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As compound semiconductor devices become more advanced, their ultimate performance becomes increasingly dependent on nano to micro scale structural defects in their constituent materials. These defects, such as dislocations, stacking faults, and anti-phase boundaries often act as recombination centers in devices, limiting efficiency. Characterization of these defects – their type (such as edge vs. screw vs. mixed dislocations), their relative densities, how they interact with other defects, what growth conditions are more or less likely to propagate them, how they relate to device efficiencies and performance, etc. – is vital to mitigate their negative effects and even sometimes exploit their potential benefits depending on the desired application. In this contribution, we will discuss the use of electron channeling contrast imaging (ECCI) as an ideal characterization method for compound semiconductors and devices as well as some recent applications which include ECCI characterization in tandem with other characterization methods for a broader material understanding. Transmission electron microscopy (TEM) is a widely used technique for characterizing extended defects and local crystal orientation. However, TEM analysis usually entails thinning a sample via focused ion beam milling or chemical/mechanical polishing to around 50-100 nm, which is time consuming and only allows a relatively small area of the sample to be characterized. These drawbacks can cause bottlenecks in research progress and even render TEM impractical for certain applications. ECCI performed in a scanning electron microscope (SEM) can bypass these issues as it requires little to no sample preparation. This means ECCI can easily characterize over large areas in a fraction of the time as TEM analysis. [1-3] These benefits mean that ECCI gives more accurate analysis by not missing larger trends that can be missed with the small samples in TEM as well as by not introducing artifacts from the sample preparation. Here we highlight applications of ECCI for characterization of two very different compound semiconductors of growing importance -- Cd3As2 and GaN. Firstly, we consider Cd3As2 thin films of use as topological quantum materials, where the Cd3As2 can be either a compound semiconductor or topological Dirac semimetal, depending on the structural phase or mode of use. The ECCI of MOCVD-grown Cd3As2 shown in this work finds that the developmental thin films are not completely single crystalline. [4] Moreover, two different spatial scales of domain formation are seen. At lower magnification, we see roughly circular-shaped larger domains that are ~ 2-5 mm in diameter. Some of these domains are more prominently defined and feature single dark spots at their center, which at first suggests threading dislocations, but turns out to be occasional defect-pits formed in concert with numerous, indistinct threads. At higher magnification, we see much smaller dot-like or speck-like domains that are ~ 100 nm or less in diameter. The wide-area electron channeling patterns used to orient the sample for the ECCI imaging were visible, but rather indistinctly resolved, which is consistent with small crystallographic misorientations of the ensemble of domains composing the film. Secondly, we consider bulk GaN wafers entering use in a wide variety of functional applications, including GaN high-voltage diodes for power electronics. We will show novel “star” defects appearing in the GaN substrates when characterized by both ECCI and high-resolution electron backscattered diffraction (EBSD). [5] Through a more complete structural understanding of these star defects, we seek to improve understanding of the impact of these defects on device performance, which may in turn facilitate development of strategies for their mitigation or removal. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Wide-bandgap semiconductors have a significant advantage over conventional Si-based electronics by leveraging materials properties to achieve higher breakdown voltage, lower on-resistance, and high-frequency operation. For electric vehicle drivetrains, this translates to higher efficiency and power density, resulting in more miles driven per charge. This move towards wide-bandgap power electronics is necessary to achieve the U.S. Department of Energy (DOE) power electronics density target of 100 kW/L. Vertical gallium nitride-based power devices are expected to exceed Si and even SiC-based systems with the promise of increased performance and power density. Compared to lateral GaN devices, a vertical topology promotes more efficient scaling towards high-power applications, where both high voltage and high current are necessary. This talk describes our team’s effort towards developing vertical GaN MOSFETs. The results of Sandia’s first-generation device demonstrator serve as a milestone in the path of producing devices rated for 1200-V and 100-A operation. The vertical GaN trench MOSFET is unique compared to Si- or SiC-alternatives in that the doped layers comprising the source and body regions are grown by epitaxy rather than formed by ion implantation. Challenges with selective-area doping in GaN add additional complexity to the design of a trench MOSFET. In addition, the lack of a high-quality native oxide in GaN means that the gate dielectric must be deposited rather than thermally grown. The devices produced at Sandia rely on atomic-layer-deposited thin films for the gate dielectric (primarily Al2O3 or SiO2). First-generation results demonstrate devices capable of 400 mA/mm drain current, 108 on/off ratio, and a positive threshold voltage near 8 V. More recently, devices capable of blocking 500 V in the off-state have been demonstrated. Device failure in the off-state results from high fields in the gate dielectric, which can be minimized by reducing the trench etch depth or by increasing the voltage rating of the drift region. However, further shielding of the gate dielectric to achieve substantially higher off-state voltages requires significant changes to the device architecture which are reliant on selective-area doping. In addition, device-killing defects either from the starting substrate, the epitaxy, or defects introduced during processing limit yield for large-area devices and present a substantial obstacle to scale devices for high-current operation. Hence, methods for reducing cell pitch and increasing packing density are highly valued. In this talk, we will discuss the path forward for achieving higher breakdown voltages and high-current operation using GaN-specific strategies to achieve better performing devices, as well as some of the challenges for vertical GaN development. This work provides a foundational platform for developing next-generation power electronics that employ wide bandgap, gallium nitride semiconductors. This work was supported by the Electric Drivetrain Consortium managed by Susan Rogers of DOE’s Vehicle Technologies Office. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Figure 1

Vertical GaN power devices have emerged to become promising candidates for next-generation high power applications due to superior material properties such as high breakdown voltage, low on-resistance, and high mobility compared to devices based on Si and SiC. GaN-based p-n junction switching devices enable higher voltage power with significantly higher efficiencies with added advantages of reduced size and weight systems. A technological limitation of GaN, however, has been the inability to achieve high p-type doping in a planar, vertical device. Here, we will focus on recent developments to achieve high p-type efficiency though ion implantation, novel high temperature annealing schemes, and the importance of defects and morphology in native substrates and epitaxial layers GaN epitaxial layers for subsequent p-type ion implantation can be grown on sapphire substrates but vertical devices are optimal when using GaN homoepitaxial structures. Unlike most other semiconductor substrates, GaN substrates are not grown from the melt. Hydride vapor phase epitaxy or ammonothermal growth are predominantly used to produce substrates and these substrates will experience higher temperatures during subsequent epitaxy and ion implant activation annealing than occurs during substrate formation. In both types of substrates, the threading dislocation distribution can vary by orders of magnitude – these variations correlate well with leakage currents in vertical devices. The activation of the implanted p-type dopants requires high temperature annealing. For most semiconductors, an annealing temperature of ~ 2/3 the melting point leads to a high activation fraction (> 95%) of the implanted species. The implantation process introduces large concentrations of native defects as well as the p-type implant (typically Mg), so it is necessary to remove the defects and for the dopants to locate on substitutional sites (Ga sites for Mg). We demonstrate how the formation of inverted domain defects at temperatures below 1400 °C correlate with Mg-compound formation, whereas annealing temperatures 1400 °C and above do not lead to the formation of such compounds but rather high Mg activation efficiencies. These recent developments, partly through the ARPA-E PNDIODES program as well as international efforts, have brought understanding of the key processing steps and substrate requirements to achieve high activation efficiency p-type doping for planar, vertical device structures in a scalable framework.

High critical electric field (EC) and a large saturation velocity in ultra-wide bandgap transistors (UWBG) such as Al-rich AlGaN and ß-Ga2O3 is stimulating research into these devices [1,2]. This research is aimed at producing devices that may one day excel in power switching [3] and RF applications [4]. Figure-of-merit (FOM) analysis supports the possibility that Al-rich AlGaN transistors will outperform other WBG and UWBG transistors [5,6]. In spite of this promise, the UWBG devices do not yet perform well enough reach the promise evidenced by the FOM analysis. The favorable resolution of several important issues may be the key to broad acceptance of Al-rich AlGaN transistors. Until that time, Al-rich AlGaN transistors will begin to find utility in less mainstream uses. Recent developments are encouraging for the acceptance of Al-rich AlGaN in harsh environments. This talk will describe multiple research efforts to develop Al-rich AlGaN into the material of choice for next-generation power electronics applications and compare these research results to other WBG transistors where appropriate. These transistors generally benefit from the materials and fabrication similarity to AlGaN/GaN. They are grown epitaxially in the same metal organic chemical vapor deposition reactors and on mostly the same substrates. They will also benefit from similar fabrication infrastructure that is in place for AlGaN/GaN. Likewise, the AlGaN alloys support a wide diversity of device types. Depending on the targeted application, the AlGaN transistors may be depletion-mode (normally-on) or enhancement-mode (normally-off) transistors. Diversity in device types also includes various types of device designs that have been investigated, including metal-semiconductor field effect transistors, high electron mobility transistors (HEMTs), metal-insulator-semiconductor transistors, metal-oxide transistors, and polarization doped transistors. We will also describe the progress in maturing this transistor technology along a number of fronts. They are improving towards high current density at a rapid rate through efforts to improve the mobility, efforts to improve the Ohmic contacts, and by means of aggressively scaling the device dimensions. The particular strengths of the Al-rich AlGaN transistor include high electric field at breakdown, which is already more than 2-times that of GaN, the high ION/IOFF ratio in the transistor, now exceeding 109 in multiple devices, and in their high temperature operation with multiple reports in the 300-500°C range. A recent comparison of an Al0.85Ga0.15N/Al0.7Ga0.3N HEMT against AlGaN/GaN showed a superior ION/IOFF ratio at 500°C and also shows a favorable extrapolation to the 500-800°C temperature range for Al0.7Ga0.3N but not for GaN [7]. Such results are beginning to suggest that AlGaN may one day become the preferred transistor for operation at extreme temperatures. Details regarding ongoing work to optimize device performance for Al-rich AlGaN transistors will be presented in the talk. [1] A.G. Baca et al., J. Vac. Science and Technology, 40, 1600501 (2020). [2] M. Higashiwaki and G. H. Jessen, Appl. Phys. Lett. 112, 060401 (2018). [3] R. J. Kaplar et al., J. Solid-State Sci. Technol., 6, Q3061 (2017). [4] R. J. Kaplar et al., Proc. GOMAC, Albuquerque, NM, USA (2019), pp. 587-591. [5] Coltrin et al., J. Solid-State Sci. Technol., 6, S3114 (2017). [6] S. Bajaj et al., Appl. Phys. Lett. 105, 263503 (2014). [7] P. H. Carey et al., J. Electron Devices Society, 7, 444 (2019). This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

High breakdown voltage and low on-state resistance are important factors that have enabled improved performance and efficiency of power switching transistors. Based on wide-bandgap (WBG) semiconductors, these transistors have recently entered commercial applications. The next generation of power transistors utilize a new class of semiconductor materials with bandgaps (Eg ) greater than that of GaN (3.4 eV), the “ultra” wide-bandgap (UWBG) semiconductors, Eg = 3.4 – 6.2 eV [1-3]. In particular, Al-rich AlGaN alloys and b-Ga2O3 represent two UWBG semiconductors with considerable recent interest [2,3]. The large bandgap of an UWBG semiconductor is an important factor in determining the critical electric field (EC , scaling as approximately as Eg 2.5 [2]), which not only affects breakdown, but also confers operability advantages in harsh environments embodied by 500°C temperatures [4]. We will discuss the progress and challenges for maturing the Al-rich AlGaN transistors (the percent of Al approaches 100%, e.g. AlN) to the point where they represent viable technology choices for extreme environments, and perhaps more broadly. After presenting the highlights of the Al-rich transistor research, attention will be given to reliability considerations, which are motivated by the ability to influence early technology decisions. Thus, the purpose of reliability research into emerging technologies is to provide early data that impacts material and process choices for the Al-rich transistors and not to attempt reliability determinations, per se. Step-stress experiments are one of the main tools available for undertaking early reliability analysis. Examples of both enhancement-mode [5] and depletion-mode HEMT step-stress experiments will be described and analyzed. Such experiments allow quick examination of multiple types of stress, including, those absent of drain current and those with varying concentrations of hot electrons, all while sampling progressively increasing magnitude of gate- or drain-bias derived electric fields. In this manner, one discovers parametric boundaries affecting degradation and may identify the most relevant follow-up experiments. Step-stress experiments may involve both reversible parametric effects and permanent ones. The reversible effects are those that heal or re-occur with abrupt changes in bias conditions and are associated with carrier capture and emission processes. They have the potential to confound the reliability analysis because considerable parametric changes can be observed. They must be accounted for so that proper interpretation of potential degradation is made. They also have the potential to reveal important details of how the capture and emission processes affect device operation. Although many examples of these experiments will highlight parallels with conventional AlGaN/GaN transistors, some notable differences unique to Al-rich semiconductors will be described as well. [1] J.Y. Tsao et al., Advanced Electronic Materials, 4, 1600501 (2018). [2] R. J. Kaplar et al., ECS J. Solid State Science and Technology, 6, Q3061 (2017). [3] M. Higashiwaki and G. H. Jessen, Appl. Phys. Lett. 112, 060401 (2018). [4] P. H. Carey et al., J. Electron Devices Society, 7, 444 (2019). [5] A.G. Baca et al., Int. Reliability Physics Symp., P.WB.2 (2019). This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Superior electronic material properties of GaN semiconductors compared to Si will enable the foundational diodes and transistors for the next revolution in power management technology. To date, high-performance GaN diodes have been demonstrated with breakdown voltages near 5 kV and specific on-resistances of less than 5 mΩ-cm2 [1]. However, a viable GaN vertical power switching transistor is the missing element necessary to realize advanced power management systems utilizing GaN. While GaN transistors are now ubiquitous in RF systems, their lateral device architecture inherently limits these devices to niche applications in power systems. Rather, high-voltage systems rely on vertical device architectures where current flow is normal to the semiconductor surface. Vertical power electronics devices such as junction field effect transistors (JFETs) require selective areas of p-type semiconductor surrounded by lightly-doped n-type semiconductor. For Si power devices, p-type regions are implanted in n-type drift regions to create p/n junctions that act as a gate and pinch-off current. Ion implantation presents challenges in GaN and requires specialized equipment and processes, including high-pressure and high-temperature annealing, for dopant activation [2]. Our previous work using metal-organic chemical vapor deposition (MOCVD) where p-type GaN was regrown on a lightly-doped, non-etched n-type drift layer demonstrated planar p/n diodes with the same electrical performance as continuously-grown devices. However, when dry-etching processes are used before the regrowth of the p-type GaN layer, as would be needed to define a regrown area, planar diodes demonstrate higher forward and reverse leakage currents, and lower breakdown voltage. Deep Level Optical Spectroscopy (DLOS) was used to identify the source of the higher forward and reverse leakage currents observed in diodes with p-type anodes re-grown on inductively coupled plasma (ICP) dry-etched n-type drift layers. We will report DLOS studies of various diode structures that show the formation of a mid-gap trap located ~ 1.7 eV below the conduction band in dry-etched, lightly doped (Nd-Na = 2x1016 cm-3) drift layers. We also will show that various wet-chemical treatments of the dry-etched n-type drift layer prior to p-anode regrowth results in diodes with 1 to 2 orders of magnitude lower reverse leakage current. DLOS measurements of regrown diodes showed a 3 times reduction in the density of the mid-gap deep level trap following wet-chemical treatment. These studies establish the connection between high reverse and forward leakage currents with residual damage caused by dry-etching and that this damage can be mitigated by with wet-chemical treatments prior to regrowth of the p-anode. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. H. Ohta et al., J. J. Appl. Phys., 57, 04FG09 (2018). T.J. Anderson et al., Elec. Lett., 50(3), 197 (2014).

After decades of research, the wide-bandgap (WBG) semiconductors GaN and SiC are beginning to enjoy commercial success for power electronics applications. Both high breakdown voltage and low on-state resistance are important factors that have enabled improved performance and efficiency of power switching transistors based on these WBG semiconductors. However, truly dramatic improvements in power device performance for extreme applications are likely to require a new class of semiconductor materials with bandgaps greater than that of GaN (3.4 eV), the so-called “ultra” wide-bandgap (UWBG) semiconductors. The critical electric field (EC) affecting breakdown scales approximately as Eg 2.5 [1], making the UWBG semiconductors capable of extremely high voltage operation. In particular, Al-rich AlGaN alloys approaching AlN with its 6.2 eV bandgap have an estimated EC that may be nearly 5x that of GaN, and nearly 2x that of ß-Ga2O3, another UWBG material that is currently receiving increased research interest. Since the first report of an AlGaN-channel transistor [2], progress has been steady and an increasing number of approaches to realizing Al-rich devices have been evaluated. High electron mobility transistors have been realized at channel concentrations as high as 85% Al [3]. However, research into this material for power applications is in its early days, and many challenges remain to be addressed in epitaxial growth, defect physics, fundamental material properties, device processing, and how these relate to device design. This talk will describe Sandia National Laboratories’ efforts to develop Al-rich AlGaN into the material of choice for next-generation power electronics applications. Research will be motivated by comparing power switching figures of merit against ß-Ga2O3 and other UWBG semiconductors being considered for future power electronics applications. The latest results in AlGaN-channel HEMTs will be described, including improvements in current density, recent approaches to realizing enhancement-mode operation, and analysis of the device characteristics over a range of temperature and bias conditions. Details regarding ongoing work to optimize device performance for Al-rich AlGaN transistors will be presented in the talk and final proceedings paper. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. [1] R. J. Kaplar, et al., ECS J. Solid State Science and Technology, vol. 6, p. Q3061 (2017). [2] T. Nanjo, et al., Appl. Phys. Lett. 92, 263502 (2008). [3] A.G. Baca, et al., Appl. Phys. Lett. 109, 033509 (2016).

Due to their large bandgaps (≥ 3.4 eV) compared to Si and SiC, AlGaN alloys have superior material properties that are appealing for the next generation of high-voltage power devices. Most notable is that the critical electrical field scales as the bandgap to the 2.0-2.5 power. Thus, diodes employing wide bandgap semiconductors are expected to operate at higher breakdown voltages with thinner and more heavily-doped drift regions, leading to lower resistive losses compared to Si-based devices. Additionally, the formation of AlGaN-based heterojunctions and the utilization of polarization fields offer design options not possible for devices based on materials such as SiC and diamond. Noteworthy demonstrations of high voltage (> 3 kV) PN and Schottky barrier diodes in both GaN and AlGaN alloys confirm the potential of III-Nitride materials for power devices. However, full exploitation of the efficiency gains promised by III-Nitrides for power management systems also requires vertically-conducting transistors for current switching, in addition to diodes. Analogous to power diodes, vertically-conducting power transistors such as D-MOSFETs employ a thick n-type drift layer to stand-off high reverse voltage, but unlike simple PN diodes, have the p-type material limited to spatially defined regions over the surface of the device. “Selective area p-doping” is typically accomplished in SiC and Si devices by implantation and activation of p-type dopants. However, implantation processing in III-Nitride structures has to date only yielded PN junctions that exhibit significantly higher reverse current leakage than continuously-grown PN junctions. Selective-area epitaxial regrowth of p-type material in regions etched into the drift layer offers an alternative to implantation. However, regrown pn junctions on plasma-etched III-Nitrides with low reverse leakage current have not been demonstrated. In this presentation we will discuss the advances in epitaxial p-regrowth for forming pn junctions on inductively-coupled plasma (ICP) etched n-type GaN and Al0.3Ga0.7N drift layers. The role and sources of unintentionally-introduced Si at the regrown interface will be presented. Additionally, approaches to ameliorate damage and defect states introduced by ICP etching through wet-chemical and in-situ reactor thermal treatments prior to p-regrowth will be related to current-voltage characteristics of regrown diodes. While high voltage GaN-based diodes have been fabricated on vertically-conducting GaN substrates, AlGaN-based high voltage diodes with thick drift layers (≥ 5 μm) have been fabricated on AlN and sapphire substrates. The insulating nature of these substrates necessitates planar device geometries that result in high ohmic losses, heating, and degraded device performance. However, the thick drift layer and the smaller lattice constant of AlGaN alloys present significant challenges for realizing vertically-conducting AlGaN power devices on GaN substrates. Here we report a method for fabricating vertically-conducting AlxGa1-xN PN junctions grown on conducting GaN substrates. An n-type GaN substrate is patterned with parallel, sub-micron-wide mesas separated by trenches etched > 0.5 μm deep prior to AlxGa1-xN overgrowth using metal-organic vapor phase epitaxy. We find that growth temperature, V/III ratio, and group-III molar flux can be used to manipulate the crystalline facets formed along the etched pattern prior to AlxGa1-xN coalescence. Using the described process, coalesced planar AlxGa1-xN epitaxial layers were achieved with less than 6 μm of overgrowth for Al mole fractions up to 0.45, which is the highest Al mole fraction reported to date. Cathodoluminescence measurements of an overgrown Al0.3Ga0.7N epilayer indicate that the density of threading dislocations is mid x 108 cm-2 which is similar to that measured for Al0.3Ga0.7N epilayers overgrown on patterned Al0.3Ga0.7N on sapphire. The current-voltage characteristics of vertically-conducting AlGaN PN diodes grown on GaN substrates will be presented. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program managed by Dr. Isik Kizilyalli. This work was also supported, in part, by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525

Semiconductor materials with wide bandgaps, including GaN and AlxGa1-xN, offer many performance advantages for power electronic devices compared to conventional Si-based devices. These include larger critical electric fields enabling higher reverse breakdown voltages, fast turn-off and turn-on transitions allowing higher-frequency operation, and high thermal conductivity making higher power density device operation possible. Various device architectures have been demonstrated in these materials, with laterally-oriented AlxGa1-xN/GaN high electron mobility transistors (HEMTs) maturing first for radio-frequency applications. More recently, lateral architectures are also starting to be seen in power switching applications. Vertical GaN PN diodes have been demonstrated with excellent forward and reverse electrical behavior including high current capacity, low forward specific on-resistance, high reverse breakdown voltage, and fast reverse recovery time. Selective-area doping control in GaN would enable traditional vertical power devices including merged PN-Schottky (MPS) diodes, junction field effect transistors (JFETs), and more advanced device architectures common in Si and SiC, thereby greatly extending the operating functionality and capability of GaN power devices. This talk will report on our work for two advanced GaN device technologies, including fundamental studies of selective-area doping control and the demonstration of a photoconductive semiconductor switch (PCSS). Both areas of work share the common goal of increasing the power density of switching power converters. Selective-area doping control using epitaxial regrowth processes by metal-organic chemical-vapor deposition (MOCVD) will be presented. Literature reports of GaN PN diodes where the p-layer is formed by epitaxial regrowth have, to date, indicated lower breakdown voltages and/or higher reverse leakage currents compared to continuously-grown PN junctions. The measurement of a “Si spike” at the regrown interface is commonly observed for GaN using characterization methods such as secondary ion mass spectroscopy (SIMS). Our initial studies to examine possible Si sources and the effects of interfacial Si on electrical performance have indicated that the MOCVD reactor is not a major source of Si. Additionally, intentional Si doping at the interface, with similar levels as measured by SIMS in regrown structures ([Si] = 6E16 cm-3 and [Si] = 5E17 cm-3), shows no noticeable degradation in forward- or reverse-bias electrical performance in planar c-plane devices fabricated on GaN substrates (Figure 1). However, regrown GaN diodes formed on m-plane GaN substrates have shown worse electrical performance than c-plane diodes. The increased chemical reactivity of the m-plane and the resulting increase in impurity uptake is likely a contributing factor, and studies are underway to identify the specific impurities responsible. Mitigation strategies to manage the challenges of the m-plane are also being investigated. Further, fabrication-induced damage due to dry-etching processes was shown to increase reverse leakage currents by at least an order of magnitude in planar, c-plane diodes. We will report on these results and the study of various surface treatment methods to reduce these leakage currents in planar, c-plane diodes. The details of a laterally-oriented, GaN PCSS switch will also be presented. PCSS technology is of interest for applications including high-voltage pulsed-power, electrical grid protection, and increased electrification through new build-outs of high-efficiency/performance power systems. PCSS devices have been demonstrated in several semiconductor materials, but GaN promises an improvement in switch capability due to its advantageous material properties. The device is operated by optically triggering photocarriers in a high-field area generated between two electrodes. Using commercially-available, semi-insulating GaN wafers, surface electrodes were fabricated with a 600 µm gap, and the device was optically triggered using a sub-bandgap 532 nm laser source. Low-voltage (low field) operation of the device demonstrated a linear photoconductive response. Increasing the field over a threshold value of 10-15 kV/cm demonstrated a “high-gain” or “lock-on” operating mode where current continues to flow after the laser source is turned off, if a sufficient field exists (Figure 2). This operating mode has been observed for other semiconductors, including GaAs, and is desirable for high-efficiency switch operation because of the low optical energy needed to turn on the switch. High-gain switching has been demonstrated with PCSS powers of 150 kW (1250 V, 120 A) triggered by optical energies as low as 35 µJ. Further increases in switch power are expected with device improvements. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the IDEAS and PNDIODES programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. Figure 1

Wide bandgap materials, such as GaN and SiC, have been rapidly maturing for use in power electronic applications. Recently, there has been a push to investigate materials with ultra-wide bandgaps (Eg > 3.4eV), such as Ga2O3 and AlGaN, in an attempt to achieve higher critical electric field, as it scales with the bandgap to the >2 power. Increasing the Al-content of AlGaN has been pursued for several applications, including both power electronics and UV optoelectronics, as the bandgap, and thus critical electric field, of the material increases with increasing Al-content. Though decades have been spent maturing GaN and low Al-content AlGaN for power, RF, and optoelectronic applications, the path for fabricating devices utilizing Al-rich AlGaN is not as straightforward as one may hope. An overview of Sandia National Laboratories’ device development with Al-rich AlGaN will be presented. In addition, this talk will report on the intricacies faced during fabrication of Al-rich AlGaN devices, including both the challenges and advancements that AlGaN can present such as the effects of Al content on dry etch characteristics (e.g. surface morphology and etch rates), effects of surface cleaning and preparation, as well as metallization schemes and characteristics for both Schottky and Ohmic contacts. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

The recent availability of high quality bulk GaN substrates with low threading dislocation density has made vertical power device structures an attractive possibility. Due to the low probability of a threading dislocation occurring within the active area of devices grown upon bulk GaN, reliability is driven by bulk or surface effects. Past work upon the reliability of Ni-GaN Schottky interfaces has suggested that interdiffusion of the Ni into the GaN can occur under thermal stress, adversely affecting the effective energy height of the barrier and increasing the ohmic nature of the interface. Conditions similar to such thermal stressing can be found at typical operating points in high current power devices used in power electronic applications. Our work examines the influence of high current density stress upon the reliability of vertical bulk GaN Schottky diodes. The vertical diodes studied were fabricated with Pd Schottky metallization with an Au ohmic overlayer. To study the effects of surface chemistry, reliability is compared between devices fabricated with different acidic and basic surface preparations, using HCl and KOH respectively. The fabricated devices were subject to life testing under varying levels of constant current density, up to 10 kA/cm^2. The lifetesting system was designed as a stress-measure-stress system, allowing for the evaluation of device parameters in-situ during testing. Electrical parameters indicating the state of the Schottky barrier were evaluated continuously during the measurement points of testing. In addition, a measurement of barrier inhomogeneity, evaluated using the Tung model, was also used as an indication of the condition of the Schottky interface. The microstructure of the interface before and after stress was examined using S/TEM imaging of representative lamellae samples of the stressed and unstressed devices. EDS imaging was used to determine the degree of diffusion between the Schottky metallization and the GaN substrate. Results of microstructure imaging are correlated with the electrical measurements taken to determine the responsible mechanisms for device degradation.

SiC- and GaN-based power semiconductor devices have in the past few years enabled great improvements in the efficiency and power density of switching power converters. A wide variety of SiC devices (e.g. MOSFETs, JFETs, BJTs, thyristors, and PiN/Schottky/JBS/MPS diodes) are now available from a number of manufacturers, and the same is true for GaN HEMTs. Thus, while vertical GaN devices are not yet mature, new research in the field is increasingly turning to the “ultra” wide-bandgap (UWBG) semiconductors, including diamond, gallium oxide, and aluminum gallium nitride (AlGaN), due to the expected scaling of the critical electric field as the bandgap to the 2.0-2.5 power. Notably, AlGaN is an alloy system, so that heterostructures are available, and it is also a polar material, which enables polarization doping. Both of these benefits significantly expand the range of device architectures that may be considered, compared to materials that do not have these properties; this is especially significant for UWBG materials, all of which have energetically deep impurity dopants that do not fully ionize at room temperature. This talk will report on both vertical PiN diodes and lateral HEMTs composed of Al-rich Al x Ga1-x N (x ≥ 0.7, EG ≥ 5.2 eV) designed as prototype power switching devices. The talk will begin with an overview of the properties of AlGaN that motivate its application to power switching, as well as an analysis of conduction and switching loss mechanisms as a function of AlGaN bandgap for various device types (PiN diodes, Schottky diodes, etc.) These results will be compared to similar metrics for SiC, GaN, and UWBG materials other than AlGaN. The talk will then present results on several PiN diode structures. All of the structures to be presented have thick (5-8 mm) Al0.7Ga0.3N drift regions doped in the 1-3×1016 cm-3 range, and were grown on thick (1.3 mm) sapphire substrates; due to the insulating nature of the substrate, the cathode contact is to a heavily-doped n-layer on the front side of the wafer in a so-called “quasi-vertical” configuration. The diodes differ in the design of the p-type anode. Homojunction diodes suffer from low free carrier density in the p-Al0.7Ga0.3N due to the low fraction of Mg than is ionized in material of such a wide bandgap (EA > 400 meV). Thus, two alternate approaches to achieve appreciable hole density were examined: heterojunction diodes in with the anode is composed of Al0.3Ga0.7N, in which reasonable Mg activation can be achieved; and diodes utilizing anodes in which the composition is graded from Al0.7Ga0.3N down to GaN. In order to avoid premature edge and/or surface breakdown, all diodes studied utilized a junction termination extension formed by nitrogen implantation into the p-region. The talk will conclude with a presentation of a second class of device, which is a power switching HEMT utilizing an Al0.85Ga0.15N/Al0.70Ga0.30N heterostructure. This heterostructure, which as a 25 nm thick barrier, is characterized by a pinch-off voltage of -4.5 V, a channel mobility of 250 cm2/Vs, and a channel sheet charge density of 6×1012 cm-2. The key challenge for Al-rich HEMTs has been the formation of Ohmic source and drain contacts; the current density in previous HEMTs reported by our group has been limited by the quasi-rectifying nature of these contacts. The HEMTs in the present work utilize planar source and drain contacts, as opposed to the etched-and-regrown contacts that we have reported previously; for those previous contacts, the regrown material was heavily-doped n-type GaN. The planar contacts in this study are composed of a Ti/Al/Ni/Au stack deposited directly on the Al0.85Ga0.15N barrier. While the current-voltage characteristics of these contacts are still quasi-rectifying, they have enabled a current density of 46 mA/mm, which is more than ten times greater than that achieved in our previous-generation AlGaN-channel HEMT, which had comparable channel sheet resistance. An effective breakdown field of ~170 V/um was achieved in a field-plated device with 4 um gate-to-drain spacing. This effective breakdown field exceeds that typically reported for GaN-channel HEMTs (~100 V/um), demonstrating the potential of AlGaN-channel HEMTs for high-performance power switching devices. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

RF amplifiers and power electronics transistors in III-N semiconductors are increasingly important in emerging applications, as are photo-transistors for the purpose of UV light detection. Increasing the Al fraction in AlGaN-based HEMT devices enables transistors with higher breakdown voltages and a potentially favorable breakdown - specific on-resistance tradeoff. However, the advantages of higher aluminum content comes with a major challenge: Ohmic contacts with acceptable contact resistance become increasingly problematic with increasing Al content in the channel and barrier layer. We report sub 10-4 W-cm2 specific contact resistivity (rc) for a 30% channel, improving on the best prior result of approximately 2x10-4 W-cm2for AlGaN channels of at least 30% Al. We investigated five different Ohmic contact metallizations consisting of X/Al/Y/Au where X = Ti, Nb/Ti, Zr, or V, and Y = Ni, Mo, or V (Table 1) as function of alloying conditions. The MOCVD-grown structure (Figure 1) is on sapphire substrates and consists of a 1.6 µm AlN buffer layer, a 4.15 µm thick Al0.3Ga0.7N channel layer and a 50 nm thick Al0.45Ga0.55N barrier with Rsheet of 3500 Ω/□ and ns= 5x1012/cm2. Representative current-voltage plots measured on circular TLM structures are shown in Figure 2. Ohmic contacts with average specific contact resistance (pc) of 7x10-5 Ω-cm2 (3.3x10-5 Ω-cm2) extracted from device (TLM) data were achieved with a conventional Ti/Al/Ni/Au metal stack and a multi-temperature stepped-anneal process. Structural characterization was carried out on the Ti/Al/Ni/Au contacts, to compare Ohmic and Schottky conditions. TEM was used to characterize the interaction between the contact metals and the AlGaN, revealing an interface consisting primarily of Ti-Au with limited indication of spiking behavior. Using the optimized Ohmic contact process, HEMTs with 10 µm source-to-drain spacing and 2 µm gate lengths were fabricated; current densities of 78 mA/mm were achieved, and parasitic loading of RON was reduced. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1

As the performance of GaN- and SiC-based power devices becomes limited by fundamental material properties, semiconductors with larger bandgaps are appealing due to their superior material properties. Most notable is that the critical electrical field scales as the bandgap to the 2.0-2.5 power. Thus, diodes employing wide bandgap semiconductors are expected to operate at higher breakdown voltages with thinner and more heavily-doped drift regions, leading to lower resistive losses. AlGaN alloys with their much larger bandgap compared to SiC and GaN are thus appealing for high voltage pn diodes and transistors. Additionally, the formation of AlGaN-based heterojunctions and the utilization of polarization fields offer design options not possible for devices based on materials such as SiC and diamond. Here we report on the growth and performance of pn diodes fabricated from Al0.30Ga0.70N (Eg=4.1eV) and Al0.74Ga0.26N (Eg=5.3eV) alloys, illustrating the potential for AlGaN alloys as a viable material for next-generation power diodes. Quasi-vertical pn diodes consisting of Al0.30Ga0.70N epilayers were grown by metalorganic vapor phase epitaxy on 1.3 mm thick sapphire substrates. The added stiffness of such thick sapphire substrates (3x thicker than typical sapphire) reduces wafer bow and eliminates cracking in epitaxial structures up to 19 μm thick. Thick epitaxial structures are needed to achieve the > 4 microns thick drift layers necessary for multi-kilovolt breakdown voltages. A series of pn diode structures were grown where the thickness of the lightly doped drift layer (No = 2-5x1016 cm-3) was varied from 4.3 to 11 μm. All structures were completed with a 0.35 μm thick Mg-doped Al0.30Ga0.70N p-layer followed by a compositional grade to a p-GaN contact layer. Diodes employed a junction edge termination structure around the p-contact to reduce the peak electric field at the contact metal to avoid premature breakdown. Breakdown voltages from 1600 to 3000V were achieved in reverse bias and a forward turn-on voltage of 6 to 8 volts was recorded. To our knowledge, this is the first vertical AlGaN-based pn diode exhibiting a breakdown voltage in excess of 1 kilovolt. The differential specific on-resistance was higher than expected based on consideration of the drift region only (typically 16-20 mΩ-cm2), which is tentatively attributed to the resistance due to lateral current spreading in these front-contacted, quasi-vertical diodes. An effective critical electric field of 5.9 MV/cm was determined from the reverse current-voltage characteristics based on a punch-through, as dictated by the doping and thickness of the drift region. We have also investigated quasi-vertical pn diodes consisting of Al0.74Ga0.26N epilayers to exploit the expected higher critical electric field in higher aluminum mole fraction alloys. These diodes consisted of 4.5 μm thick AlN epilayers grown on sapphire followed by a heavily doped n-type contact layer. Next a lightly doped (No = 2-4x1016 cm-3) 5 or 8 μm thick drift layer was grown. The combination of a thick AlN buffer and Al0.74Ga0.26N layers resulted in x-ray diffraction linewidths of 164-197 (237–284) arc seconds for the (0002) and (10-12) reflections for the drift layer, respectively. By optimizing the growth conditions used for the drift layer, the density of electron compensating defects can be reduced to ~ 1x1016 cm-3 — a level reported for GaN pn diodes. Unlike GaN diodes where drift layer doping at mid to low 1015 cm-3 is necessary for breakdown voltages of several kilovolts, the free electron concentration in these Al0.74Ga0.26N pn diodes is significantly higher than the level of compensation, making doping much easier to control and illustrating the benefit of higher critical electric field. The drift layer was followed by p-type AlGaN and finally a compositional grade to a p-GaN contact layer. It is the p-AlGaN layer that is a significant challenge in Al-rich pn diodes. Unlike pn diodes based on Al0.30Ga0.70N epilayers, simple homojunction pn diodes are not practical in Al-rich AlGaN alloys due to the low hole concentration owing to the very deep (Ea > 400 meV) acceptor level. Consequently, other approaches must be considered for hole transport to the pn junction. We will report on the current-voltage characteristics of pn heterojunction diodes where a Mg-doped p-type Al0.30Ga0.70N layer is grown on the n-type Al0.74Ga0.26N drift layer. Additionally, the current-voltage characteristics of pn diodes where holes are generated in the p-region by field ionization of Mg acceptors will also be presented. Finally, practical pn diodes require specific on-resistance lower than what is inherent for front-contacted quasi-vertical device architectures grown on insulating substrates. We will report an approach that has realized vertically conducting AlGaN pn diodes employing AlGaN overgrowth on conducting GaN substrates.

As the wide-bandgap materials SiC and GaN mature and become commercially available, “Ultra” Wide-Bandgap (UWBG) semiconductors with bandgaps exceeding 3.4 eV are gaining interest as the next-generation materials that will enable dramatic leaps in power electronics performance. This is because the Figure-of-Merit (FOM) comparing breakdown voltage (VB) and area-normalized “specific” on-resistance (Ron,sp), defined as VB 2/Ron,sp, scales as the cube of the critical electric field for vertical devices and the square of the critical electric field for lateral devices. The critical electric field for avalanche breakdown in turn scales approximately as the 5/2 power of the bandgap, implying a very strong dependence of FOM on bandgap. Representative UWBG materials include diamond, Ga2O3, and Al x Ga1-x N. The AlGaN system is particularly attractive due to the availability of heterostructures as well as the ability to utilize polarization doping; both of these properties mitigate the issue of energetically deep impurity dopants and consequent incomplete ionization which universally affects UWBG semiconductors. Thus, our group’s focus has been on the development of AlGaN for the fabrication of next-generation power devices with both vertical and lateral architectures. Quasi-vertical AlGaN PiN diodes were grown on thick (1.3 mm) sapphire substrates using Metal Organic Vapor Phase Epitaxy (MOVPE). The initial epitaxial layers consist of an AlN layer followed by an unintentionally-doped AlGaN layer to facilitate the transition from the substrate to the active device layers. Dislocation density is 1-2×109 cm-2; additional reduction in dislocation density is possible using patterned AlGaN and overgrowth techniques. The transition layer is followed by a heavily-doped n-type AlGaN:Si contact layer and the thick lightly-doped n-type AlGaN:Si drift layer. The drift layer is critical since it supports the high voltage in the reverse-bias blocking mode and is also responsible for the on-state resistance in the forward-bias conducting mode. This is followed by a p-type AlGaN:Mg anode layer and finally graded to a very thin GaN:Mg p+ contact layer. Ion implantation into the p-layers is utilized to form a Junction Termination Extension (JTE) to prevent premature breakdown. A mesa etch is performed down to the n-contact layer, and Ohmic contacts (TiAlMoAu) are deposited; this is necessary since the substrate is insulating and, due to the lateral current transport in this layer, the device configuration is termed “quasi-vertical” (the conduction through the pn junction and drift layer is in the vertical direction). The top Ohmic contact is PdAu, the surface is passivated with silicon nitride, and a shallow trench isolates the JTE from the mesa sidewalls. For a device utilizing Al0.3Ga0.7N for the active layers, and with a 4.3 um thick drift region doped in the mid-1016 cm-3 range, VB exceeded 1.5 kV (Figure 1a); combined with the measured Ron,sp of 16 mOhm·cm2, a FOM of approximately 150 MW/cm2 is achieved. To our knowledge this is the first time a vertical kV-class AlGaN PiN diode has been demonstrated, and the FOM is the highest reported for an AlGaN diode. Further, the current density under forward bias exceeds 3.5 kA/cm2, and the critical electric field for the Al0.3Ga0.7N is estimated to be 6.0 MV/cm. Lateral High Electron Mobility Transistors (HEMTs) were also grown and fabricated in Al-rich AlGaN alloys. The epitaxial layers are likewise grown on sapphire substrates by MOVPE. Following the growth of an AlN nucleation layer, a 400 nm thick Al0.85Ga0.15N buffer/channel layer is grown, which is capped by a 48 nm thick AlN barrier layer. All layers are unintentionally doped. A combination of mercury-probe capacitance-voltage and contactless resistance measurements indicate a sheet resistance of approximately 4200 Ohm/square and a channel mobility of 250 cm2/V·s, and the pinch-off voltage is approximately -4.0 V. Circular HEMTs were fabricated using a six-mask photolithographic process. The key step in the process is the fabrication of the source and drain Ohmic contacts, for which n+ GaN:Si is re-grown in trenches etched through the AlN barrier, and a TiAlNiAu metal stack is afterwards deposited. The Schottky gate is NiAu and the surface is passivated with silicon nitride; no field plates are utilized. Drain current vs. drain voltage curves are shown in Figure 1b and clear gate control is evident. However, the current density is much lower than expected based on the measured sheet resistance and known device geometry, indicating that further optimization of the Ohmic contacts is necessary (the contact resistivity is estimated to be 0.1 Ohm·cm2). Despite this, the devices show excellent performance with Ion/Ioff > 107 and VB > 800 V, and to our knowledge are the highest-bandgap transistors ever reported. Figure 1

The AlGaN alloy system has been the material system of choice for development of solid-state UV emitters and has promising properties for high-voltage power electronics. The performance and reliability of both optical emitters and power devices are critically dependent on the density of threading dislocations. While the binary substrates, GaN and AlN, are becoming commercially available, only a limited range of AlGaN compositions can be grown pseudomorphicly to device-relevant thicknesses of a micron or more. This his restricts the range of heterostructures available for designing advanced devices, and creates a need for a ternary-like pseudo-substrate with a tunable lattice constant. We present a method for fabricating crack-free AlxGa1-xN templates over the entire alloy range with threading dislocation densities (TDD) in the low 108 cm-2 ranges. We employ AlxGa1-xN overgrowth of previously grown AlxGa1-xN/AlN/sapphire templates that have been patterned with parallel trenches on a 2 μm pitch consisting of submicron-wide-mesas.1 Following overgrowth, the remaining threading dislocations are uniformly distributed over 2-inch-wafer-size areas enabling large area devices such as LEDs and PIN diodes to consist of only low TDD material. As a result, laser diodes do not need to be aligned to the underlying trench pattern. Wafer bow is reduced to less than 15 μm and epilayer cracking is essentially eliminated by using 1.3 mm-thick sapphire substrates (three times thicker than typical 2-inch wafers). The utility of the described overgrowth process for various alloy compositions is demonstrated by growing 5 to 10 μm thick Al0.3Ga0.7N, Al0.7Ga0.3N and AlN epilayers on sapphire substrates. While AlN substrates continue to improve, they are still relatively immature, expensive, and small in area compared to GaN substrates. Using the described overgrowth process and a modest growth temperature of 1100 °C, AlN epilayers grown on sapphire have achieved FWHM linewidths < 300 arc seconds for the (10-11) x-ray diffraction reflection and an RMS roughness of 1 Å over a 10 x 10 μm2region. Such x-ray linewidth and surface roughness values are comparable to AlN grown in specialized MOCVD reactors capable of growth in excess of 1300 °C. These low TDD AlGaN layers have been used for various devices. For example, the AlN overgrowth templates are used to demonstrate 2DEG formation in MODFETs formed by Al0.85Ga0.15N / Al0.7Ga0.3N heterostructures. Additionally, UV laser diodes were grown on Al0.3Ga0.7N overgrowth templates that achieved room-temperature, pulsed-current operation with emission at 352 nm and peak output powers > 2.5 mW per facet. Finally, we will also discuss the potential of using >10 μm thick Al0.3Ga0.7N overgrowth templates for PIN diodes. [1.] Allerman, et. al. J. Crystal Growth 38876 (2014). This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Dramatic improvements in Size, Weight, and Power (SWaP) for power conversion systems have recently been enabled by the adoption of Silicon Carbide (SiC) and Gallium Nitride (GaN) based power switching devices. Such improvements are of high interest for both civilian and defense applications. However, despite the significant progress in these Wide-Bandgap (WBG) materials and the dramatic advantages conferred by them relative to the incumbent Silicon (Si) based technology, various challenges related to performance and reliability remain to be solved. For example, SiC MOSFETs have suffered from gate oxide reliability problems, while non-MOS SiC devices may not be normally-off; similarly, GaN-channel HEMTs are intrinsically normally-on, and normally-off operation requires complex gate stack processing that may degrade long-term device reliability. Lateral GaN HEMTs are also limited in breakdown voltage, and do not avalanche and must thus be over-designed to ensure reliable operation. Improvement in WBG material and device technology has slowed as the technology has matured and become commercially available, and fundamental material properties now limit the performance. Thus, dramatic leaps in power electronics performance require a new generation of materials, the so-called “Ultra” Wide-Bandgap (UWBG) materials with bandgaps larger than 3.4 eV. Representative materials in this category include Aluminum Nitride (AlN), Diamond, and Gallium Oxide (Ga2O3). This talk will focus on our efforts to develop the Aluminum Gallium Nitride (AlGaN) system with Al composition in the range of 30-100% into a viable material for next-generation power electronics. The basic advantages of AlGaN relative to Si, SiC, and GaN will be discussed, as evidenced by the superior unipolar Figure-of-Merit (FOM) for AlN as demonstrated in Figure 1. Here, AlN is shown to have a theoretical order-of-magnitude improvement relative to GaN; in the plot, high breakdown voltage (VB, horizontal axis) and low specific on-resistance (Ron,sp, vertical axis) are desired. This FOM is based on a number of basic material parameters such as mobility and critical electric field, which will be covered in the talk. Fundamental challenges associated with substrates, epitaxial growth, doping, and point and extended defects will be discussed. Vertical and lateral (heterostructure-based) device architectures will likewise be covered, and the advantages and limitations imposed by the properties of the UWBG AlGaN materials will be examined. In particular, it will be demonstrated that an intrinsically normally-off device that exceeds the capability of GaN (as measured by the FOM) is viable. Experimental results at both the material and device levels will be presented. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1