Search

Herein, the reliability of commercial ultraviolet‐C (UV‐C) light‐emitting diodes (LEDs) subjected to constant current stress is reported. Electrical, optical, and spectral analyses are carried out on UV‐C LEDs with an emission peak of 275 nm and a nominal optical power of 12 mW at 100 mA. Degradation tests are carried out at the maximum rated current, the double and at three times the maximum. The LED lifetime is found to be inversely proportional to the third power of the stress current density, indicating that the degradation mechanism might be activated by high‐energy electrons arising from Auger–Meitner recombination. Electrical characterization indicates an increase in defect‐related leakage, whereas the spectral analysis identifies a variation in two emission peaks which can be ascribed to a defect density increase in the active region and the p‐gallium nitride (GaN) layer of the LEDs. A final remark comes from the strong dependence of lifetime on operating current: increasing current to lower the number of LEDs in a system is not an optimized strategy. In fact, this has a substantial impact on system lifetime, thus lowering the total number of permitted disinfections. [ABSTRACT FROM AUTHOR]

The ongoing problems in western countries connected to the global energy supply urgently force the research community to strive in finding new methodologies for boosting the functional properties of earth‐abundant raw materials, for example, the largely available cupreous oxide. In this work, the authors focus on the surface sensitization of this metal oxide semiconductor with an argon plasma treatment, that promotes, during photoelectrochemical hydrogen evolution, the formation of metallic copper nanostructures. Interestingly, these copper‐based hierarchical nano‐branches, having inherent plasmonic properties, are at the origin of the improved shelf‐life of the modified Cu2O photocathode, as is demonstrated by advanced structural and photophysical analyses. This proposed photophysical mechanism for an operando electrode stabilization suggests that a self‐healing process can occur within the Cu2O/plasmonic Cu heterostructure. These findings pave the way to the implementation of new, easy‐to‐make strategies to improve the properties of low‐cost, low‐toxicity energy materials. [ABSTRACT FROM AUTHOR]

Herein, the results are reviewed concerning reliability of high‐electron mobility transistors (HEMTs) based on GaN, which currently represent the technology of choice for high‐efficiency microwave and millimeter‐wave power amplifiers. Several failure mechanisms of these devices are extensively studied, including converse piezoelectric effects, formation of conductive percolation paths at the edge of gate toward the drain, surface oxidation of GaN, time‐dependent breakdown of GaN buffer, and of field‐plate dielectric. For GaN HEMTs with scaled gate length, the simultaneous control of short‐channel effects, deep‐level dispersion, and hot‐electron‐induced degradation requires a careful optimization of epitaxial material quality and device design. [ABSTRACT FROM AUTHOR]

Costs and toxicity concerns are at the center of a heated debate regarding the implementation of perovskite solar cells (PSCs) into commercial products. The first bottleneck could be overcome by eliminating the top metal electrode (generally gold) and the underlying hole transporting material and substituting both with one single thick layer of conductive carbon, as in the so‐called carbon‐based PSCs (C‐PSCs). The second issue, related to the presence of lead, can be tackled by resorting to other perovskite structures based on less toxic metallic components. An interesting case is that of the double perovskite Cs2AgBiBr6, which at present still lacks the outstanding optoelectronic performances of the lead‐based counterparts but is very stable to environmental factors. In this work, the processing of carbon electrodes onto Cs2AgBiBr6‐based C‐PSCs was reported, starting from an additive‐free isopropanol ink of a carbon material obtained from the hydrothermal recycling of waste tires and employing a high‐throughput ultrasonic spray coating method in normal environmental conditions. Through this highly sustainable approach that ensures a valuable step from an end‐of‐life to an end‐of‐waste status for used tires, devices were obtained delivering a record open circuit voltage of 1.293 V, which might in the future represent ultra‐cheap solutions to power the indoor Internet of Things ecosystem. [ABSTRACT FROM AUTHOR]

Herein, the main factors and mechanisms that limit the reliability of gallium nitride (GaN)‐based light‐emitting diodes (LEDs) are reviewed. An overview of the defects characterization techniques most relevant for wide‐bandgap diodes is provided first. Then, by introducing a catalogue of traps and deep levels in GaN and computer‐aided simulations, it is shown which types of defects are more detrimental for the radiative efficiency of the devices. Gradual degradation mechanisms are analyzed in terms of their specific driving force: a separate analysis of recombination‐enhanced processes, driven by nonradiative recombination and/or temperature‐assisted processes, such as defects or impurity diffusion, is presented. The most common lifetime estimation methods and standards adopted for solid‐state luminaires are also reported on. Finally, the paper concludes by examining which are the typical degradation and failure mechanisms exhibited by LEDs submitted to electrical overstress. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study presents a novel system to investigate the on-wafer level dynamic properties of GaN-based power transistors in hard-switching application conditions. The system is able to analyse devices with an on-resistance (RDSON) in the range from few ohms to hundreds of ohms, and can be effectively used to improve the development process of GaN high electron mobility transistors (HEMTs) power devices at the wafer level. Contrary to the conventional double-pulse setup, where a resistive load is usually used in combination with a very low duty cycle, the dynamic RDSON is acquired during realistic operating conditions, in a boost converter circuit. Consequently, the authors' system is able to study not only the field-activated trapping processes, but also those induced by hard-switching conditions, i.e. promoted by hot electrons and self-heating. The maximum working voltage (600 V) and the minimum RDSON measurement time after turn-on (2 µs) allow evaluating the operation limit of the devices in a voltage/frequency range close to real switching conditions. Working on the wafer level allows a more realistic assessment of the dynamic RDSON behaviour before the packaging phase, which is very important to improve the production and development process of GaN-HEMT devices. [ABSTRACT FROM AUTHOR]

This article reviews most recent results on the reliability of vertical GaN‐based devices, by presenting a few case studies focused on the stability and degradation of high‐voltage GaN‐on‐GaN diodes and of GaN‐based field‐effect transistors (FETs). With regard to diodes, two relevant stress conditions are investigated. The first is operation at high forward current that can induce a degradation of the electrical properties of the devices, mostly consisting in an increase in the operating voltage, well correlated to a decrease in the electroluminescence signal emitted by the diodes. This degradation process is ascribed to the diffusion of hydrogen from the highly p‐type doped regions toward the junction, with consequent compensation of the acceptor (Mg) dopant. The second stress regime investigated on diodes is avalanche: specifically, it is shown that polarization‐doped GaN devices may show avalanche capability, and the stability of diodes in avalanche regime is investigated in detail. With regard to transistors, the analysis is focused on GaN‐on‐GaN vertical Fin‐FETs. First, the stability of the threshold voltage under positive gate stress is analyzed, and the role of interface/oxide traps is discussed by experimental characterization. Then, the degradation under positive gate or high‐drain stress is investigated, to provide information on the dominant degradation processes. [ABSTRACT FROM AUTHOR]

Herein, the development of gallium nitride on silicon (GaN‐on‐Si) heterostructures targeting 1200 V power applications is reported. In particular, it is shown that the insertion of superlattices (SLs) into the buffer layers allows pushing the vertical breakdown voltage above 1200 V without generating additional trapping effects as compared with a more standard optimized step‐graded AlGaN‐based epistructure using a similar total buffer thickness. DC characterizations of fabricated transistors by means of back‐gating measurements reflect both the enhancement of the breakdown voltage and the low trapping effects up to 1200 V. These results show that a proper buffer optimization and the insertion of SL pave the way to GaN‐on‐Si lateral power transistors operating at 1200 V with low on‐resistance and low trapping effects. [ABSTRACT FROM AUTHOR]

GaN-based transistors are promising devices for power switching applications, but their unique properties require careful design of the circuit in order to achieve the best performance and reliability. The first part will focus on the characterisation of deep-level effects in enhancement- and depletion-mode HEMTs, which limit the performance of the transistor. Threshold voltage instabilities in MIS-HEMT structures, caused by charge trapping under the gate, and their dependence on SiN acting as the gate dielectric will be reviewed. ON-resistance increase, caused by charge trapping in the gate-drain access region, will then be discussed in normally off gate injection transistors, as well as the effect of the switching frequency on the trapping-de-trapping equilibrium condition. The second part of the paper will discuss possible failure modes related to operating conditions, starting with the oxidation of the surface and the inverse piezoelectric effect at high gate-drain voltage. To have a complete picture of high-field effects, the gate dielectric and passivation robustness will be investigated. Time-dependent degradation will then be analysed in both gate reverse and forward-bias conditions. Finally, failures related to off-state stress and to power dissipation and electric field during an ESD event in AlGaN/GaN-HEMTs will be discussed. [ABSTRACT FROM AUTHOR]

In a combined experimental and numerical investigation, we present the effects of trap-assisted tunneling on the sub-threshold forward bias characteristics of a blue InGaN/GaN single-quantum-well LED test structure grown on a SiC substrate. The different role of donor- and acceptor-like traps has been studied, for the information it can provide on the role played by point defects. Using the energy Et and trap density Nt as the only tunneling-related fitting parameters, the behavior of the measured I( V) curves is well reproduced by our model over a wide current and temperature range. The very good agreement between simulations and experiments suggests that trap-assisted forward tunneling is one of the most relevant contributions to the current flow below the optical turn-on of the diode. [ABSTRACT FROM AUTHOR]

In this work we report on the three dominant trapping mechanisms affecting the dynamic performance of a double-heterostructure GaN-based MIS-HEMT grown on silicon substrate. In the OFF-state, with high drain voltage and pinched-off 2DEG, the dominant mechanism is the charge-trapping in the gate-drain access region caused by the transversal drain-to-substrate potential. This effect causes the dynamic increase of the ON-resistance, and is positively temperature-dependent, thus of great concern for high-temperature operation. In the SEMI-ON-state, due to the presence of high VDS and relatively high IDS, an additional trapping mechanism emerges, involving the injection of hot electrons from the 2DEG into trap states located in the GaN-buffer or in the AlGaN barrier. This mechanism, critical in hard-switching operations, affects both the ON-resistance and the VTH. Finally, when the gate is positively biased (gate overdrive state) trapping of electrons happens in the gate dielectric layer(s), leading to strong metastable VTH instabilities. [ABSTRACT FROM AUTHOR]