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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 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]

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]

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]

We present an extensive investigation of the charge-trapping processes in vertical GaN nanowire FETs with a gate-all-around structure. Two sets of devices were investigated: Gen1 samples have unipolar (n-type) epitaxy, whereas Gen2 samples have a p-doped channel and an n-p-n gate stack. From experimental results, we demonstrate the superior performance of the transistor structure with a p-GaN channel/Al2O3 gate insulator in terms of dc performance. In addition, we demonstrate that Gen2 devices have highly stable threshold voltage, thus representing ideal devices for power electronic applications. Insight into the trapping processes in the two generations of devices was obtained by modeling the threshold voltage variations via differential rate equations. [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]

This work describes an experiment on degradation mechanisms of InGaN light-emitting diode (LED) test structures which do not fulfill the requirements of longlife products. We present a combined capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), electroluminescence (EL), and cathodoluminescence (CL) study of short-term instabilities of InGaN/GaN LEDs submitted to low current aging tests at room temperature. In the early stages of the aging tests, the EL and CL characterizations showed an optical power decrease, more prominent at low current levels. The C-V profiles indicated that the stress induced an apparent charge increase, well related to the deep level changes detected by DLTS and to the optical power decrease. It is supposed that the main cause of the degradation is the generation of nonradiative paths, due to the generation/propagation of defects activated by carrier transport. [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.

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]

Electroluminescence (EL) characterization of InGaN/GaN light-emitting diodes (LEDs), coupled with numerical device models of different sophistication, is routinely adopted not only to establish correlations between device efficiency and structural features, but also to make inferences about the loss mechanisms responsible for LED efficiency droop at high driving currents. The limits of this investigative approach are discussed here in a case study based on a comprehensive set of currentand temperature-dependent EL data from blue LEDs with low and high densities of threading dislocations (TDs). First, the effects limiting the applicability of simpler (closed-form and/or one-dimensional) classes of models are addressed, like lateral current crowding, vertical carrier distribution nonuniformity, and interband transition broadening. Then, the major sources of uncertainty affecting state-ofthe- art numerical device simulation are reviewed and discussed, including (i) the approximations in the transport description through the multi-quantum-well active region, (ii) the alternative valence band parametrizations proposed to calculate the spontaneous emission rate, (iii) the difficulties in defining the Auger coefficients due to inadequacies in the microscopic quantum well description and the possible presence of extra, non-Auger high-current-density recombination mechanisms and/or Auger-induced leakage. In the case of the present LED structures, the application of three-dimensional numerical-simulation-based analysis to the EL data leads to an explanation of efficiency droop in terms of TD-related and Auger-like nonradiative losses, with a C coefficient in the 10-30 cm6/s range at room temperature, close to the larger theoretical calculations reported so far. However, a study of the combined effects of structural and model uncertainties suggests that the C values thus determined could be overestimated by about an order of magnitude. This preliminary attempt at uncertainty quantification confirms, beyond the present case, the need for an improved description of carrier transport and microscopic radiative and nonradiative recombination mechanisms in device-level LED numerical models. [ABSTRACT FROM AUTHOR]

This paper reports on an extensive analysis of the degradation of AlGaN/GaN high electron mobility transistors (HEMTs) submitted to on-state and off-state stresses. The results of this study indicate that: (i) the analyzed devices do not show any degradation when submitted to off-state stress tests up to 100 V; (ii) during on-state stress, devices show a significant decrease in drain current and in the electroluminescence (EL) signal; (iii) degradation rate is strongly dependent on the intensity of the EL signal measured during stress on the devices, which is related to the concentration of hot electrons in the channel. Based on the experimental evidence collected within this paper, the on-state degradation of HEMTs is ascribed to the trapping of negative charge in the gate-drain access region, triggered by hot electrons. By considering the intensity of the EL signal as a measure of the stress acceleration factor, we have derived an acceleration law for the hot-electron induced degradation. [ABSTRACT FROM AUTHOR]

This paper describes a detailed analysis of the time-dependent degradation kinetics of GaN-based high electron mobility transistors submitted to reverse-bias stress. We show that: (1) exposure to reverse-bias may induce recoverable changes in gate leakage and threshold voltage, due to the accumulation of negative charge within the AlGaN layer, and of positive charge at the AlGaN/GaN interface. (2) Permanent degradation consists in the generation of parasitic leakage paths. Several findings support the hypothesis that permanent degradation is due to a defect percolation process: (2(a)) for sufficiently long stress times, degradation occurs even below the 'critical voltage' estimated by step stress experiments; (2(b)) before permanent degradation, gate current becomes noisy, indicating an increase in defect concentration; and (2(c)) time to breakdown strongly depends on the initial defectiveness of the samples. [ABSTRACT FROM AUTHOR]

Physical mechanisms causing the efficiency droop in InGaN/GaN blue light-emitting diodes and remedies proposed for droop mitigation are classified and reviewed. Droop mechanisms taken into consideration are Auger recombination, reduced active volume effects, carrier delocalization, and carrier leakage. The latter can in turn be promoted by polarization charges, inefficient hole injection, asymmetry between electron and hole densities and transport properties, lateral current crowding, quantum-well overfly by ballistic electrons, defect-related tunneling, and saturation of radiative recombination. Reviewed droop remedies include increasing the thickness or number of the quantum wells, improving the lateral current uniformity, engineering the quantum barriers (including multi-layer and graded quantum barriers), using insertion or injection layers, engineering the electron-blocking layer (EBL) (including InAlN, graded, polarization-doped, and superlattice EBL), exploiting reversed polarization (by either inverted epitaxy or N-polar growth), and growing along semi- or non-polar orientations. Numerical device simulations of a reference device are used through the paper as a proof of concept for selected mechanisms and remedies. [ABSTRACT FROM AUTHOR]

This paper reports an extensive analysis of the degradation of deep-ultraviolet light-emitting diodes submitted to dc stress test. The study was carried out by means of combined electrical and optical characterization techniques. Results described in the paper indicate that the following: (i) stress can induce a significant decrease in the optical power emitted by the devices; (ii) optical power decrease is more prominent at low measuring current levels, thus suggesting that degradation is related to an increase in the concentration of defects; (iii) stress induces a significant increase in the green-yellow parasitic emission of the devices; and (iv) stress causes a localized increase in the apparent charge distribution profiles in the active region of the devices. Experimental evidence collected within this work suggest that degradation is due to an increase in the defectiveness in the active layer of the devices, with subsequent worsening of their radiative efficiency. [ABSTRACT FROM AUTHOR]

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]

We report on a detailed investigation of the degradation of AlGaN/GaN Schottky diodes grown on silicon, submitted to high reverse-bias. The analyzed devices have a vertical structure; thanks to this feature, it was possible (i) to characterize the effects of stress by means of capacitance-voltage (C-V) measurements, therefore, identifying and localizing the trap states generated as a consequence of the stress tests; (ii) to accurately control the intensity and distribution of the electric field over stress time. Results indicate that stress induces an increase in the leakage current, which is well correlated to the increase of a new capacitance peak in the C-V characteristics. Based on experimental data and bidimensional simulations, degradation is ascribed to the generation of donor traps in the GaN buffer, close to the AlGaN/GaN interface. [ABSTRACT FROM AUTHOR]