particularly, a new system is presented for the on-the-fly characterization of on-state resistance (RON) and threshold voltage (VTH) drifts that occur during switching operations. This system was employed for the characterization of SiC and GaN devices, highlighting the VTH instabilities in SiC (Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) and the RON degradation in GaN High Electron Mobility Transistors (HEMTs). In the former, traps at the SiC/SiO2 interface are identified as the main cause of the instability, while in the latter case, acceptor traps in the buffer layer are considered as the main cause of the increase in on-resistance due to the Drain voltage applied in off-state. These traps are introduced by Carbon doping that is normally used to make the buffer semi-insulating and reduce leakage in GaN devices for power applications. In case of RF applications, this dopant can be replaced by a different dopant species such as Iron. Such doping, however, also introduces trap states into the GaN-buffer which are in turn responsible for drain current collapse. To study this type of trap states, both pulsed measurements and current transient measurements can be used. The first technique was applied to highlight the presence of trapping phenomena and investigate the effect of said deep levels on the breakdown voltage stability of tested devices. The second technique, on the other hand, was used to investigate the effect of the electric field and self-heating on the emission dynamics from such traps. In particular, it is shown how an increase in temperature in the device, due to self-heating, leads to an acceleration of the emission from such trap states. The observed correlation between temperature and the dynamics of traps allowed the implementation of a novel technique for estimating the temperature within GaN devices, the accuracy of which is in line with state-of-the-art techniques

A Novel Temperature Estimation Technique Exploiting Carrier Emission from Buffer Traps

We propose a novel technique for temperature estimation in electron devices based on the mutual correlation between emission time constant from traps ( τ ) and temperature ( T ). Arrhenius equation is employed as the physical model relating τ and T The reference system used to present the technique is AlGaN/GaN high electron mobility transistors (HEMTs) with Fe-doping in the buffer. Drain Current Transients (DCTs) are used for extracting the emission time constant ( τ ) from Fe traps and non-linear regression through Trust Region Reflective (TRR) optimization algorithm is used to learn the model parameters from data and infer device temperature. Electro-thermal device simulations are employed for validating the proposed technique, showing that this method is able to provide an improved accuracy with respect to conventional electrical techniques (e.g., McAlister method) promoting it as a valid alternative to state of-the-art optical techniques in GaN HEMTs

Evaluation of VTH and RON Drifts during Switch-Mode Operation in Packaged SiC MOSFETs

In this paper, we investigate the evolution of threshold voltage (VTH) and on-resistance (RON) drifts in the silicon carbide (SiC) power metal-oxide-semiconductor field-effect transistors (MOSFETs) during the switch-mode operation. A novel measurement setup for performing the required on-the-fly characterization is presented and the experimental results, obtained on commercially available TO-247 packaged SiC devices, are reported. Measurements were performed for 1000 s, during which negative VTH shifts (i.e., VTH decrease) and negative RON drifts (i.e., RON decrease) were observed. To better understand the origin of these parameter drifts and their possible correlation, measurements were performed for different (i) gate-driving voltage (VGH) and (ii) off-state drain voltage (VPH). We found that VTH reduction leads to a current increase, thus yielding RON to decrease. This correlation was explained by the RON dependence on the overdrive voltage (VGS–VTH). We also found that gate-related effects dominate the parameter drifts at low VPH with no observable recovery, due to the repeated switching of the gate signal required for the parameter monitoring. Conversely, the drain-induced instabilities caused by high VPH are completely recoverable within 1000 s from the VPH removal. These results show that the measurement setup is able to discern the gate/drain contributions, clarifying the origin of the observed VTH and RON drifts