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

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

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

New electronic components based on AlN material for future power applications

Les semiconducteurs à large bande interdite tels que le GaN et SiC sont des matériaux de choix pour les applications de forte puissance. En effet, les propriétés du matériau GaN, notamment la haute densité et mobilité des électrons du gaz bidimensionnel des hétérostructures associées permettent de réaliser un excellent compromis entre la résistance à l’état passant (Ron) et la tension de claquage. De plus, les récents progrès en matière de croissance de GaN sur substrat silicium (111) laissent espérer l’intégration future de composants de forte puissance à bas coût avec des technologies matures de type CMOS. Afin de repousser davantage les limites des transistors à haute mobilité électronique (HEMT) en GaN pour la conversion de puissance, l’un des défis est de repousser la tenue en tension de cette filière. Dans ce cadre, nous avons, tout d’abord, étudié électriquement les couches tampons (buffer) par décomposition de l’empilement. Plusieurs hétérostructures ont été analysées dont la croissance a été stoppée à différents stades. De cette manière, nous avons été en mesure d'évaluer séparément le processus de conduction et de claquage de la couche de nucléation d'AlN, du buffer AlGaN et de l'empilement des couches jusqu'à une couche GaN dopée carbone. Une seconde étude a permis de développer un buffer à base de super-réseaux (pairs AlN/GaN ultrafins). Afin de mettre en évidence les avantages obtenus avec ce type de buffer une comparaison des caractérisations électriques avec un buffer standard a été réalisée.Ensuite, nous avons développé une approche innovante basée sur l’introduction d’une couche épaisse d’AlN au sein de tranches gravées suivie d’un dépôt par électrolyse de cuivre épais en face arrière. Le matériau AlN constitue une barrière de potentiel après le dépôt de l’électrode métallique sur la face-arrière, étape indispensable dans les convertisseurs de puissance de type DC/DC par exemple. Après avoir vérifié le bénéfice de cette solution en terme de tension de claquage, nous avons analysé son impact sur les pièges et les contraintes mécaniques.Enfin, partant du principe que l'électronique à base de matériaux à grands gaps tels que le GaN et le SiC arrivent à maturité, les matériaux à ultra large bande interdite tels que l'AlN (6,2 eV) ou l’AlGaN riche en Al, pourraient permettre de repousser les limites en tension ou en température. En outre, l'utilisation d'un buffer AlN permettrait à la fois d'augmenter le confinement des électrons dans le canal du transistor mais aussi d'améliorer la dissipation thermique. Nous avons donc mené une étude préliminaire sur différentes configurations de transistors à base d’AlN et de canaux en AlGaN.Wide band gap semiconductors such as GaN and SiC are the materials of choice for high power applications. Indeed, the properties of GaN-based material, namely the high density and electron mobility of the two-dimensional gas of the associated heterostructures, make it possible to achieve an excellent on-state resistance (Ron) and breakdown voltage trade-off. In addition, recent progress in the growth of GaN on silicon substrate (111) paves the way for the future integration of high power, low cost devices with mature CMOS technologies. In order to further push the limits of GaN high electron mobility transistors (HEMTs) for power conversion, one of the challenges is to increase the blocking voltage of these types of devices. In this frame, we first studied the buffer layers by implementing a stack decomposition. Several heterostructures were analyzed and the growth was stopped at different stages. In this way, we were able to evaluate separately the conduction and breakdown process of the AlN nucleation layer, the AlGaN buffer and the entire layer stack including a carbon-doped GaN layer. In a second study, a buffer based on superlattices (ultrathin AlN/GaN pairs) was developed. In order to highlight the advantages obtained with this type of buffer, a comparison with a standard buffer was carried out.Then, we developed an innovative approach based on the introduction of a thick layer of AlN within etched trenches followed by a thick copper electrodeposition on the backside of devices. The AlN material provides a potential barrier after the deposition of the metal electrode. After verifying the benefit of this solution in terms of breakdown voltage, we analyzed its impact on traps and mechanical stresses.Finally, assuming that electronics based on wide bandgap materials such as GaN and SiC are maturing, ultra wide band gap materials such as AlN (6.2 eV) or Al-rich AlGaN, could allow us to further push the limits in terms of voltage or temperature. In addition, the use of an AlN buffer would both increase electron confinement in the transistor channel and improve the thermal dissipation. We therefore conducted a preliminary study on various configurations of AlN-based transistors and AlGaN channels

Nouveaux composants électroniques à base du matériau AlN pour les futures applications de puissance

Wide band gap semiconductors such as GaN and SiC are the materials of choice for high power applications. Indeed, the properties of GaN-based material, namely the high density and electron mobility of the two-dimensional gas of the associated heterostructures, make it possible to achieve an excellent on-state resistance (Ron) and breakdown voltage trade-off. In addition, recent progress in the growth of GaN on silicon substrate (111) paves the way for the future integration of high power, low cost devices with mature CMOS technologies. In order to further push the limits of GaN high electron mobility transistors (HEMTs) for power conversion, one of the challenges is to increase the blocking voltage of these types of devices. In this frame, we first studied the buffer layers by implementing a stack decomposition. Several heterostructures were analyzed and the growth was stopped at different stages. In this way, we were able to evaluate separately the conduction and breakdown process of the AlN nucleation layer, the AlGaN buffer and the entire layer stack including a carbon-doped GaN layer. In a second study, a buffer based on superlattices (ultrathin AlN/GaN pairs) was developed. In order to highlight the advantages obtained with this type of buffer, a comparison with a standard buffer was carried out.Then, we developed an innovative approach based on the introduction of a thick layer of AlN within etched trenches followed by a thick copper electrodeposition on the backside of devices. The AlN material provides a potential barrier after the deposition of the metal electrode. After verifying the benefit of this solution in terms of breakdown voltage, we analyzed its impact on traps and mechanical stresses.Finally, assuming that electronics based on wide bandgap materials such as GaN and SiC are maturing, ultra wide band gap materials such as AlN (6.2 eV) or Al-rich AlGaN, could allow us to further push the limits in terms of voltage or temperature. In addition, the use of an AlN buffer would both increase electron confinement in the transistor channel and improve the thermal dissipation. We therefore conducted a preliminary study on various configurations of AlN-based transistors and AlGaN channels.Les semiconducteurs à large bande interdite tels que le GaN et SiC sont des matériaux de choix pour les applications de forte puissance. En effet, les propriétés du matériau GaN, notamment la haute densité et mobilité des électrons du gaz bidimensionnel des hétérostructures associées permettent de réaliser un excellent compromis entre la résistance à l’état passant (Ron) et la tension de claquage. De plus, les récents progrès en matière de croissance de GaN sur substrat silicium (111) laissent espérer l’intégration future de composants de forte puissance à bas coût avec des technologies matures de type CMOS. Afin de repousser davantage les limites des transistors à haute mobilité électronique (HEMT) en GaN pour la conversion de puissance, l’un des défis est de repousser la tenue en tension de cette filière. Dans ce cadre, nous avons, tout d’abord, étudié électriquement les couches tampons (buffer) par décomposition de l’empilement. Plusieurs hétérostructures ont été analysées dont la croissance a été stoppée à différents stades. De cette manière, nous avons été en mesure d'évaluer séparément le processus de conduction et de claquage de la couche de nucléation d'AlN, du buffer AlGaN et de l'empilement des couches jusqu'à une couche GaN dopée carbone. Une seconde étude a permis de développer un buffer à base de super-réseaux (pairs AlN/GaN ultrafins). Afin de mettre en évidence les avantages obtenus avec ce type de buffer une comparaison des caractérisations électriques avec un buffer standard a été réalisée.Ensuite, nous avons développé une approche innovante basée sur l’introduction d’une couche épaisse d’AlN au sein de tranches gravées suivie d’un dépôt par électrolyse de cuivre épais en face arrière. Le matériau AlN constitue une barrière de potentiel après le dépôt de l’électrode métallique sur la face-arrière, étape indispensable dans les convertisseurs de puissance de type DC/DC par exemple. Après avoir vérifié le bénéfice de cette solution en terme de tension de claquage, nous avons analysé son impact sur les pièges et les contraintes mécaniques.Enfin, partant du principe que l'électronique à base de matériaux à grands gaps tels que le GaN et le SiC arrivent à maturité, les matériaux à ultra large bande interdite tels que l'AlN (6,2 eV) ou l’AlGaN riche en Al, pourraient permettre de repousser les limites en tension ou en température. En outre, l'utilisation d'un buffer AlN permettrait à la fois d'augmenter le confinement des électrons dans le canal du transistor mais aussi d'améliorer la dissipation thermique. Nous avons donc mené une étude préliminaire sur différentes configurations de transistors à base d’AlN et de canaux en AlGaN

Next generation of GaN-on-Silicon power devices

International audienc

Low Buffer Trapping Effects above 1200 V in Normally off GaN-on-Silicon Field Effect Transistors

International audienceWe report on the fabrication and electrical characterization of AlGaN/GaN normally off transistors on silicon designed for high-voltage operation. The normally off configuration was achieved with a p-gallium nitride (p-GaN) cap layer below the gate, enabling a positive threshold voltage higher than +1 V. The buffer structure was based on AlN/GaN superlattices (SLs), delivering a vertical breakdown voltage close to 1.5 kV with a low leakage current all the way to 1200 V. With the grounded substrate, the hard breakdown voltage transistors at VGS = 0 V is 1.45 kV, corresponding to an outstanding average vertical breakdown field higher than 2.4 MV/cm. High-voltage characterizations revealed a state-of-the-art combination of breakdown voltage at VGS = 0 V together with low buffer electron trapping effects up to 1.4 kV, as assessed by means of substrate ramp measurements

GaN-on-silicon transistors with reduced current collapse and improved blocking voltage by means of local substrate removal

International audienceWe report on the demonstration of low trapping effects above 1200 V of GaN-on-silicon transistors using a local substrate removal (LSR) followed by a thick backside ultra-wide-bandgap AlN deposition. Substrate ramp measurements show reduced hysteresis up to 3000 V. It has been found that the LSR approach not only enable to extend the operation voltage capabilities of GaN-on-Silicon HEMTs with low on-resistance but also allow to reduce trapping effects directly affecting their dynamic behavior. This work points out that a large part of the electron trapping under high bias occurs at the AlN nucleation layer and Si substrate interface

The Effects of AlN and Copper Back Side Deposition on the Performance of Etched Back GaN/Si HEMTs

International audienc

AlGaN channel high electron mobility transistors with regrown ohmic contacts

International audienceHigh power electronics using wide bandgap materials are maturing rapidly, and significantmarket growth is expected in a near future. Ultra wide bandgap materials, which have an even largerbandgap than GaN (3.4 eV), represent an attractive choice of materials to further push the performancelimits of power devices. In this work, we report on the fabrication of AlN/AlGaN/AlN high-electronmobility transistors (HEMTs) using 50% Al-content on the AlGaN channel, which has a much widerbandgap than the commonly used GaN channel. The structure was grown by metalorganic chemicalvapor deposition (MOCVD) on AlN/sapphire templates. A buffer breakdown field as high as5.5 MV/cm was reported for short contact distances. Furthermore, transistors have been successfullyfabricated on this heterostructure, with low leakage current and low on-resistance. A remarkablethree-terminal breakdown voltage above 4 kV with an off-state leakage current below 1μA/mm wasachieved. A regrown ohmic contact was used to reduce the source/drain ohmic contact resistance,yielding a drain current density of about 0.