http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

Photovoltaic (PV) energy has grown at an average annual rate of 60% in the last five years, surpassing one third of the cumulative wind energy installed capacity, and is quickly becoming an important part of the energy mix in some regions and power systems. This has been driven by a reduction in the cost of PV modules. This growth has also triggered the evolution of classic PV power converters from conventional singlephase grid-tied inverters to more complex topologies to increase efficiency, power extraction from the modules, and reliability without impacting the cost. This article presents an overview of the existing PV energy conversion systems, addressing the system configuration of different PV plants and the PV converter topologies that have found practical applications for grid-connected systems. In addition, the recent research and emerging PV converter technology are discussed, highlighting their possible advantages compared with the present technology. Solar PV energy conversion systems have had a huge growth from an accumulative total power equal to approximately 1.2 GW in 1992 to 136 GW in 2013 (36 GW during 2013) [1]. This phenomenon has been possible because of several factors all working together to push the PV energy to cope with one important position today (and potentially a fundamental position in the near future). Among these factors are the cost reduction and increase in efficiency of the PV modules, the search for alternative clean energy sources (not based on fossil fuels), increased environmental awareness, and favorable political regulations from local governments (establishing feed-in tariffs designed to accelerate investment in renewable energy technologies). It has become usual to see PV systems installed on the roofs of houses or PV farms next to the roads in the countryside. Grid-connected PV systems account for more than 99% of the PV installed capacity compared to

An Overview of Recent Research and Emerging PV Converter Technology

Photovoltaic (PV) energy has grown at an average annual rate of 60% in the last five years, surpassing one third of the cumulative wind energy installed capacity, and is quickly becoming an important part of the energy mix in some regions and power systems. This has been driven by a reduction in the cost of PV modules. This growth has also triggered the evolution of classic PV power converters from conventional single-phase grid-tied inverters to more complex topologies to increase efficiency, power extraction from the modules, and reliability without impacting the cost. This article presents an overview of the existing PV energy conversion systems, addressing the system configuration of different PV plants and the PV converter topologies that have found practical applications for grid-connected systems. In addition, the recent research and emerging PV converter technology are discussed, highlighting their possible advantages compared with the present technology.

Grid-Connected Photovoltaic Systems: An Overview of Recent Research and Emerging PV Converter Technology

GaN-on-Si power switching transistors that use carbon-doped epitaxy are highly vulnerable to dynamic $\text{R}_{ON}$ dispersion, leading to reduced switching efficiency. In this paper, we identify the causes of this dispersion using substrate bias ramps to isolate the leakage paths and trapping locations in the epitaxy and simulation to identify their impact on the device characteristics. It is shown that leakage can occur both vertically and laterally, and we suggest that this is associated not only with bulk transport, but also with extended defects as well as hole gases at heterojunctions. For exactly the same epitaxial design, it is shown using a “leaky dielectric” model that depending on the leakage paths, dynamic $\text{R}_{ON}$ dispersion can vary between insignificant and infinite. An optimum leakage configuration is identified to minimize dispersion requiring a resistivity which increases with depth in the buffer stack. It is demonstrated that leakage through the undoped GaN channel is required over the entire gate to drain gap, and not just under the contacts, in order to fully suppress dispersion.

/pdf/leaky-dielectric-model-for-the-suppression-of-dynamic-r-2ho0i94ufm.pdf

“Leaky Dielectric” Model for the Suppression of Dynamic $R_{\mathrm{ON}}$ in Carbon-Doped AlGaN/GaN HEMTs

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.

https://hal.archives-ouvertes.fr/hal-03421528/document

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

Pulse behavior of insulated-gate double-field-plate power AlGaN/GaN HEMTs with C-doped buffers showing small current-collapse effects and dynamic RDS,on increase can accurately be reproduced by numerical device simulations that assume the CN-CGa autocompensation model as carbon doping mechanism. Current-collapse effects much larger than experimentally observed are instead predicted by simulations if C doping is accounted by dominant acceptor states. This suggests that buffer growth conditions favoring CN-CGa autocompensation can allow for the fabrication of power AlGaN/GaN HEMTs with reduced current-collapse effects. The drain-source capacitance of these devices is found to be a sensitive function of the C doping model, suggesting that its monitoring can be adopted as a fast technique to assess buffer compensation properties.

Influence of Buffer Carbon Doping on Pulse and AC Behavior of Insulated-Gate Field-Plated Power AlGaN/GaN HEMTs

Effects of residual C impurities and Ga vacancies on the dynamic instabilities of AlN/AlGaN/GaN metal insulator semiconductor high electron mobility transistors are investigated. Secondary ion mass spectroscopy, positron annihilation spectroscopy, and steady state and time-resolved photoluminescence (PL) measurements have been performed in conjunction with electrical characterization and current transient analyses. The correlation between yellow luminescence (YL), C- and Ga vacancy concentrations is investigated. Time-resolved PL indicating the CN ON complex as the main source of the YL, while Ga vacancies or related complexes with C seem not to play a major role. The device dynamic performance is found to be significantly dependent on the C concentration close to the channel of the transistor. Additionally, the magnitude of the YL is found to be in agreement with the threshold voltage shift and with the on-resistance degradation. Trap analysis of the GaN buffer shows an apparent activation energy of ∼0.8...

Impact of residual carbon impurities and gallium vacancies on trapping effects in AlGaN/GaN metal insulator semiconductor high electron mobility transistors

Effects of residual C impurities and Ga vacancies on the dynamic instabilities of AlN/AlGaN/GaN metal insulator semiconductor high electron mobility transistors are investigated. Secondary ion mass spectroscopy, positron annihilation spectroscopy, steady state and time-resolved photoluminescence (PL) measurements have been performed in conjunction with electrical characterization and current transient analyses. The correlation between yellow luminescence (YL), C- and Ga vacancy concentration is investigated. Time-resolved PL indicating the C$_{\mathrm{N}}$O$_{\mathrm{N}}$ complex as the main source of the YL, while Ga vacancies or related complexes with C seem not to play a major role. The device dynamic performance is found to be significantly dependent on the C concentration close to the channel of the transistor. Additionally, the magnitude of the YL is found to be in agreement with the threshold voltage shift and with the on-resistance degradation. Trap analysis of the GaN buffer shows an apparent activation energy of $\sim$0.8eV for all samples, pointing to a common dominating trapping process and that the growth parameters affect solely the density of trap centres. It is inferred that the trapping process is likely to be directly related to C based defects.

Impact of Residual Carbon Impurities and Gallium Vacancies on Trapping Effects in AlGaN/GaN MIS-HEMTs

Threshold voltage instabilities observed in GaN HEMTs designed for power switching applications when submitted to either DC or pulsed testing are here presented and interpreted. Main results can be summarized as follows: i) two acceptor trap levels, characterized by two well distinct time constants, are present in the UID GaN channel and C-doped GaN buffer respectively and behave as electron and hole traps respectively; ii) the trapped charge is modulated by the high voltage biasing of the gate and drain terminals; iii) when empty, channel electron traps induce a negative threshold-voltage shift, while buffer hole traps induce a positive threshold-voltage shift; iv) when the device is pulsed from off- to on-state conditions, trap charge/discharge dynamics induces negative and positive threshold-voltage instabilities over distinct time scales.

Gianmauro Pozzovivo

Papers

Influence of Buffer Carbon Doping on Pulse and AC Behavior of Insulated-Gate Field-Plated Power AlGaN/GaN HEMTs

Impact of residual carbon impurities and gallium vacancies on trapping effects in AlGaN/GaN metal insulator semiconductor high electron mobility transistors

Impact of Residual Carbon Impurities and Gallium Vacancies on Trapping Effects in AlGaN/GaN MIS-HEMTs

Threshold voltage instabilities in D-mode GaN HEMTs for power switching applications

GaN Power Semiconductors for PV Inverter Applications ¿ Opportunities and Risks