We report ultra-wide bandgap (UWBG) AlGaN heterostructure field-effect transistors (HFETs) exhibiting a high breakdown field (>5.3 MV/cm) and a low contact resistance (∼1.55 Ω mm), tailored for high-power radio frequency applications. A split-doped barrier architecture, employing two distinct doping concentrations, is shown to enhance both the breakdown field and contact resistance. This design enables a state-of-the-art combination of maximum drain current (487 mA/mm) and breakdown field, along with a high cutoff frequency of 7.2 GHz. These results demonstrate a viable pathway to push device performance toward the material limits while minimizing contact resistance in UWBG AlGaN HFETs, paving the way for next-generation high-power, high-frequency applications.

We report on the demonstration of ferroelectric ScAlN/AlGaN high-electron mobility transistors (HEMTs). The device exhibited a maximum drain density, ID, of ∼16 mA/mm with excellent electrostatic control at room temperature, demonstrating the integration of a molecular beam epitaxy-grown Sc0.15Al0.85N barrier with an Al0.50Ga0.50N channel, which was grown using metal-organic chemical vapor deposition. A counterclockwise hysteresis transfer curve was observed with a tunable threshold voltage range of 6.4 V, a high on/off current ratio (ION/IOFF) of ∼106, and a low subthreshold swing (SS) ∼65 mV/dec at drain voltage (VDS) = 1 V. The maximum ID was observed to increase with temperature up to 400 °C, suggesting thermally activated transport, driven by thermionic emission over the n+-GaN/Al0.50Ga0.50N heterobarrier. These results indicate that ferroelectric ScAlN/AlGaN HEMT is a promising candidate for the next-generation nonvolatile, reconfigurable power, and high-temperature memory applications.

We report on the design and demonstration of ultra-wide bandgap (UWBG) AlGaN-channel metal-insulator heterostructure field effect transistors (HEFTs) for high-power, high-frequency applications. We find that the integration of gate dielectrics and field plates greatly improves the breakdown field in these devices, with state-of-art average breakdown field of 5.3 MV/cm (breakdown voltage > 260 V) with an associated maximum current density of 342 mA/mm, and cut-off frequency of 9.1 GHz. Furthermore, low trap-related impact was observed from minimal gate and drain lag estimated from pulsed I-V characteristics. The reported results provide the potential of UWBG AlGaN HEFTs for the next generation high-power radio frequency applications.

Abstract Epitaxial regrowth processes are presented for achieving Al‐rich aluminum gallium nitride (AlGaN) high electron mobility transistor (HEMTs) with p‐type gates with large, positive threshold voltage for enhancement mode operation and low resistance Ohmic contacts. Utilizing a deep gate recess etch into the channel and an epitaxial regrown p‐AlGaN gate structure, an Al0.85Ga0.15N barrier/Al0.50Ga0.50N channel HEMT with a large positive threshold voltage (VTH = +3.5 V) and negligible gate leakage is demonstrated. Epitaxial regrowth of AlGaN avoids the use of gate insulators which can suffer from charge trapping effects observed in typical dielectric layers deposited on AlGaN. Low resistance Ohmic contacts (minimum specific contact resistance = 4 × 10−6 Ω cm2, average = 1.8 × 10−4 Ω cm2) are demonstrated in an Al0.85Ga0.15N barrier/Al0.68Ga0.32N channel HEMT by employing epitaxial regrowth of a heavily doped, n‐type, reverse compositionally graded epitaxial structure. The combination of low‐leakage, large positive threshold p‐gates and low resistance Ohmic contacts by the described regrowth processes provide a pathway to realizing high‐current, enhancement‐mode, Al‐rich AlGaN‐based ultra‐wide bandgap transistors.

This work reports on high current density 1.2 kV class HfO2-gated vertical GaN trench metal-oxide-semiconductor field-effect transistors (MOSFETs). An output current density of 330 mA mm(-1) is reported at a drain bias of five volts, which, to our knowledge, is over ten-times the highest reported values for 1.2 kV class GaN or SiC MOSFETs. This work also showcases a significant achievement in demonstrating substantially thick (100 nm) HfO2 on GaN with simultaneous low leakage current (0.5 nA at 2 MV cm(-1)), a high breakdown strength (5.2 MV cm(-1)), and a high recorded dielectric constant (22.0).

We report a comparative study of three rectifying gate metals, W, Pd, and Pt/Au, on ultrawide bandgap Al0.86Ga0.14N barrier/Al0.7Ga0.3N channel high electron mobility transistors for use in extreme temperatures. The transistors were electrically characterized from 30 to 600 °C in air. Of the three gate metals, the Pt/Au stack exhibited the smallest change in threshold voltage (0.15 V, or 9% change between the 30 and 600 °C values, and a maximum change of 42%), the highest on/off current ratio (1.5 × 106) at 600 °C, and a modest forward gate leakage current (0.39 mA/mm for a 3 V gate bias) at 600 °C. These favorable results showcase AlGaN channel high electron mobility transistors' ability to operate in extreme temperature environments.

GaN/InGaN microLEDs are a very promising technology for next-generation displays. Switching control transistors and their integration are key components in achieving high-performance, efficient displays. Monolithic integration of microLEDs with GaN switching devices provides an opportunity to control microLED output power with capacitive (voltage)-controlled rather than current-controlled schemes. This approach can greatly reduce system complexity for the driver circuit arrays while maintaining device opto-electronic performance. In this work, we demonstrate a 3-terminal GaN micro-light emitting transistor that combines a GaN/InGaN blue tunneling-based microLED with a GaN n-channel FET. The integrated device exhibits excellent gate control, drain current control, and optical emission control. This work provides a promising pathway for future monolithic integration of GaN FETs with microLED to enable fast switching, high-efficiency microLED display and communication systems.

This paper presents the design, material growth and fabrication of AlGaN laser structures grown by plasma-assisted molecular beam epitaxy. Considering hole transport to be the major challenge, our ultraviolet-A diode laser structures have a compositionally graded transparent tunnel junction, resulting in superior hole injection and a low contact resistance. By optimizing active region thickness, a five-fold improvement in photoluminescence intensity is obtained compared to that of our own non-optimized test structures. The electrical and optical characteristics of processed devices demonstrate only spontaneous emission with a peak wavelength at 354 nm. The devices operate up to a continuous-wave current density of 11.1 kA cm −2 at room temperature, which is the highest reported for laser structures grown on AlGaN templates. Additionally, they exhibit a record-low voltage drop of 8.5 V to achieve this current density.

PN diodes with multi-kilovolt breakdown voltages have been demonstrated to great than 6 kV in GaN, validating the device potential predicted by the intrinsic material properties of III-nitride semiconductors. However, power control circuits also require switching transistors such as junction field effect transistors (JFETs) and practical diodes such as merged PIN Schottky (MPS) diodes. Such devices usually employ selective areas of p-type semiconductor surrounded by n-type material and require the formation of PN junctions with low reverse leakage current. Selective-area p-type doping in Si and SiC based power devices is typically achieved using implantation and thermal annealing to form the p-type areas. Ion implantation of Mg in GaN has demonstrated p-type conductivity, but this process requires specialized equipment for the high-pressure and high-temperature activation of Mg dopants. We have investigated selective-area-regrowth (SArG) to epitaxially grow p-type GaN as an alternative to dopant implantation. Successful PN diode formation by SArG requires a process to remove residual crystalline damage resulting from the ex-situ inductively coupled plasma (ICP) etch typically used to form the p-well, as well as a method to remove the elevated level of Si found at the regrowth interface of a surface that had been exposed to air. For the case of SArG of p-GaN on air-exposed, blanket ICP etched n-type GaN, we present the novel use of a fluorine-based precursor (XeF2) for in-situ etching of GaN in the MOCVD chamber. Unlike chlorine (TBCl and CCl4) and bromine (CBr4) -based precursors we have studied; we obtained a smooth surface following in-situ fluorine etching of air-exposed GaN. Schottky barrier diodes (SBDs) formed by shadow mask evaporation on in-situ fluorine etched n-GaN that had been previously ICP etched showed reverse leakage currents to -40 V that are equal to those of SBDs formed on as-grown n-GaN layers. The in-situ fluorine/ICP etched Schottky diodes had reverse leakage currents more than 3 orders of magnitude lower than those formed on n-GaN layers that had only experienced ICP etching. This suggests that the in-situ fluorine etch was effective at removing the residual damage from the ICP etch process. Furthermore, we discuss the use of in-situ fluorine etching to reduce the concentration of Si at the regrowth interface required to form PN junctions with low reverse leakage current. This work was supported in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program directed by Dr. Isik Kizilyalli, and in part by the US Department of Energy (DOE) Vehicle Technologies Office (VTO) under the Electric Drive Train Consortium. 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.

Vertical gallium nitride (GaN) power devices continue to garner interest in multiple power conversion applications requiring a medium-voltage (1.2 – 20 kV) capability. Currently, silicon carbide (SiC) is addressing this voltage range, however, with a comparable critical electric field and superior mobility, GaN is expected to offer advantages in applications where fast switching and avalanche breakdown response times are desired. While uses in electric vehicles, solid-state transformers, and renewable energy conversion are being actively explored, the potential of a vertical GaN device for electric grid protection in the form of an electromagnetic pulse arrestor is a unique proposition that requires very fast transient capabilities (<1 µs pulse widths with rise-times on the order of 10 ns). However, vertical GaN devices are significantly less mature than present SiC offerings. Specifically, low-doped, thick epitaxial growth of GaN via metal-organic chemical vapor deposition (MOCVD) still presents many challenges, and advancements in processing, manufacturability, and failure analysis are needed. In this work, we describe our efforts to address the above issues and advance the state-of-the-art in vertical GaN PN diode development. We have successfully demonstrated an MOCVD-grown, 50 µm thick, low-doped (<10 15 cm -3 ) drift region on a GaN substrate that was processed into relatively large-area (1 mm 2 ) PN diodes capable of achieving a 6.7 kV breakdown. Temperature-dependent breakdown was observed, consistent with the avalanche process. The devices consisted of a 4-zone step-etched junction termination extension (JTE), where the breakdown region was visualized via electroluminescence (EL) imaging. Ongoing work aims to scale the current capability of the medium-voltage diodes through a parallel interconnect design that negates defective or poor performing diodes. Further investigation of edge termination structures was explored using a bevel approach, where we studied the relationship between the bevel angle and p-doping. It was found that a very shallow angle of only 5° accompanied by a 500 nm p-region consisting of 3×10 17 cm -3 Mg concentration resulted in a consistent 1.2 kV breakdown for an 8 µm thick, 1.6×10 16 cm -3 doped drift region. EL imaging confirmed uniform breakdown, and temperature dependence was demonstrated. The bevel approach was then implemented on a diode structure with a 20 µm thick drift region capable of 3.3 kV breakdown, where an unclamped inductive switching (UIS) test was performed to evaluate the impact of a field plate design on avalanche uniformity and ruggedness. A parallel effort to establish a foundry process for vertical GaN devices has been underway. Initially, this focus was on comprehensive studies of GaN wafer metrology using capacitance-voltage (C-V) mapping, optical profilometry, and x-ray diffraction (XRD) mapping. A machine learning algorithm was implemented to identify defective regions and produce a yield prediction for each GaN wafer prior to processing. A hybrid edge termination structure consisting of implanted guard rings (GR) and JTEs was developed in coordination with a controlled experiment that varied the anode thickness, and therefore the remaining p-GaN after implantation. It was observed that thinner p-GaN regions under the JTE/GR region resulted in a significant (>100x) reduction in leakage current under reverse-bias conditions. This process has resulted in 1.2-kV-class devices with up to 18 A forward current for a 1 mm 2 device with a specific on-resistance of 1.2 mOhm-cm 2 . The foundry effort has since been extended to 3.3-kV-class devices that utilize 25 µm thick drift layers with ~2-4×10 15 cm -3 doping. These devices have demonstrated up to 3.8 kV breakdown with leakage currents <1 nA up to 3 kV. More than 40 wafers have been processed to date, resulting in >20,000 devices. Statistical variations in I-V and C-V characteristics will be discussed. Packaging process development and analysis are underway to develop electrical stress procedures and identify fundamental failure mechanisms. Finally, a pulse arrested spark discharge (PASD) setup, capable of up to 15 kV pulsed operation in 100 V steps, was implemented to quantify the time response of avalanche breakdown. Initial results on a packaged 800 V device showed a ~1 ns response time during breakdown, which reinforces the potential EMP grid protection applicability. This work was supported by the ARPA-E OPEN+ Kilovolt Devices Cohort directed by Dr.Isik Kizilyalli. 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 of the United States Government.

•Lines 7-8: Furthermore, state-of-the-art vertical GaN devices including fin-JFETs, CAVETs, and MOSFETs have been reported with blocking voltages up to 1.6 kV [ 4, 5, 6].•Lines 99-101: Consequently, no leakage ramp-up or avalanche behavior was observed. On average, devices only reached 30% of the theoretical blocking voltage before failing. Several factors can reduce the robustness of the gate dielectric such as material defects, non-uniform thickness, and interface roughness. Further associated challenges will be discussed Section 4.•Lines 148-149: It should be noted that initial reports on regrown devices such as fin JFETs are emerging [ 4 ], however, the leakage currents are excessively high compared to traditional commercial offerings.

Localized lattice distortions in GaN substrates can serve as nucleation sites for epitaxial macro-steps and macro-terraces. These detrimental macro-scale features give rise to optically hazy homoepitaxial GaN surfaces. After nucleating, these macro-features grow laterally along the surface and coalesce, leading to significant coverage of the wafer surface. Dot-core GaN substrates consisting of a periodic array of cores were used as a defect-engineered system, where dislocations are intentionally concentrated at the cores. The high density of threading dislocations at the cores induced localized lattice distortions. These distortions are associated predominantly with lattice tilt on the order of hundreds of arcsec across ∼0.5 mm laterally along the wafer surface. The resulting macro-features that nucleated at these localized distorted sites were made up of macro-terraces with lengths ranging ∼30–∼150 μm and macro-step heights ranging ∼200–∼400 nm. Another source of localized distortion was threading screw dislocations or GaN nanopipes that resulted in spiral growth and hillock formation. Based on x-ray topography and optical microscopy measurements, we speculate that the coalescence of hillocks evolves into macro-terraces and macro-steps. While previous studies focused on the substrate miscut as a means to control macro-feature formation, we show that localized lattice tilt from defects is another important contributor to macro-feature formation.

Molecular beam epitaxy-grown GaN/AlGaN-based active regions were optimized by varying quantum-well widths to yield increased photoluminescence intensity at UV-A wavelengths. The optimized gain medium was then used in electrically-pumped laser structures with transparent tunnel junctions.

In this work, we demonstrate two-junction UV LEDs enabled by transparent tunnel junctions. Low voltage-drop tunnel junctions were realized in Al 0.3 Ga 0.7 N layers through a combination of high doping and compositional grading. Capacitance and current–voltage measurements confirmed the operation of two junctions in series. The voltage drop of the two-junction LED was 2.1 times that of an equivalent single-junction LED, and the two-junction LED had higher external quantum efficiency (147%) than the single junction.

This work reports on the optimal dose for a step-etched single-zone junction termination extension by means of a multi-point study on etch depth. Breakdown and device characteristics are reported on over one hundred devices for each dataset to determine a statistically significant representation of the population. Electroluminescence imaging during avalanche breakdown confirms the point at which the JTE switches from full depletion to partial depletion, which corresponds to the maximum breakdown.

Ultra-violet (UV) light emitting diodes operating at 339 nm using transparent interband tunnel junctions are reported. Tunneling-based ultraviolet light emitting diodes were grown by plasma-assisted molecular beam epitaxy on 30% Al-content AlGaN layers. A low tunnel junction voltage drop is obtained through the use of compositionally graded n and p-type layers in the tunnel junction, which enhance hole density and tunneling rates. The transparent tunnel junction-based UV LED reported here show a low voltage drop of 5.55 V at 20 A/cm(2) and an on-wafer external quantum efficiency of 1.02% at 80 A/cm(2).