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We demonstrate the avalanche capability and the existence of breakdown walkout in GaN-on-GaN vertical devices with polarization doping. By means of combined electrical and optical characterization, we demonstrate the following original results: 1) vertical p-n junctions with polarization doping have avalanche capability; 2) stress in avalanche regime induces an increase in breakdown voltage, referred to as breakdown walkout; 3) this process is fully-recoverable, thus being related to a trapping mechanism; 4) temperature-dependent measurements of the breakdown walkout identify $\text{C}_{N}$ defects responsible for this process; and 5) capacitance deep level transient spectroscopy (C-DLTS) and deep level optical spectroscopy (DLOS) confirm the presence of residual carbon in the devices under test. A possible model to explain the avalanche walkout is then proposed. [ABSTRACT FROM AUTHOR]

Describes a failure mechanism of AlGaAs/InGaAs pseudomorphic high electron mobility transistors after carrier dc accelerated testing. Characterization of hot carrier effects by means of gate current measurements and electroluminescence spectroscopy; Measurements of direct current, pulsed and low-frequency ac; Electron traps at gate-to-drain electric fields.

The role of carbon-related traps in GaN-based ungated high-electron mobility transistor structures has been investigated both experimentally and by means of numerical simulations. A clear quantitative correlation between the experimental data and numerical simulations has been obtained. The observed current decrease in the tested structure during backgating measurements has been explained simply by means of a thermally activated hole-emission process with EA = 0.9 eV, corresponding to the distance of the acceptor-like hole-trap level from the GaN valence band. Moreover, it has been demonstrated by means of electrical measurements and numerical simulations that only a low percentage of the nominal carbon doping levels induces the observed current reduction when negative substrate bias is applied to the tested structure. [ABSTRACT FROM AUTHOR]

This paper reports an extensive analysis of the trapping and reliability issues in AlGaN/GaN metal insulator semiconductor (MIS) high electron mobility transistors (HEMTs). The study was carried out on three sets of devices with different gate insulators, namely PEALD SiN, RTCVD SiN and ALD Al 2 O 3 . Based on combined dc, pulsed and transient measurements we demonstrate the following: (i) the material/deposition technique used for the gate dielectric can significantly influence the main dc parameters (threshold current, subthreshold slope, gate leakage) and the current collapse; and (ii) current collapse is mainly due to a threshold voltage shift, which is ascribed to the trapping of electrons at the gate insulator and/or at the AlGaN/insulator interface. The threshold voltage shift (induced by a given quiescent bias) is directly correlated to the leakage current injected from the gate; this demonstrates the importance of reducing gate leakage for improving the dynamic performance of the devices. (iii) Frequency-dependent capacitance–voltage (C–V) measurements demonstrate that optimized dielectric allow to lower the threshold-voltage hysteresis, the frequency dependent capacitance dispersion, and the conductive losses under forward-bias. (iv) The material/deposition technique has a significant impact on device robustness against gate positive bias stress. Time to failure is Weibull-distributed with a beta factor not significantly influenced by the properties of the gate insulator. The results presented within this paper provide an up-to-date overview of the main advantages and limitations of GaN-based MIS HEMTs for power applications, on the related characterization techniques and on the possible strategies for improving device performance and reliability. [ABSTRACT FROM AUTHOR]

This paper describes a detailed analysis of the effects of high temperatures on the optical performance and structural characteristics of GaN-based LED structures with a high threading dislocation density. Results show that, as a consequence of storage at 900 °C in N2 atmosphere, the samples exhibit: (i) an increase in the efficiency of GaN and quantum-well luminescence, well correlated to an increase in carrier lifetime; (ii) a decrease in the parasitic luminescence peaks related to Mg acceptors, which is correlated to the reduction in the concentration of Mg in the p-type region, detected by Secondary Ion Mass Spectroscopy (SIMS); (iii) a diffusion of acceptor (Mg) atoms to the quantum well region; (iv) a reduction in the yield of Rutherford Backscattering Spectrometry (RBS)-channeling measurements, possibly due to a partial re-arrangement of the dislocations, which is supposed to be correlated to the increase in radiative efficiency (see (i)). [ABSTRACT FROM AUTHOR]

In this paper, we review and add additional data and understandings on our findings on the two most common failure modes of GaN-based HEMTs: 1) permanent gate leakage current increase and 2) output current drop. We suggested that they have different origins and one is not necessarily correlated to the other. Yet, they can both concur to the device degradation. First, we demonstrate that the phenomenon of gate leakage current increase has a voltage-accelerated degradation kinetic. Therefore, the identification of the critical voltage for leakage increase is meaningless. We demonstrate that the time-to-breakdown tBD data are Weibull distributed and we prove that they represent intrinsic failures. According to our data, this phenomenon is not related to the inverse piezoelectric effect. Finally, a new degradation model for the gate leakage current increase based on the percolation path theory is proposed. Second, we show that the permanent output current drop is a consequence of the relaxation of AlGaN layer. This occurs by means of formation of crystallographic defects as described by the inverse piezoelectric degradation model. Finally, we show an excellent stability of devices with reduced Al content in the AlGaN barrier, proving the crucial role of strain in the reliability of AlGaN/GaN HEMTs. [ABSTRACT FROM AUTHOR]

Slow trapping phenomenon in AlGaN/GaN HEMTs has been extensively analyzed and described in this paper. Thanks to a detailed investigation, based on a combined pulsed and transient investigation of the current/voltage characteristics (carried out over on an 8-decade time scale), we report a detailed description of the properties of trap levels located in the gate-drain surface, and in the region under the gate of AlGaN/GaN HEMTs. More specifically, the following, relevant results have been identified: (i) the presence of surface trap states may determine a significant current collapse, and reduction of the peak transconductance. During a current transient measurement, the emission of electrons trapped at surface states proceeds through hopping, as demonstrated by means of temperature-dependent measurements. The activation energy of the de-trapping process is equal to 99 meV. (ii) The presence of a high density of defects under the gate may induce a significant shift in the threshold voltage, when devices are submitted to pulsed transconductance measurements. The traps responsible for this process have an activation energy of 0.63 eV, and are detected only on samples with high gate leakage, since gate current allows for a more effective charging/de-charging of the defects. [ABSTRACT FROM AUTHOR]

In this paper, an extensive description of the main characteristics and possible applications of a double-halo metal–oxide–semiconductor (MOS) device is presented. In particular, the details concerning the prototype fabrication through a standard complementary MOS (CMOS) process and the obtained experimental results are reported. Extensive numerical device simulation has been carried out in order to deeply understand the new structure. Furthermore, to gain insight on the device behavior, an electrical model to be used in SPICE-like circuit simulation tools has been developed and verified. As shown by our analysis, digital integrated circuits employing the new technology introduce, with respect to standard CMOS ones, a drastic reduction of both the device number and the parasitic capacitances, leading to a significant improvement of the circuit performance. [ABSTRACT FROM AUTHOR]

Long-term ON-state and OFF-state high-electric-field stress results are presented for unpassivated GaN/AlGaN/GaN high-electron-mobility transistors on SiC substrates. Because of the thin GaN cap layer, devices show minimal current-collapse effects prior to high-electric-field stress, despite the fact that they are not passivated. This comes at the price of a relatively high gate-leakage current. Under the assumption that donor-like electron traps are present within the GaN cap, two-dimensional numerical device simulations provide an explanation for the influence of the GaN cap layer on current collapse and for the correlation between the latter and the gate-leakage current. Both ON-state and OFF-state stresses produce simultaneous current-collapse increase and gate-leakage-current decrease, which can be interpreted to be the result of gate-drain surface degradation and reduced gate electron injection. This study shows that although the thin GaN cap layer is effective in suppressing surface-related dispersion effects in virgin devices, it does not, per se, protect the device from high-electric-field degradation, and it should, to this aim, be adopted in conjunction with other technological solutions like surface passivation, prepassivation surface treatments, and/or field-plate gate. [ABSTRACT FROM AUTHOR]

An extensive characterization of the on-state breakdown characteristics of GaAs based MESFETs and HEMTs has been carried out by means of dc and pulsed measurements and of circuit simulations. A computer-controlled, three-terminals Transmission Line Pulse (TLP) system with 50-100 ns pulse width and sub-ns risetime has been developed, which allows automated pulsed measurements of device I-V characteristics. The TLP system has been adopted for nondestructive measurements of the on-state breakdown characteristics of GaAs MESFETs and HEMTs up to unprecedented values of gate current density (I[sub G]/W = 30 mA/mm has been reached), in strong avalanche conditions. The device behavior in strong avalanche conditions is dominated by a parasitic bipolar effect (PBE) similarly to SOI and bulk Si MOSFETs. By taking into account this and other parasitic effects, an equivalent circuit model, suitable for SPICE simulations has been developed. The proposed model is capable of predicting the exact behavior of the gate and drain currents in both weak and strong avalanche conditions. [ABSTRACT FROM AUTHOR]

This paper reviews most recent results concerning reliability of InP-based and metamorphic high electron mobility transistors (HEMTs). Thanks to research work carried out in the last 10 years, a deeper understanding of failure mechanisms of these devices has been achieved, and process and technology solutions have been found for the control of premature breakdown (related to the reduced energy gap of InGaAs) and of parasitic effects, such as “kink” effects, and transconductance frequency dispersion. After a brief description of impact-ionization effects in InGaAs, an analysis of failure modes and mechanisms of InP and metamorphic HEMTs is carried out, including hot-carrier-induced degradation, gate sinking and Schottky/ohmic contact interdiffusion, hydrogen effects, and donor compensation due to fluorine atoms indiffusion. Results show that reliability of these devices is continuously improving, opening the way for applications in microwave and millimeter-wave systems. [Copyright &y& Elsevier]

The operating voltage of advanced microwave devices is currently limited by impact-ionization and breakdown effects. An extended study of the effects of hot carriers on the breakdown behavior and reliability of GaAs-based and InP-based HEMTs (high electron mobility transistors) has been carried out by means of pulsed measurements, electroluminescence spectroscopy and microscopy and Monte Carlo simulations. We show that the parasitic bipolar effect, which explains kink effects in HEMTs, can also induce regenerative phenomena eventually leading to on-state breakdown. Results concerning the effects of hot carrier aging on GaAs-based and InPbased HEMTs are summarized. [ABSTRACT FROM AUTHOR]

Analyzes carrier transport phenomena occuring in pseudomorphic AlGaAs/InGaAs high electronic mobility transistor (HEMT). Significance of hot-electron phenomena in the design power of AlGaAs/InGaAs pseudomorphic HEMT; Development of techniques to improve hot-electron characteristics and breakdown voltage of HEMT; Sample description and experimental apparatus; Electrical measurements.

Presents information on a study which focused on the on-state and off-state breakdown of GaInAs/InP composite-channel high electron mobility transistors. Device structure and performance; On-state and off-state breakdown characteristics; Discussion of the results; Conclusions.

We report on the hot-electrons induced degradation in AlGaAs/GaAs high electron mobility transistors (HEMTs), consisting of a decrease in the drain current and an increase in the parasitic drain resistance. The amount of the degradation is proportional to the impact-ionization rate which is related to the electron energy. Transconductance dispersion measurements and drain current deep level transient spectroscopy (DLTS) have been used to identify interface traps which are located at the AlGaAs/GaAs interface in the gate-drain access region and are the causes of the observed degradation. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

InGaN-based multi-quantum well (MQW) solar cells are promising devices for photovoltaics (e.g., for tandem solar cells and concentrator systems), space applications, and wireless power transfer. In order to improve the efficiency of these devices, the factors limiting their efficiency and stability must be investigated in detail. Due to the complexity of a MQW structure, compared with a simple pn junction, modeling the spectral response of these solar cells is not straightforward, and ad hoc methodologies must be implemented. In this paper, we propose a model, based on material parameters and closed-formula equations, that describes the shape of the quantum efficiency of InGaN/GaN MQW solar cells, by taking into account the layer thickness, the temperature dependence of the absorption coefficient, and quantum confinement effects. We demonstrate (i) that the proposed model can effectively reproduce the spectral response of the cells; in addition, (ii) we prove that the bulk p-GaN layer absorbs radiation, but the carriers photogenerated in this region do not significantly contribute to device current. Finally, we show that (iii) by increasing the temperature, there is a redshift of the absorption edge due to bandgap narrowing, which can be described by Varshni law and is taken into account by the model, and a lowering in the extraction efficiency due to the increase in recombination (mostly Shockley–Read–Hall) inside the quantum wells, which is also visible by decreasing light intensity. [ABSTRACT FROM AUTHOR]

Degradation of InGaN–GaN LEDs has been the subject of intense investigations in the past few years. While current- and temperature-induced degradation processes have been described, the impact of photon-induced degradation has not been investigated in detail in the literature. This paper aims at improving the understanding of the mechanisms responsible for the degradation of the InGaN subject to high photon densities by stressing the devices under a high-intensity laser beam in open-circuit conditions (i.e., in the absence of external current). We analyzed the degradation by means of electrical, optical, and deep-level characterization techniques. First, we demonstrate the existence of optically induced degradation processes in GaN LEDs: from photoluminescence measurements, we observed a decrease in the luminescence after stress, more prominent in the region irradiated during stress. Second, we ascribe this effect to a decrease in internal quantum efficiency due to the generation of non-radiative defects within the active region. Third, by steady-state photocapacitance measurements, we reveal the presence of a shallow level with an energy of EC–2.2 eV, which can be ascribed to gallium vacancies and its complexes with oxygen and nitrogen and can be related to the increase in yellow luminescence. [ABSTRACT FROM AUTHOR]

Understanding the properties of N-implanted β-Ga2O3 is fundamental for the optimization of doping and isolation structures based on gallium oxide. This paper reports an extensive analysis of the impact of thermal annealing on the concentration and properties of deep levels in N-implanted β-Ga2O3 Schottky barrier diodes by means of capacitance isothermal transient spectroscopy. Samples with annealing temperatures from 800 to 1200 °C were considered. The original results presented in this paper demonstrate the following: (a) The instability of current–voltage characteristics detected for all the samples under test can be attributed to the presence of three electron traps with activation energies of 0.6, 0.7, and 1 eV, consistent with previous reports in β-Ga2O3. (b) The detected traps are not the nitrogen level but intrinsic defects whose concentration is increased by the implantation process. (c) The concentration of deep levels decreases as the annealing temperature increases, demonstrating that the annealing process can effectively restore the quality of the material while keeping the conductivity decrease related to the presence of the nitrogen. Finally, (d) we demonstrate that the residual leakage and the turn-on voltage shift are correlated with the Arrhenius signature of the detected deep levels. An interpretation is proposed to explain the measurement results. [ABSTRACT FROM AUTHOR]

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the fabrication of power devices. Among the semiconductors 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 spontaneous 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 properties 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. [ABSTRACT FROM AUTHOR]

We present an electro-optical method for the extrapolation of the nonradiative and Auger recombination coefficients in InGaN/GaN Light-emitting diodes (LEDs). The method has the advantage of permitting the extrapolation of the recombination parameters of packaged devices, contrary to conventional techniques based on the analysis of quasibulk structures. For the analyzed devices, the average values of the nonradiative and Auger recombination coefficients have been determined to be equal to 2.3×107 s-1 and 1.0×10-30 cm6 s-1, respectively. These results are consistent with previous reports based on the analysis of quasibulk structures and on theoretical simulations. The method described in this paper constitutes an efficient tool for the analysis of the recombination dynamics in GaN-based LEDs. The results obtained within this work support the hypothesis on the importance of Auger recombination in determining the so-called efficiency droop in LED structures. [ABSTRACT FROM AUTHOR]

We present a detailed model of the static and dynamic gate leakage current in lateral β-Ga2O3 MOSFETs with an Al2O3 gate insulator, covering a wide temperature range. We demonstrate that (i) in the DC regime, current originates from Poole–Frenkel conduction (PFC) in forward bias at high-temperature, while (ii) at low temperature the conduction is dominated by Fowler–Nordheim tunneling. Furthermore, (iii) we modeled the gate current transient during a constant gate stress as effect of electron trapping in deep levels located in the oxide that inhibits the PF conduction mechanism. This hypothesis was supported by a TCAD model that accurately reproduces the experimental results. [ABSTRACT FROM AUTHOR]

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]

• Record current density for a 2 μm thick-CdTe based devices. • Cd-rich absorber is beneficial for CdSeTe/CdTe solar cells. • Reduction of deep levels (negative U-center) by Se diffusion. Recently, the introduction of a CdSe x Te 1-x (CST) buffer in the CdTe absorber has increased the photon absorption at both short wavelengths in the visible range (by removal of the parasitic absorption of the CdS layer) and at the infrared range (by reduction of the absorber band gap). But, moreover, the efficiency improvement is also attributed to a longer minority carrier lifetime when Se is added. This paper presents CdSe x Te 1-x /CdTe devices fabricated by evaporation at low substrate temperature and with a reduced absorber thickness (2 µm). These cells achieve record current densities, exceeding 28 mA/cm2 for this absorber thickness with efficiencies close to 15 %. We have compared CdSe x Te 1-x /CdTe devices made with different approaches and we have found that the performance improvement is due to a lower amount of interstitial Te in the absorber, suggesting that the increase of the minority carrier lifetime is connected with a change of the stoichiometry due to the selenium diffusion. [ABSTRACT FROM AUTHOR]

This tutorial paper focuses on the physical origin of thermal droop, i.e., the decrease in the luminescence of light-emitting diodes (LEDs) induced by increasing temperature. III-nitride-based LEDs are becoming a pervasive technology, covering several fields from lighting to displays, from automotive to portable electronics, and from horticulture to sensing. In all these environments, high efficiency is a fundamental requirement, for reducing power consumption and system cost. Over the last decade, a great deal of effort has been put in the analysis of the efficiency droop, the decrease in LED internal quantum efficiency (IQE) induced by high current density. On the other hand, an IQE decrease is observed also for increasing temperature, a phenomenon usually referred to as thermal droop. For commercial LEDs, the IQE decrease related to thermal droop can be comparable to that of efficiency droop: for this reason, understanding thermal droop is a fundamental step for making LEDs capable of operating at high temperature levels. In several fields (including street lighting, automotive, photochemical treatments, projection, entertainment lighting, etc.), compact and high-flux light sources are required: typically, to reduce the size, weight, and cost of the systems, LEDs are mounted in compact arrays, and heat sinks are reduced to a minimum. As a consequence, LEDs can easily reach junction temperatures above 85–100 °C and are rated for junction temperatures up to 150–175 °C (figures from commercially available LED datasheets: Cree XHP70, Osram LUW HWQP, Nichia NVSL219CT, Samsung LH351B, and LedEngin LZP-00CW0R) and this motivates a careful analysis of thermal droop. This paper discusses the possible physical causes of thermal droop. After an introduction on the loss mechanisms in junctions, we will individually focus on the following processes: (i) Shockley–Read–Hall (SRH) recombination and properties of the related defects; (ii) Auger recombination and its temperature dependence, including the discussion of trap-assisted Auger recombination; (iii) impact of carrier transport on the thermal droop, including a discussion on carrier delocalization, escape, and freeze out; (iv) non-SRH defect-related droop mechanisms. In addition, (v) we discuss the processes that contribute to light emission at extremely low current levels and (vi) the thermal droop in deep ultraviolet LEDs, also with reference to the main parasitic emission bands. The results presented within this paper give a tutorial perspective on thermal droop; in addition, they suggest a pathway for the mitigation of this process and for the development of LEDs with stable optical output over a broad temperature range. [ABSTRACT FROM AUTHOR]

The long-term stability of ultraviolet (UV)-C light-emitting diodes (LEDs) is of major importance for many applications. To improve the understanding in this field, we analyzed the degradation of AlGaN-based UVC LEDs and modeled the variation of electrical characteristics by 2D simulations based on the results of deep-level optical spectroscopy (DLOS). The increase in the forward leakage current observed during ageing was ascribed an increase in trap-assisted tunneling. The analysis of the degradation kinetics suggests the role of a defect diffusion process, possibly involving impurities coming from the p-type layers. [ABSTRACT FROM AUTHOR]

Radio frequency microelectromechanical system (RF MEMS) switches require a lifetime of 20 years, during which the devices must operate without failure. During operation, the RF MEMS switches are subjected to prolonged actuation which may induce the problems of charge trapping and mechanical relaxation leading to stiction of the moving membrane. In this paper, we suggest an innovative recovery procedure that allows to avoid the stiction. At regular intervals, the switch is opened and an electrical current forced through the suspended membrane can effectively fully recover the switch. Using the topography analysis, we show that the restoring mechanism is based on the recovery of the mechanical properties of the moving bridge during the current flow. The validity of the procedure is demonstrated with the application of the recovery system on several devices for 20 consecutive cycles. [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]

In this paper, we extensively characterize and model the threshold voltage instability in lateral β-Ga2O3 MOSFETs with Al2O3 gate dielectric. Specifically, (i) the results indicate that the threshold voltage instability originates from electron trapping at gate dielectric border traps close to the Al2O3/β-Ga2O3 interface. (ii) Logarithmic kinetics were detected for both stress and recovery by means of a innovative fast-capacitance experimental setup, over more than seven decades of time; (iii) a generalized model, which is capable of accurately reproducing the experimental results, was proposed to explain this trend. [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]

Trapping phenomena degrade the dynamic performance of wide-bandgap transistors. However, the identification of the related traps is challenging, especially in presence of non-ideal defects. In this paper, we propose a novel methodology (trap-state mapping) to extract trap parameters, based on the mathematical study of stretched exponential recovery kinetics. To demonstrate the effectiveness of the approach, we use it to identify the properties of traps in AlGaN/GaN transistors, submitted to hot-electron stress. After describing the mathematical framework, we demonstrate that the proposed methodology can univocally describe the properties of the distribution of trap states. In addition, to prove the validity and the usefulness of the model, the trap properties extracted mathematically are used as input for TCAD simulations. The results obtained by TCAD closely match the experimental transient curves, thus confirming the accuracy of the trap-state mapping procedure. This methodology can be adopted also on other technologies, thus constituting a universal approach for the analysis of multiexponential trapping kinetics. [ABSTRACT FROM AUTHOR]

We investigate the density of defects and the degradation rate in InGaN light-emitting diodes having identical dislocation density and epitaxial structure, but different indium content in the quantum well (QW; 12%, 16%, 20%). Our results, based on combined steady-state photocapacitance, light-capacitance voltage, and degradation measurements indicate that: (a) the density of defects in the superlattice underlayer is identical for the three wafers, indicating good and reproducible growth conditions; (b) the density of defects within the active region of the devices shows a monotonic dependence on the indium content in the QWs. These results, consistent with previous studies on the topic, prove unequivocally the important role of indium in favoring the incorporation of point defects, further clarifying the possible mechanisms of defect formation, and give a quantitative assessment of the related effect; (c) in step-stress experiments, the degradation rate was found to be much stronger for devices having high indium content in the QW. This result can be explained by considering a decrease in injection efficiency due to the generation or transport of defects, or an increment in defect-assisted Auger recombination terms due to the propagation of defects. [ABSTRACT FROM AUTHOR]

We studied the short- and long-term effects of heavy-ion strikes on silicon-on-insulator (SOI) FinFET devices manufactured in a sub 32-nm CMOS process. The degradation of the device DC characteristics after irradiation strongly depends on the incidence angle, strain, and channel type, depending on the balance between damage to the high-k gate oxide and to the buried oxide. The time-dependent dielectric breakdown (TDDB) and the device parameter degradation kinetics are affected by the LET, ion fluence, and incidence angle. A reduction in TDDB is observed in irradiated FinFETs with respect to unirradiated ones. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including one on deep ultraviolet (DUV) light emitting diodes (LEDs), one on zinc oxide (ZnO) LEDs, and one on blue phosphorescent organic LEDs (OLEDs).

Abstract: The purpose of this work is an in depth reliability analysis of Blu-Ray InGaN laser diodes (LD) submitted to CW stress at different currents and temperatures. During reliability tests, LD devices exhibit a gradual threshold current increase, while slope efficiency is almost not affected by the ageing treatment. Furthermore we demonstrate that: (i) the degradation rate shows a linear correlation with stress current level; (ii) the Ith increase is correlated to the decrease in non-radiative lifetime (τnr ); (iii) stress temperature acts as an accelerating factor for LD degradation; (iv) pure thermal storage does not significantly degrade LDs characteristics. [Copyright &y& Elsevier]

We studied the permanent effects of heavy-ion strikes on decananometer triple-gate SOI devices. We highlighted the role of the geometry and the three-dimensional architecture in the response to heavy ions. Heavy-ion strikes in state-of-the-art Triple-Gate FETs may have measurable permanent effects, due to microdose in the buried oxide, breakdown of the gate oxide, or interface state generation in the side oxide/body interface. This last effect is particularly interesting since it is related to the verticality of multigate transistors. [ABSTRACT FROM PUBLISHER]

High-electric-field degradation phenomena are investigated in GaN-capped AIGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate—source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate—source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters. [ABSTRACT FROM AUTHOR]

Abstract: This paper analyses the thermally-activated failure mechanisms of GaN LED test-structures related with the presence of a hydrogen rich SiN passivation layer, by comparing the electrical and optical behaviour of samples with and without passivation during thermal stress. The analysis was carried out by means of electroluminescence, cathodoluminescence, emission microscopy and current–voltage measurements. Thermal treatment induced degradation only on the samples with passivation: identified degradation modes were an efficiency decrease exponential in time, emission crowding, and a forward voltage increase. On the other side, thermal treatment did not change the behaviour of the LEDs without passivation. An interpretation for the degradation of the passivated samples is the following: as a consequence of passivation deposition, a considerable amount of hydrogen is incorporated in the passivation layer. Heating at 250 ∘C allows this hydrogen to interact with the LED surface, thus worsening the transport properties of -GaN and of the -ohmic contact, and then the current and emission distribution, inducing the observed degradation and emission crowding. The activation energy of the degradation process was found to be equal to 1.3 eV. Comparison between spectral electroluminescence and cathodoluminescence measurements shows how the mechanism mentioned above is not the only ageing cause and the thermal worsening of QW confinement and/or the creation of nonradiative centers possibly contribute to the LED damage. [Copyright &y& Elsevier]

Defects can significantly modify the electro-optical characteristics of InGaN light-emitting diodes (LEDs); however, modeling the impact of defects on the electrical characteristics of LEDs is not straightforward. In this paper, we present an extensive investigation and modeling of the impact of defects on the electrical characteristics of InGaN-based LEDs, as a function of the thickness of the quantum well (QW). First, we demonstrate that the density of defects in the active region of III-N LEDs scales with increasing thickness of the InGaN QW. Since device layers with high indium content tend to incorporate more defects, we ascribed this experimental evidence to the increased volume of defects-rich InGaN associated to thicker InGaN layers. Second, we demonstrate that the current–voltage characteristics of the devices are significantly influenced by the presence of defects, especially in the sub turn-on region. Specifically, we show that the electrical characteristics can be effectively modeled in a wide current range (from pA to mA), by considering the existence of trap-assisted tunneling processes. A good correspondence is obtained between the experimental and simulated electrical characteristics (I–V), by using—in the simulation—the actual defect concentrations/activation energies extracted from steady-state photocapacitance, instead of generic fitting parameters. [ABSTRACT FROM AUTHOR]

Dilute bismides (%Bi ∼ 1%–3%) are excellent candidates for the fabrication of optoelectronic devices, thanks to the strong reduction in the bandgap with increasing bismuth content, the weak temperature dependence of the bandgap, and the small Auger recombination coefficient for high bismuth concentrations. Since Shockley–Read–Hall recombination may significant impact on the electrical and optical properties of the devices, describing the defects within the semiconductor material is of paramount importance for the optimization of device technology. To this aim, this paper investigates the deep levels within the bandgap in un-annealed n-type GaAsBi grown by molecular beam epitaxy at low temperature (390 °C) by means of deep level transient spectroscopy and filling time-dependent measurements. The original results described in the paper indicate (a) the presence of four majority and two minority carrier traps, and (b) that, from the analysis of the charge-capture kinetics, the dominant electron and hole traps originate, respectively, from a dislocation-related and a point defect with capture barrier of 0.48 eV. In addition, (c) we demonstrate that the incorporation of bismuth in GaAs generates only one additional electrically active defect with concentration higher than the sensitivity of the measurement setup, because most of the relevant traps found in GaAsBi layers have been identified in pure GaAs grown at low temperature. Finally, (d) the trap concentration is relatively constant along the growth direction, indicating a uniform defect density. [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]

Over the last 20 years, the technological improvements of CdTe solar cell have been impressive, and today the performance of these cells is comparable with that of silicon cells. Behind these important results, there have been important research efforts, aimed at exploring every single aspect related to CdTe cell optimization. One of these is reliability, that is fundamental in the perspective of massive commercialization. The aim of this review paper is to explore the main aspects related to the operation and reliability of CdTe solar cells, and describe the most relevant results presented in the literature on these topics. The paper is divided in two parts. The first is dedicated to a general introduction of CdTe issues. In this part, we report the most relevant details on CdTe solar cells from a reliability perspective, including (a) a description of the CdTe solar cells development on the years, (b) a description of the main production methods, (c) an overview of the main mathematical model to describe a solar cell, and (d) the main normative requirements related to solar cell reliability. The second section describes the most important papers in the field of CdTe solar cell reliability, along with two recent case studies, that can be used as an example to understand the complexity of the involved processes. The results are critically discussed by considering the most recent literature on the topic, to provide a general overview to the reader willing to approach the topic. [ABSTRACT FROM AUTHOR]

In this paper we report on the study of sintered ceramic composites based on a low melting glass in which ZnO nanocrystals and commercial YAG:Ce3+ phosphor are embedded. A low T g is necessary to avoid high temperature sintering which can damage the optical properties of the embedded phosphor, while ZnO is introduced to increase the thermal conductivity of the system. The compositions have been optimized in terms of stability, sintering efficiency and thermal conductivity. Selected samples were optically characterized using a GaN high power multimode 450 nm Laser Diode, with a maximum output power of 1.6 W at 1.5 A. [ABSTRACT FROM AUTHOR]

In this work, we report a contactless temperature measurement method based on the photoluminescence (PL) of an AlInGaP light-emitting diode (LED) structure under open-circuit conditions. The proposed method has been evaluated between 90 and 460 K, but a broader temperature range is possible. The system is based on the temperature-dependent PL variation of a commercial AlInGaP LED designed to emit at 630 nm at room $T$. The sample is placed inside a temperature-controlled cryostat equipped with a transparent glass window. After exciting the sample by means of a green 532-nm laser, the emitted PL spectrum has been collected by an optical fiber and acquired by means of a charge-coupled device (CCD) array spectrometer. In the first part of the work, the most effective calibration is discussed as a function of the properties of the sampled spectrum; both PL emission peak and spectral width have been considered to calculate the optimal estimated temperature. The effect of spectrometer resolution on temperature estimation has been evaluated by comparing and analyzing the results from two spectrometers with different pixel densities. The effects of excitation intensity and angular positioning have also been evaluated. Finally, uncertainty due to calibration, together with measurement accuracy and precision, has been calculated over 50 acquisitions resulting on average in ±1, ±0.7, and ±0.3 K, respectively, on a temperature range of 300 K. [ABSTRACT FROM AUTHOR]

In this article, 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: 1) different dissipated power (PD,steady) at constant drain-to-source bias (VDS,steady) or 2) constant PD,steady at different VDS,steady’s. We found that: 1) an increase in PD,steady yields an acceleration in the thermally activated emission process, consistently with the temperature rise induced by SHE and 2) on the other hand, the field-effect turned out to be negligible within the investigated voltage range, indicating the absence of the 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. [ABSTRACT FROM AUTHOR]

RON degradation due to stress in GaN-based power devices is a critical issue that limits, among other effects, long-term stable operation. Here, by means of 2-D device simulations, we show that the RON increase and decrease during stress and recovery experiments in carbon-doped AlGaN/GaN power metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) can be explained with a model based on the emission, redistribution, and retrapping of holes within the carbon-doped buffer (“hole redistribution” in short). By comparing simulation results with front- and back-gating OFF-state stress experiments, we provide an explanation for the puzzling observation of both stress and recovery transients being thermally activated with the same activation energy of about 0.9 eV. This finds a straightforward justification in a model in which both RON degradation and recovery processes are limited by hole emission by dominant carbon-related acceptors that are energetically located at about 0.9 eV from the GaN valence band. [ABSTRACT FROM AUTHOR]

• CdSe is applied for the first time as buffer layer on vacuum evaporated Sb 2 Se3. • Higher current densities are obtained with CdSe. • Diffusion of S in Sb 2 Se 3 is demonstrated. • Se loss in the absorber for devices with CdS is shown. • Se Sb antisite is observed with CdSe is applied as buffer layer. • Sb 2 Se 3 show high stability after 7000 h under 1 sun and 80 °C. In this work, for the first time, CdSe is applied as a buffer layer on thermally evaporated Sb2Se3-based solar cells on superstrate configuration. The influence of this buffer layer on the growth of Sb 2 Se 3 and the performance of the finished devices have been analyzed and compared with the typical CdS/Sb 2 Se 3 junction. Selenium vacancies in Sb 2 Se 3 thin film act as recombination centres, for this reason applying an excessive amount of Se is beneficial to the performance of the devices. In the case of the CdS/Sb 2 Se 3 structure, the high concentration gradient enhances the selenium diffusion into the CdS and the sulphur diffusion into the Sb 2 Se 3 resulting in an increased number of defects. The replacement of CdS with CdSe would allow us to avoid this detrimental effect. In this work, we show that CdSe improves the external quantum efficiency in the entire light spectrum, increasing the average current density of the devices up to 2 mA/cm2 in comparison to CdS/Sb 2 Se 3. Also, the fill factor improves while the Voc slightly decreases due to the narrower band gap. Moreover, CdSe/Sb 2 Se 3 samples demonstrate excellent stability under accelerated stability tests. [ABSTRACT FROM AUTHOR]

We analyze the impact of scaling on the off-state, three-terminal, lateral breakdown of 100 V E-mode p-GaN/AlGaN/GaN HEMTs for low-voltage/low on-resistance applications. To this aim, we compared device structures with different SiO 2 dielectric thickness below the field plate, and varying gate-to-drain spacing and field-plate dimensions. The results indicate that: a) the breakdown voltage depends on the geometry and on the thickness of the dielectric under the field plate (t Si O 2 ) ; b) scaling the dielectric thickness increases the sensitivity of breakdown voltage to the gate-drain distance, while preserving device robustness (breakdown voltages above 150 V for a 100 V technology); c) for the devices with thinner dielectric, breakdown voltage scales linearly with gate drain distance, and with field plate length. A further analysis of the data reveals that the critical parameter in terms of reliability is the distance between the field-plate edge and the drain contact; d) scaling of the dielectric thickness does not enhance charge trapping phenomena. In summary, the results provide guidelines for scaling GaN HEMT device dimensions, while preserving reliability and immunity to charge trapping phenomena. • Breakdown voltage depends on geometry and on the thickness of the dielectric under the field plate; • Scaling dielectric thickness increases the sensitivity of breakdown voltage to the gate-drain distance preserving robustness; • Wafer with thinner dielectric is sensitive to either a variation in gate-drain spacing, or in field plate length; • Scaling of the dielectric thickness does not enhance charge trapping phenomena. [ABSTRACT FROM AUTHOR]

The aim of this paper is to improve the understanding of gallium nitride (GaN) high electron mobility transistors (HEMTs) submitted to hard switching operation, with focus on the hot-electron phenomena. This is becoming a hot-topic both for the scientific community and for the industry. The analysis is carried out through a cross-comparison of three different experimental techniques: conventional Pulsed-IV characterization, a novel pulsed-drain current transient (P-DCT) method, and a custom-developed hard switching test protocol. Hard switching analysis was performed through a novel system able to test the device in hard-switching conditions with an unprecedented turn-on slew-rate of 25 V ns−1 on-wafer level. This allows μs to investigate the impact of hard switching in terms of (i) locus trajectory, (ii) dissipated power, and (iii) dynamic increase. Furthermore, the accumulation of switching stress is assessed by repeating the experiment with increasing frequency, from 1 kHz to 100 kHz. The extensive cross-analysis offers a novel insight on the degradation mechanisms occurring in power GaN HEMTs. The results collected within this paper allow: (1) to evaluate the dynamic behavior under both soft- and hard-switching stress, thus differentiating off-state and semi-on-state stress; (2) to pinpoint hot-electrons as the main cause of the current collapse observed in semi-on; (3) by comparing the results obtained from P-DCT and Hard Switching Analysis we demonstrate that the hot-electron trapping is a very fast process which can happen in few ns. The related trapping and de-trapping kinetics are investigated in detail. The results described within this paper provide novel insight on the important role of hot-electrons in the dynamic increase during hard switching operations. [ABSTRACT FROM AUTHOR]

In this article, pulsed measurements of thermal resistance in GaN-based high-electron mobility transistors (HEMTs) on silicon, with different gate geometries and gate-to-drain extensions, are analyzed and modeled. Simple expressions for the thermal resistance of silicon-on-insulator (SOI) MOSFETs, which take into account the gate width and channel length, can be adapted to model the thermal resistance of these GaN-based HEMTs. Narrow width effects and the increase in the heat flow through the gate as the channel length increases were correctly reproduced. In addition, numerical simulations were performed to explain the reduction obtained in thermal resistance as the gate-to-drain extension increases. Our approach can also be applied easily to other well-established models using circuit simulators. [ABSTRACT FROM AUTHOR]