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

Theranostic Biomarkers for Schizophrenia

Schizophrenia is a highly heritable, chronic, severe, disabling neurodevelopmental brain disorder with a heterogeneous genetic and neurobiological background, which is still poorly understood. To allow better diagnostic procedures and therapeutic strategies in schizophrenia patients, use of easy accessible biomarkers is suggested. The most frequently used biomarkers in schizophrenia are those associated with the neuroimmune and neuroendocrine system, metabolism, different neurotransmitter systems and neurotrophic factors. However, there are still no validated and reliable biomarkers in clinical use for schizophrenia. This review will address potential biomarkers in schizophrenia. It will discuss biomarkers in schizophrenia and propose the use of specific blood-based panels that will include a set of markers associated with immune processes, metabolic disorders, and neuroendocrine/neurotrophin/neurotransmitter alterations. The combination of different markers, or complex multi-marker panels, might help in the discrimination of patients with different underlying pathologies and in the better classification of the more homogenous groups. Therefore, the development of the diagnostic, prognostic and theranostic biomarkers is an urgent and an unmet need in psychiatry, with the aim of improving diagnosis, therapy monitoring, prediction of treatment outcome and focus on the personal medicine approach in order to improve the quality of life in patients with schizophrenia and decrease health costs worldwide

Bologna guidelines for diagnosis and management of adhesive small bowel obstruction (ASBO) : 2017 update of the evidence-based guidelines from the world society of emergency surgery ASBO working group

Background: Adhesive small bowel obstruction (ASBO) is a common surgical emergency, causing high morbidity and even some mortality. The adhesions causing such bowel obstructions are typically the footprints of previous abdominal surgical procedures. The present paper presents a revised version of the Bologna guidelines to evidence-based diagnosis and treatment of ASBO. The working group has added paragraphs on prevention of ASBO and special patient groups. Methods: The guideline was written under the auspices of the World Society of Emergency Surgery by the ASBO working group. A systematic literature search was performed prior to the update of the guidelines to identify relevant new papers on epidemiology, diagnosis, and treatment of ASBO. Literature was critically appraised according to an evidence-based guideline development method. Final recommendations were approved by the workgroup, taking into account the level of evidence of the conclusion. Recommendations: Adhesion formation might be reduced by minimally invasive surgical techniques and the use of adhesion barriers. Non-operative treatment is effective in most patients with ASBO. Contraindications for non-operative treatment include peritonitis, strangulation, and ischemia. When the adhesive etiology of obstruction is unsure, or when contraindications for non-operative management might be present, CT is the diagnostic technique of choice. The principles of non-operative treatment are nil per os, naso-gastric, or long-tube decompression, and intravenous supplementation with fluids and electrolytes. When operative treatment is required, a laparoscopic approach may be beneficial for selected cases of simple ASBO. Younger patients have a higher lifetime risk for recurrent ASBO and might therefore benefit from application of adhesion barriers as both primary and secondary prevention. Discussion: This guideline presents recommendations that can be used by surgeons who treat patients with ASBO. Scientific evidence for some aspects of ASBO management is scarce, in particular aspects relating to special patient groups. Results of a randomized trial of laparoscopic versus open surgery for ASBO are awaited.Peer reviewe

Fast-switching Tri-Anode Schottky Barrier Diodes for monolithically integrated GaN-on-Si power circuits

Tri-Anode GaN Schottky Barrier Diodes (SBDs) have recently shown excellent DC performance with low turn-on voltage and large breakdown thanks to their 3D contact structure around the two-dimensional electron gas (2DEG) channel. However, the 3D nature of the Tri-Anode structure is also often believed to hinder the device switching performance. In this work, we demonstrate that, on the contrary, the Tri-Anode architecture significantly enhances the device switching performance with respect to conventional planar SBDs, as shown by a substantial decrease in the recovery charge and an improvement in frequency response. The Tri-Anode SBDs excellent static and dynamic performance is then applied to a real circuit to demonstrate a monolithically integrated high-frequency Full Bridge Rectifier. These results show the potential of Tri-Anode SBDs for high-efficiency and fast-switching power integrated circuits

Performance of GaN Power Devices for Cryogenic Applications Down to 4.2 K

Gallium nitride (GaN) power devices are employed in an increasing number of applications thanks to their excellent performance. Nevertheless, their potential for cryogenic applications, such as space, aviation, and superconducting systems, has not yet been fully explored. In particular, little is known on the device performance below liquid nitrogen temperature (77 K) and the behavior of popular GaN architectures such as gate injection transistor and Cascode below room temperature has not yet been reported. Most importantly, it is still unclear how the different device loss contributions, i.e., conduction, soft- and hard-switching losses, change at cryogenic temperatures. In this letter, we investigate and compare the performance of four GaN commercial power devices in a wide temperature range between 400 and 4.2 K. All of the tested devices can successfully operate at cryogenic temperatures with an overall performance improvement. However, different GaN HEMT technologies lead to significant variations in device gate control and loss mechanisms, which are discussed based on the device structure. The presented results prove the promising potential of the GaN technology for low-temperature applications and provide precious insights to properly design power systems operating under cryogenic temperatures and maximize their efficiency

Novel Slanted Field Plate Technology for GaN HEMTs by Grayscale Lithography on Flowable Oxide

We report a novel process to achieve slanted field plate (S-FP), which is a field plate with a gradual increase thickness from gate edge towards drain - utilizing grayscale lithography on flowable oxide (FOX) in single process step, in which developed FOX works as field plate dielectric. GaN-on-Si MOSHEMTs are fabricated by this technique. The breakdown voltage shows a significantly improvement by S-FP as a result of more uniform electric field distribution in the drift region. The S-FP MOSHEMT exhibits outstanding performance with a breakdown voltage (V BR ) of 832 V ( L GD = 5μm, at 1 μA/mm), state-of-the-art ON-resistance (R ON ) of 4.5 Ω·mm, and high-power figure-of-merit (FOM = V BR 2 /R ON,SP ) of 1.24 GW/cm 2 . This approach using a simple and flexible process to engineer the electric field in the MOSHEMT, offering a powerful technology for future advances in GaN high power devices

Enhancement-mode Multi-channel AlGaN/GaN Transistors with LiNiO Junction Tri-Gate

Multi-channel GaN power device, consisting of stacking multiple two-dimensional-electron-gas (2DEG) channels, has been demonstrated to achieve unprecedented on-state performance while maintaining high breakdown voltage (VBR). However, the large carrier density (Ns) makes it more challenging to achieve high positive threshold voltages (VTH) on multi-channel epitaxies. In this work, we demonstrate enhancement-mode (e-mode) multi-channel GaN transistors based on conformally deposited p-type LiNiO over tri-gates to form a multi-channel junction gate structure. Compared to the normal MOS gate, the p-type LiNiO junction gate provides an additional depletion of the channels to yield a more positive VTH, reaching a maximum VTH of 1.2 V (defined at 1 μA/mm). Moreover, high-quality LiNiO provided excellent on-state performance in multi-channel tri-gate devices with a stable operation at high temperature, which present small VTH shift and hysteresis, and low off-state leakage current. The e-mode devices in this work presented a small specific RON (RON, sp) of 0.62 mΩ∙cm-2 along with a hard breakdown voltage (VBR) of 920 V. This work demonstrates the potential of LiNiO for high-performance e-mode power devices

High-Voltage Normally-off Recessed Tri-Gate GaN Power MOSFETs With Low on-Resistance

In this letter, we present normally-off GaN-on-Si MOSFETs based on the combination of tri-gate with a short barrier recess to yield a large positive threshold voltage (VTH) while maintaining a low specific on resistance (R ON,SP ) and high current density (ID). The trigate structure offered excellent channel control, enhancing VTH from +0.3 V for the recessed to +1.4 V for the recessed tri-gate, along with a much reduced hysteresis in VTH, and a significantly increased transconductance (gm). Additional conduction channels at the sidewalls of the tri-gate trenches compensated the degradation in ON-resistance (R ON ) from the gate recess, resulting in a small R ON of 7.32 ± 0.26 Q·mm for L GD of 15 μm and an increase in the maximum drain current (I max D ). In addition, the tri-gate inherently integrates a gate-connected field-plate (FP), which improved the breakdown voltage (V BR ) and reduced the degradation in dynamic R ON . With proper passivation techniques, these devices could be very promising as high-performance power switches for future power applications