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There is increasing interest in the alpha polytype of Ga2O3 because of its even larger bandgap than the more studied beta polytype, but in common with the latter, there is no viable p-type doping technology. One option is to use p-type oxides to realize heterojunctions and NiO is one of the candidate oxides. The band alignment of sputtered NiO on α-Ga2O3 remains type II, staggered gap for annealing temperatures up to 600°C, showing that this a viable approach for hole injection in power electronic devices based on the alpha polytype of Ga2O3. The magnitude of both the conduction and valence band offsets increase with temperature up to 500°C, but then are stable to 600°C. For the as-deposited NiO/ α-Ga2O3 heterojunction, ΔEV=-2.8 eV and ΔEC= 1.6 eV, while after 600°C annealing the corresponding values are ΔEV=-4.4 eV and ΔEC= 3.02 eV. These values are 1-2 eV larger than for the NiO/β-Ga2O3 heterojunction.

The fabrication and switching characteristics of large diameter (1mm) Ga2O3 heterojunction rectifiers with bilayers of NiO as the p-type layer are reported. The band alignment shows this is a staggered-gap system. Selective wet and dry etching was developed to allow fabrication of vertical geometry heterojunction rectifiers. A forward current of 2.5 Amperes was achieved, with a reverse breakdown voltage of 2.7 kilovolts. The on/off ratio was above 108 over a broad range of switching voltages, with a measurement-limited reverse recovery time of 100 nanoseconds.

NiO/ β-(Al x Ga1-x )2O3 /Ga2O3 heterojunction lateral geometry rectifiers with diameter 50-100 µm exhibited maximum reverse breakdown voltages >7kV, showing the advantage of increasing the bandgap using the β-(Al x Ga1-x )2O3 alloy. This Si-doped alloy layer was grown by Metal Organic Chemical Vapor Deposition with an Al composition of ~21 %. On state resistances were in the range 50-2180 Ω.cm2, leading to power figures-of-merit up to 0.72 MW.cm-2. The forward turn-on voltage was in the range 2.3-2.5 V, with maximum on/off ratios >700 when switching from 5V forward to reverse biases up to -100V. Transmission line measurements showed the specific contact resistance was 0.12 Ω.cm2. The breakdown voltage is among the highest reported for any lateral geometry Ga2O3-based rectifier.

In this study, the focused ion beam (FIB) Ga+ irradiation damage on β-Ga2O3 Schottky diode structure under different ion energies were tested, and the effective recovery of diode characteristics after rapid thermal annealing (RTA) post-annealing were also studied. β-Ga2O3 based Schottky diodes have attracted increasing attention because of the prospects for use in next-generation high-power electronics. Focused ion beam (FIB) machining is one of the newest processing techniques, which is mainly used in semiconductor and chip manufacture and transmission electron microscopy (TEM) sample preparation. During FIB sample preparation, the energetic Ga ions are incident onto the sample to induce physical sputtering of the material, which in turn, modifies its chemical composition, crystallinity and electrical properties. It has been found in many studies that 20-30 nm amorphous layer can form on silicon after irradiation with 30 keV Ga ions. From our previous study, the FIB ion beam induced damages could cause significant degradation on diode on-resistance and turn-on voltage. Milling at lower energy has been used to reduce the electrical damage. However low energy milling is more time consuming and less precise. Short-time, high-temperature post-annealing treatments, such as RTA has been proved could remove the crystal damage. Thus, there is a great need to determine the maximal ion beam energy and post-annealing temperature for FIB without degrading diode characteristics. In this work, energies ranging from 2 keV to 30 keV were employed to mill off a part of Ga2O3 epi-layer and Schottky diodes were fabricated on milled surface. Forward IV characteristic shows less degradations with lower ion beam energies. For the diodes exposed to 2, 5, and 10 keV Ga+ ion beams exhibited significant recovery of diode on resistance and turn-on voltage after 300°C RTA annealing. For the diodes exposed to 30 keV Ga+ ion beams recovered significantly after 400 °C annealing. In conclusion, decreasing the FIB energy shows effective alleviations on the FIB damage, and post annealing treatment effectively recovered the damage from Ga+ implantation. This is the first time the thermal annealing treatment was performed for Ga2O3 Schottky diode recovering from FIB cutting, it is an effective and simple way to mitigate the degradation, especially for higher ion beam energies. This work could broaden the FIB’s usability and release tremendous potential as a specimen preparation and imaging tool in materials science applications. Figure 1

In this work, we demonstrate the rejuvenation of Ti/4H-SiC Schottky barrier diodes after forward current-induced degradation, at room temperature and in a few seconds, by exploiting the physics of high-energy electron interactions with defects. The diodes were intentionally degraded to a 42% decrease in forward current and a 9% increase in leakage current through accelerated electrical stressing. The key feature of our proposed rejuvenation process is very high current density electrical pulsing with low frequency and duty cycle to suppress any temperature rise. The primary stimulus is, therefore, the electron wind force, which is derived from the loss of the momentum of the high energy electrons upon collision with the defects. Such defect-specific or “just in location” mobilization of atoms allows a significant decrease in defect concentration, which is not possible with conventional thermal annealing that requires higher temperatures and longer times. We show evidence of rejuvenation with additional improvement in leakage current (16%) and forward current (38%) beyond the pristine condition. Transmission electron microscopy, geometric phase analysis, Raman spectroscopy, and energy dispersive x-ray-spectroscopy reveal the enhancement of defects and interfaces. The ultrafast and room temperature process has the potential for rejuvenating electronic devices operating in high power and harsh environmental conditions.

There are numerous applications for deep UV AlGaN Light-Emitting Diodes (LEDs) in virus inactivation, air and water purification, sterilization, bioagent detection and UV polymer curing. The long-term stability of these LEDs is also of interest for long-duration space missions such as the Laser Interferometer Space Antenna (LISA), the first gravitational wave detector in space. We review the literature on long-term aging of these devices as a function of drive current, temperature and dc versus pulsed operation. The LEDs typically show a gradual decline in output power (up to 50%) over extended operating times (>100 h) and the rate of decline is mainly driven by current and temperature. Experimentally, the degradation rate is dependent on the cube of drive current density and exponentially on temperature. The main mechanism for this decline appears to be creation/migration of point defects. Pre-screening by considering the ratio of band edge-to-midgap emission and LED ideality factor is effective in identifying populations of devices that show long lifetimes (>10,000 h), defined as output power falling to 70% of the initial value.

While radiation is known to degrade AlGaN/GaN high-electron-mobility transistors (HEMTs), the question remains on the extent of damage governed by the presence of an electrical field in the device. In this study, we induced displacement damage in HEMTs in both ON and OFF states by irradiating with 2.8 MeV Au4+ ion to fluence levels ranging from 1.72 × 10 10 to 3.745 × 10 13 ions cm−2, or 0.001–2 displacement per atom (dpa). Electrical measurement is done in situ, and high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray (EDX), geometrical phase analysis (GPA), and micro-Raman are performed on the highest fluence of Au4+ irradiated devices. The selected heavy ion irradiation causes cascade damage in the passivation, AlGaN, and GaN layers and at all associated interfaces. After just 0.1 dpa, the current density in the ON-mode device deteriorates by two orders of magnitude, whereas the OFF-mode device totally ceases to operate. Moreover, six orders of magnitude increase in leakage current and loss of gate control over the 2-dimensional electron gas channel are observed. GPA and Raman analysis reveal strain relaxation after a 2 dpa damage level in devices. Significant defects and intermixing of atoms near AlGaN/GaN interfaces and GaN layer are found from HRTEM and EDX analyses, which can substantially alter device characteristics and result in complete failure.

This paper presents a pulse-stimulus sensor readout circuit for use in cardiovascular disease examinations. The sensor is based on a gold nanoparticle plate with an antibody post-modification. The proposed system utilizes gated pulses to detect the biomarker Cardiac Troponin I in an ionic solution. The characteristic of the electrostatic double-layer capacitor generated by the analyte is related to the concentration of Cardiac Troponin I in the solvent. After sensing by the transistor, a current-to-frequency converter (I-to-F) and delay-line-based time-to-digital converter (TDC) convert the information into a series of digital codes for further analysis. The design is fabricated in a 0.18-μm standard CMOS process. The chip occupies an area of 0.92 mm2 and consumes 125 μW. In the measurements, the proposed circuit achieved a 1.77 Hz/pg-mL sensitivity and 72.43 dB dynamic range.

α-Ga2O3 has the corundum structure analogous to that of α-Al2O3. The bandgap energy of α-Ga2O3 is 5.3 eV and is greater than that of β-Ga2O3, making the α-phase attractive for devices that benefit from its wider bandgap. The O–H and O–D centers produced by the implantation of H+ and D+ into α-Ga2O3 have been studied by infrared spectroscopy and complementary theory. An O–H line at 3269 cm−1 is assigned to H complexed with a Ga vacancy (VGa), similar to the case of H trapped by an Al vacancy (VAl) in α-Al2O3. The isolated VGa and VAl defects in α-Ga2O3 and α-Al2O3 are found by theory to have a “shifted” vacancy-interstitial-vacancy equilibrium configuration, similar to VGa in β-Ga2O3, which also has shifted structures. However, the addition of H causes the complex with H trapped at an unshifted vacancy to have the lowest energy in both α-Ga2O3 and α-Al2O3.

In this article, we investigate defect nucleation leading to device degradation in $\beta $ -Ga2O3 Schottky barrier diodes by operating them inside a transmission electron microscope. Such in situ approach allows simultaneous visualization and quantitative device characterization, not possible with the current art of postmortem microscopy. High current density and associated mechanical and thermal fields are shown to induce different types of crystal defects, from vacancy cluster and stacking fault to microcrack generation prior to failure. These structural defects can act as traps for carrier and cause device failure at high biasing voltage. Fundamental insights on nucleation of these defects and their evolution are important from materials reliability and device design perspectives.

The SARS-CoV-2 pandemic has had a significant impact worldwide. Currently, the most common detection methods for the virus are polymerase chain reaction (PCR) and lateral flow tests. PCR takes more than an hour to obtain the results and lateral flow tests have difficulty with detecting the virus at low concentrations. In this study, 60 clinical human saliva samples, which included 30 positive and 30 negative samples confirmed with RT-PCR, were screened for COVID-19 using disposable glucose biosensor strips and a reusable printed circuit board. The disposable strips were gold plated and functionalized to immobilize antibodies on the gold film. After functionalization, the strips were connected to the gate electrode of a metal-oxide-semiconductor field-effect transistor on the printed circuit board to amplify the test signals. A synchronous double-pulsed bias voltage was applied to the drain of the transistor and strips. The resulting change in drain waveforms was converted to digital readings. The RT-PCR-confirmed saliva samples were tested again using quantitative PCR (RT-qPCR) to determine cycling threshold (Ct) values. Ct values up to 45 refer to the number of amplification cycles needed to detect the presence of the virus. These PCR results were compared with digital readings from the sensor to better evaluate the sensor technology. The results indicate that the samples with a range of Ct values from 17.8 to 35 can be differentiated, which highlights the increased sensitivity of this sensor technology. This research exhibits the potential of this biosensor technology to be further developed into a cost-effective, point-of-care, and portable rapid detection method for SARS-CoV-2.

Space-based gravitational wave detection will be carried out by the laser interferometer space antenna (LISA), a joint European Space Agency and NASA collaboration. The configuration of this antenna will include three identical spacecraft in a triangular formation separated by [Formula: see text], flying in a drag-free formation around free-falling test masses. Charging of the test masses by cosmic ray fluxes and solar energetic particles must be compensated by photons that contain more energy than the effective work function of gold ([Formula: see text]). The UV photons will be provided by AlGaN light emitting diodes, which must operate reliably for the duration of the mission. We have tested a large number (96 for dc and pulsed testing, more than 200 for all tests) of UV LEDs over a period of up to 600 days to characterize their performance over a wide range of operating conditions, assessing the lifetime performance under dc (1–[Formula: see text] drive current) and pulsed conditions (500–100 000 pulses per second) and temperatures ranging from [Formula: see text] to [Formula: see text]. Degradation of UV light output is faster at elevated temperatures and dc conditions. Preselection of LEDs based on initial spectral ratio of peak-to-midgap emission and ideality factor provides a positive correlation with subsequent reliability. The UV LEDs used for LISA will need to support 2 years of cruise and commissioning plus a 4-year baseline science mission.

This study investigated a SARS-CoV-2 virus detection mechanism using dry-stored disposable strips. The accuracy of this sensing platform is as good as polymerase chain reaction (PCR) with a detection time of fewer than 30 s. SARS-CoV-2 antibodies were biofunctionalized on disposable strips similar to glucose detection strips to detect the presence and concentrations of SARS-CoV-2 in saliva samples. Eight 1 ms electric pulses were sent through the sensor strip with a saliva sample in its microfluidic chamber. A circuit board embedded with MOSFET was also employed to amplify the detected signals and convert the signal to digital readings displayed on an LCD screen. The COV-antibody functionalized disposable strips were stored in a dry condition for at least one day before analyzing clinical human saliva samples with known cycling threshold (Ct) values confirmed with conventional PCR tests. Results demonstrate our system is capable of showing qualitative positive or negative results within 30 s and providing quantitative SARS-CoV-2 concentrations in terms of Ct values in 5 min.

Halide vapor phase epitaxial (HVPE) Ga2O3 films were grown on c-plane sapphire and diamond substrates at temperatures up to 550 °C without the use of a barrier dielectric layer to protect the diamond surface. Corundum phase α-Ga2O3 was grown on the sapphire substrates, whereas the growth on diamond resulted in regions of nanocrystalline β-Ga2O3 (nc-β-Ga2O3) when oxygen was present in the HVPE reactor only during film growth. X-ray diffraction confirmed the growth of α-Ga2O3 on sapphire but failed to detect any β-Ga2O3 reflections from the films grown on diamond. These films were further characterized via Raman spectroscopy, which revealed the β-Ga2O3 phase of these films. Transmission electron microscopy demonstrated the nanocrystalline character of these films. From cathodoluminescence spectra, three emission bands, UVL′, UVL, and BL, were observed for both the α-Ga2O3/sapphire and nc-Ga2O3/diamond, and these bands were centered at approximately 3.7, 3.2, and 2.7 eV.

Radiation damage in electronic devices is known to be influenced by physics, design, and materials system. Here, we report the effects of biasing state (such as ON and OFF) and pre-existing damage in GaN high electron mobility transistors exposed to γ radiation. Controlled and accelerated DC biasing was used to prestress the devices, which showed significant degradation in device characteristics compared to pristine devices under ON and OFF states after γ irradiation. The experiment is performed in situ for the ON-state to investigate transient effects during irradiation until the total dose reaches 10 Mrad. It shows that threshold voltage, maximum transconductance, and leakage current initially decrease with dosage but slowly converge to a steady value at higher doses. After 10 Mrad irradiation, the OFF-state device demonstrates larger RON and one order of magnitude increased leakage current compared to the ON-state irradiated device. The micro-Raman study also confirms that the ON-state operation shows more radiation hardness than OFF and prestressed devices. Prestressed devices generate the highest threshold voltage shift from −2.85 to −2.49 V and two orders of magnitude higher leakage current with decreased saturation current after irradiation. These findings indicate that high electric fields during stressing can generate defects by modifying strain distribution, and higher defect density can not only create more charges during irradiation but also accelerate the diffusion process from the ionizing track to the nearest collector and consequently degrade device performances.

Leakage of human cerebrospinal fluid (CSF) caused by trauma or other reasons presents exceptional challenges in clinical analysis and can have severe medical repercussions. Conventional test methods, including enzyme-linked immunosorbent assay and immunofixation electrophoresis testing, typically are performed at a few clinical reference laboratories, which may potentially delay proper diagnosis and treatment. At the same time, medical imaging can serve as a secondary diagnosis tool. This work presented here reports the use of a point-of-care electrochemical sensor for detection of beta-2-transferrin (B2T), a unique isomer of transferrin that is present exclusively in human CSF but is absent in other bodily fluids. Limits of detection were examined via serial dilution of human samples with known B2T concentrations down to 7 × 10(−12) g B2T/ml while maintaining excellent sensitivity. Nine human samples with varying levels of B2T were compared using up to 100 times dilution to confirm the validity of sensor output across different patient samples.

1. Plate Type Exchangers 2. Dynamic Systems 3. A Historical Survey of Research on Gallium Nitride 4. Growth of Group III Nitrides from Molecular Beams 5. Ternary Alloys 6. Optical Characterization of GaN and Related Materials 7. Theoretical Studies in GaN 8. GaAsN Alloys and GaN/GaAs Thin Layer Structures 9. The Contribution of Defects to the Electrical and Optical Properties of GaN 10. Growth of GaN Single Crystals Under High Nitrogen Pressure 11.Ion Implantation Doping and Isolation of III-Nitride Materials 12. High-Density ECR Etching of Group-III Nitrides 13. Contacts on III-Nitrides 14. III-V Nitride Based LEDs 15. III-V Nitride Electronic Devices 16. Physical Properties of the Bulk GaN Crystals Grown by the High-Pressure, High Temperature Method 17. Microstructure of Epitaxial III-V Nitride Thin Films 18. The Role of Hydrogen in GaN and Related Compounds

There is increasing interest in deep UV Light-Emitting Diodes (LEDs) for applications in water purification, virus inactivation, sterilization, bioagent detection, and UV curing, as well as charge management control in the Laser Interferometer Space Antenna (LISA), which will be the first gravitational wave detector in space. To fully understand the current state of commercial UV LEDs and assess their performance for use on LISA, large numbers of UV LEDs need to be tested across a range of temperatures while operating in air or in a vacuum. We describe a new hardware system designed to accommodate a high volume of UV LED performance tests and present the performance testing results from over 200 UV LEDs with wavelengths in the 250 nm range.

Heavily Sn-doped films of α-Ga2O3 were grown by halide vapor phase epitaxy (HVPE) on basal plane c-sapphire and on (10-12) r-sapphire substrates with and without α-Cr2O3 thin buffers prepared by magnetron sputtering and annealing in air at 500 °C for 3 h. For both substrate orientations, the use of α-Cr2O3 buffers led to three major effects. The first was a substantial decrease of the half-width of the symmetric and asymmetric x-ray reflections. The second was an order of magnitude decrease of the net donor concentration produced by flowing the same amounts of Sn into the reactor. Third, there was a reduction in the concentration of the major electron trap in the films near E c − 1 eV by more than a factor of two. These results show the major influence of sapphire substrate orientation on the electrical and structural properties of α-Ga2O3 grown by HVPE.

Performance and reliability of microelectronic devices are governed by the mechanical strain. Typically, strain engineering implies uniformly distributed strain. However, we offer a different perspective by hypothesizing that very small but localized strain (or stress) may have significant impact on the overall behavior of AlGaN/GaN high electron mobility transistors (HEMTs). Micro to nanoscale confined mechanical stress fields may develop unavoidably and are ignored because their spatial average is insignificant. We exploit high resolution techniques such as transmission electron microscopy (TEM) and micro-Raman spectroscopy to spatially resolve the stress field in GaN HEMTs to demonstrate the stress localization and then radiation effects. To study stress localization effects on the global electrical characteristics, we introduced a highly localized strain relief by milling a 20×30 μm2 micro trench about 70 μm deep on the backside of an 800×840 μm2 size HEMT die. The resulting local relaxation of in-plane residual strain was mapped using micro-Raman technique. Our results show that a decrease of only 0.02% strain can decrease the overall output saturation current up to ~20 %. The drop of output current is attributed to reduced two-dimensional electron gas (2DEG) sheet carrier density and electron mobility due to the strain relief in the device layers. However, the mechanistic process of strain relief also causes defect generation at the interfaces, which increases leakage current. Our technique for localized strain re-distribution could be an effective tool to surrogate the influence of inherent localized strain build-up across the channel of electronic devices. To study radiation effects, we exposed the HEMTs to gamma rays (up to 10 megaRads dose at 180 kiloRads per hour). We report the thermal and mechanical responses in terms of the changes in lattice strain and temperature, which were simultaneously characterized by changes in phonon frequency of E2 (high) and A1 (LO) from on- state and unpowered/pinched off reference states. Lower doses of radiation improved the electrical properties, however degradation initiated at about 1 megaRads. We observed about 16% decrease in saturation current and 6% decrease in transconductance at the highest dose. However, leakage current increase by 3 orders of magnitude was the most notable radiation effect. We observed temperature increase by 40% and mechanical stress increase by a factor of three at dose of 10 megaRads compared to the pristine devices. Spatial mapping of mechanical stress along the channel identifies the gate region as mechanically affected area, whereas the thermal degradation was mostly uniform. Transmission electron microscopy showed contrast changes reflecting high vacancy concentration in the gate region. These findings suggest that localized stress (mechanical hotspots) may increases vulnerability to radiation damage by accommodating higher concentration of defects that promote leakage current. Finally, we present experimental results on a novel concept of electron wind force driven room temperature annealing of GaN and SiC devices. This technique was applied on Gamma irradiated HEMT and SiC diodes. We demonstrate full recovery of the HEMTs irradiated to 10 mega-Rads gamma radiation. About 200% performance improvement was recorded for the SiC diodes. Our hypothesis is that mechanical stress waves are instrumental in eliminating interfacial defects.

β-Ga2O3 is attractive for high temperature applications in harsh environments that cannot be tolerated by conventional electronics. Its wide bandgap allows operation at elevated temperatures, while it is also radiation hard. Radiation tolerance is an important factor while fabricating microelectronics and typical radiation damage suffered includes total dose effects, displacement damage, and single event effects. Ga2O3 appears to be more resistant to displacement damage than GaN and SiC, as expected from a consideration of their average bond strengths. EPR results of neutron irradiated, bulk samples suggested that octahedral gallium monovacancy defects were the main defects produced. Proton irradiation introduces two main paramagnetic defects in Ga2O3, which are stable at room temperature. Charge carrier removal can be explained by Fermi-level pinning far from the conduction band minimum due to gallium interstitials (Ga i ), vacancies (VG a), and antisites (GaO). While there are no experimental or simulation results for single event effects in Ga2O3 to this point, it has become worryingly apparent that while other wide bandgap semiconductors like SiC and GaN are robust against displacement damage and total ionizing dose, they display significant vulnerability to single event effects at high Linear Energy Transfer (LET) and at much lower biases than expected. We have also analyzed the transient response of β-Ga2O3 SBDs to heavy-ion strikes via TCAD simulations to understand the effect of various structural parameters. Using field metal rings has been reported to improve the breakdown voltage of these devices and biasing those rings can help control the breakdown voltage of the device. We find that such biased rings help in the removal of the charge deposited by the ion strike.

A SARS-CoV-2 rapid detection technology employed commercially available test strips similar to the blood glucose test strips were developed with a detection time of 30 sec and the sensitivity as good as polymerase chain reaction (PCR). The analytical specificity of this technology was also investigated by testing the cross-reactivity and microbial interference on this biosensor platform using FDA suggested 31 different contaminants. Variants of SARS-CoV-2 have mutated along with the worldwide spread of the pandemic. Currently, the most common detection methods are PCR and lateral flow tests. The former takes more than an hour to obtain results and the latter has difficulty detecting the virus at low concentrations (for cycling threshold values, CT values, >25 to 27). Therefore, the demand for a fast, cost-effective, and low detection limit testing method has been significantly increased. In this work, 60 human saliva samples with 30 positive and 30 negative were collected and tested with PCR for SARS-CoV-2 prior to using our disposable strips. For this technology, the test strip was connected to the gate electrode of the MOSFET on the printed circuit board that serves to amplify signals. A synchronous double-pulsed bias voltages (around 1 ms) were sent to both the drain and gate of the MOSFET. The resulting change in drain waveforms was converted to digital readings and compared with cycling threshold (Ct) values of human samples to assess this sensor technology. The result signifies that positive human samples with a range of Ct values from 17.8 to 40 can be differentiated, along with proving none of those high-risk organisms would hinder the sensitivity of our system. Which demonstrated the potential of this system to be developed into a cost-effective and point-of-care rapid detection for SARS-CoV-2. Figure 1

In this study, we present a double-layer NiO/β-Ga2O3 p-n heterojunction diode, which exhibits high performance with breakdown voltage, on-resistance and turn-on voltage. β-Ga2O3 has been attracting great attention as one of the excellent materials for next-generation high-performance electronics, owing to its high Baliga’s figure-of-merit (BFOM), carrier mobility, critical breakdown field strength and low-cost, simple growth methods. β-Ga2O3 based unipolar devices, especially Schottky barrier diodes, have been developed rapidly; however, the lack of p-type doping in Ga2O3 remains a challenge for bipolar devices, which can more generally be utilized in practical applications. One of the possible strategies is to introduce a p-type material to construct p-n junctions with n-type β-Ga2O3. The NiO turns out to be a feasible p-type material that has a wide bandgap, adjustable doping concentration, intrinsic p-type conductivity and diverse deposition techniques to form a uniform film, such as sputtering, PLD and thermal oxidation. In this study, we fabricated the diodes from depositing Ti/Au for the back Ohmic contact metal and annealing at 550℃ in N2, followed by sputtering two NiO layers with different doping concentrations by tuning the ratio of Ar/O2. The Ni/Au contact metal was deposited onto the NiO layer after annealing at 300℃ under O2 ambient. Compared to typical NiO thicknesses of 300 to 500 nm, we turned to another trend with ultra-thin layers of NiO. The top layer NiO thickness is 10nm while the bottom layer of NiO thickness varies from 10 to 80 nm. Through the design of the ultra-thin (20 nm) double-layer NiO structure, the breakdown voltage (Vb) is substantially improved to 4.7 kV and demonstrates the on-resistance (Ron) of 11.3 mΩ·cm2, which leads to a figure-of-merit (Vb 2/Ron) of 2 GW/cm2. The high Vb is attributed to the structure of both the double-layer and the guard-ring designs. From the TCAD simulation, the peak of the electric field locates at the edge of the diodes. Increasing the doping concentration of NiO layer contact with Ni/Au can reduce the electric field on the edge of the Ohmic contact. On the other hand, the low doping concentration of the NiO contact with β-Ga2O3 can effectively move the electric field crowding on the edge to the inside of devices. In addition, the extension guard ring can also spread the electric field crowding. This work provides a desirable design strategy for p-n heterojunction, which has the highest breakdown voltage among all Ga2O3-based p-n diodes. Figure 1