Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Two-dimensional (2D) materials have attracted increasing research interest because of the abundant choice of materials with diverse and tunable electronic, optical, and chemical properties. Moreover, 2D material based heterostructures combining several individual 2D materials provide unique platforms to create an almost infinite number of materials and show exotic physical phenomena as well as new properties and applications. To achieve these high expectations, methods for the scalable preparation of 2D materials and 2D heterostructures of high quality and low cost must be developed. Chemical vapor deposition (CVD) is a powerful method which may meet the above requirements, and has been extensively used to grow 2D materials and their heterostructures in recent years, despite several challenges remaining. In this review of the challenges in the CVD growth of 2D materials, we highlight recent advances in the controlled growth of single crystal 2D materials, with an emphasis on semiconducting transition meta...

Chemical Vapor Deposition Growth and Applications of Two-Dimensional Materials and Their Heterostructures

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

/pdf/the-2018-gan-power-electronics-roadmap-2iunv619fh.pdf

The 2018 GaN power electronics roadmap

Gallium nitride (GaN) power devices are an emerging technology that have only recently become available commercially. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This paper reviews the characteristics and commercial status of both vertical and lateral GaN power devices, providing the background necessary to understand the significance of these recent developments. In addition, the challenges encountered in GaN-based converter design are considered, such as the consequences of faster switching on gate driver design and board layout. Other issues include the unique reverse conduction behavior, dynamic $R_{\mathrm {{ds}},\mathrm {{on}}}$ , breakdown mechanisms, thermal design, device availability, and reliability qualification. This review will help prepare the reader to effectively design GaN-based converters, as these devices become increasingly available on a commercial scale.

Review of Commercial GaN Power Devices and GaN-Based Converter Design Challenges

https://absimage.aps.org/image/MAR14/MWS_MAR14-2014-020819.pdf

Van der Waals heterostructures

This work studies the leakage and breakdown mechanisms of 1.2 kV GaN vertical power FinFETs with edge termination. Two competing leakage and breakdown mechanisms have been identified. The first mechanism is dominated by the electric field, with the leakage current dominated by the electric field in the drift region and destructive breakdown voltage by the peak electric field at the edge termination. The second leakage and breakdown mechanism is controlled by an energy (or potential) barrier in the fin channel. This energy barrier suffers from the drain-induced barrier lowering (DIBL) effect and is highly dependent on gate/drain biases, fin geometries, and GaN/oxide interface charges. The electrons injected into the drift region due to the DIBL effect further lead to trap-assisted space-charge-limited conduction, which results in a nondestructive early breakdown. The barrier height in the fin channel determines which mechanism is dominant; the same device could show either destructive or nondestructive breakdown at different gate biases. To enable the normally off power switching, it is important to suppress the leakage from the second mechanism and maintain a sufficiently high energy barrier in the fin channel up to high drain voltages. Finally, the key device parameters determining the energy barrier in the fin channel have been identified. The findings in this work provide critical device understanding and design guidelines for GaN vertical power FinFETs and other “junctionless” vertical high-voltage power transistors.

Leakage and breakdown mechanisms of GaN vertical power FinFETs

The output capacitance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C OSS ) loss, a loss produced when the device's output capacitor is charged and discharged, has become a concern for GaN high electron mobility transistors (HEMTs) in high-frequency applications. This work presents a new, easy-to-implement method for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C OSS loss characterization based on the unclamped inductive switching (UIS) setup. As compared to prior approaches, this method involves the device's ON-state conduction and could measure the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C OSS loss in a single pulse and the steady-state switching. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C OSS loss of three types of mainstream commercial GaN HEMTs is characterized, which exhibit some common dependencies including a non-monotonic relation with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv/dt (or resonance frequency), a linear relation with the ON-state current, a power-law relation with the peak blocking voltage, and little temperature dependence. In addition, their <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C OSS losses all show minimal distinctions in a single pulse and the steady-state switching, despite the increased on-resistance in the steady-state switching. This suggests that the traps accounting for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C OSS loss possess different de-trapping time constants as compared to the traps governing the dynamic on-resistance. Finally, a unified model is established to describe the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C OSS loss of all three types of GaN HEMTs. These results provide important references for the high-frequency application of GaN HEMTs and new insights into the physical origin of their <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C OSS loss.

Output Capacitance Loss of GaN HEMTs in Steady-State Switching

An essential ruggedness characteristic of power devices is the capability to withstand the transient overvoltage in power electronic applications. As the newly commercialized GaN HEMTs can switch faster, and under higher voltage biases, it is important to identify their true overvoltage limitations in transient switching events. For the first time, this work characterizes the transient overvoltage capability and failure mechanisms of GaN HEMTs under hard-switched turn-off conditions at increasing temperatures, by using a clamped inductive switching circuit with a variable parasitic inductance. This test method allows a flexible control over both the magnitude and the dV/dt of the transient overvoltage. The overvoltage robustness of two commercial enhancement-mode (E-mode) p-gate HEMTs was extensively studied: one with the Ohmic-type gate and the other with the Schottky-type gate. The overvoltage failure of the two devices was found to be determined by the overvoltage magnitude rather than the dV/dt. Tests were repeated at increasing temperatures (100 °C and 150 °C), and the failures of both devices were consistent with room temperature results. The two types of devices show different failure behaviors and the underlying mechanisms have been revealed by physics-based device simulations.

Hard-Switched Overvoltage Robustness of p-Gate GaN HEMTs at Increasing Temperatures

GaN high electron mobility transistors (HEMTs) have limited avalanche capability and usually fail catastrophically in voltage overshoot up to their dynamic breakdown voltage (BVdyn). This article presents the first comparative study of the parametric shift and recovery of three mainstream GaN HEMTs in repetitive overvoltage switching near their BVdyn. In each switching cycle, a voltage overshoot up to 90% of BVdyn was applied during the turn-off process. As the switching prolongs, all devices showed shifts in threshold voltage and saturation current, and these shifts saturated in less than 1-million cycles. These shifts are believed to be induced by the trapping of the holes generated in the impact ionization (I. I.). The device's poststress recovery was found to be dominated by the hole detrapping and through-gate removal, which highly depends on the gate stack. The gate injection transistor showed a fast natural recovery benefitted from the efficient hole removal through the Ohmic gate. The hole detrapping in the Schottky-type p-gate HEMT can be described by the Poole–Frenkel emission, allowing for the accelerated recoveries at negative gate bias and high temperatures. The hole removal in the metal-insulator-semiconductor (MIS) HEMT is blocked by the gate insulator, preventing a natural recovery. The MIS-HEMT can be recovered by applying positive gate and substrate biases, which facilitate the hole recombination in the channel. This article shows the good overvoltage robustness of all three GaN HEMTs and unveils effective methods for their poststress recovery, as well as suggests the significant impacts of I. I. and hole dynamics on the overvoltage ruggedness of GaN HEMTs near BVdyn.

Degradation and Recovery of GaN HEMTs in Overvoltage Hard Switching Near Breakdown Voltage

GaN HEMTs have no avalanche capability and withstand the surge energy through capacitive charging, which often causes significant voltage overshoot up to their catastrophic limit. While the single-event overvoltage failure has been recently studied for GaN HEMTs, it is still unclear on their robustness under repetitive overvoltage and surge-energy events. This work studies the robustness of the GaN gate injection transistor (GIT) under repetitive overvoltage events implemented at the device hard-switched turn-off. A clamped, inductive switching circuit with a 400 V dc bias and a controllable parasitic inductance are used to generate the overvoltage stress events with different overvoltage magnitude up to 1050 V (90% of the device destruction limit), different switching periods down to 0.33 ms, and different temperatures up to 150 °C. The electrical parameters were characterized and monitored between stress cycles. The 650-V, 31-A rated GITs show no failure or permanent degradation after 1-million overvoltage events at different switching periods, or elevated temperatures. These results suggest the great potential of GaN GITs for applications that demand stressful operating conditions such as electrical vehicle powertrains.

Robustness of GaN Gate Injection Transistors under Repetitive Surge Energy and Overvoltage

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