Vertical heterojunction NiO/β n-Ga₂O/n⁺ Ga₂O₃ rectifiers with 100 μm diameter fabricated on ∼17–18 μm thick drift layers with carrier concentration 8.8 × 10¹⁵ cm⁻³ and employing simple dual-layer PECVD SiNx/SiO₂ edge termination demonstrate breakdown voltages (V_B) up to 13.5 kV, on-voltage (V_ON) of ∼2.2 V and on-state resistance R_ON of 11.1–12 mΩ.cm². Without edge termination, the maximum V_B was 7.9 kV. The average critical breakdown field in heterojunctions was ∼7.4–9.4 MV. cm⁻¹, within the reported theoretical value range from 8–15 MV.cm⁻¹ for β-Ga₂O_3. For large area (1 mm diameter) heterojunction deives, the maximum V_B was 7.2 kV with optimized edge termination and 3.9 kV without edge termination. The associated maximum power figure-of-merit, V_B ²/R_ON is 15.2 GW·cm⁻² for small area devices and 0.65 GW.cm⁻² for large area devices. By sharp contrast, small area Schottky rectifiers concurrently fabricated on the same drift layers had maximum V_B of 3.6 kV with edge termination and 2.7 kV without edge termination, but lower V_ON of 0.71–0.75 V. The average critical breakdown field in these devices was in the range 1.9–2.7 MV. cm⁻¹, showing the importance of both the heterojunction and edge termination. Transmission electron microscopy showed an absence of lattice damage between the PECVD and sputtered films within the device and the underlying epitaxial Ga₂O₃. The key advances are thicker, lower doped drift layers and optimization of edge termination design and deposition processes.

The effect of doping in the drift layer and the thickness and extent of extension beyond the cathode contact of a NiO bilayer in vertical NiO/β-Ga2O3 rectifiers is reported. Decreasing the drift layer doping from 8 × 1015 to 6.7 × 1015 cm−3 produced an increase in reverse breakdown voltage (VB) from 7.7 to 8.9 kV, the highest reported to date for small diameter devices (100 μm). Increasing the bottom NiO layer from 10 to 20 nm did not affect the forward current–voltage characteristics but did reduce reverse leakage current for wider guard rings and reduced the reverse recovery switching time. The NiO extension beyond the cathode metal to form guard rings had only a slight effect (∼5%) in reverse breakdown voltage. The use of NiO to form a pn heterojunction made a huge improvement in VB compared to conventional Schottky rectifiers, where the breakdown voltage was ∼1 kV. The on-state resistance (RON) was increased from 7.1 m Ω cm2 in Schottky rectifiers fabricated on the same wafer to 7.9 m Ω cm2 in heterojunctions. The maximum power figure of merit (VB)2/RON was 10.2 GW cm−2 for the 100 μm NiO/Ga2O3 devices. We also fabricated large area (1 mm2) devices on the same wafer, achieving VB of 4 kV and 4.1 A forward current. The figure-of-merit was 9 GW cm−2 for these devices. These parameters are the highest reported for large area Ga2O3 rectifiers. Both the small area and large area devices have performance exceeding the unipolar power device performance of both SiC and GaN.

Optimized vertical heterojunction rectifiers with a diameter of 100 µm, consisting of sputter-deposited p-type NiO forming a p–n junction with thick (10 µm) Ga₂O₃ drift layers grown by halide vapor phase epitaxy (HVPE) on (001) Sn-doped (10¹⁹ cm⁻³) β-Ga₂O₃ substrates, exhibited breakdown voltages >8 kV over large areas (>1 cm²). The key requirements were low drift layer doping concentrations (<10¹⁶ cm³), low power during the NiO deposition to avoid interfacial damage at the heterointerface and formation of a guard ring using extension of the NiO beyond the cathode metal contact. Breakdown still occurred at the contact periphery, suggesting that further optimization of the edge termination could produce even larger breakdown voltages. On-state resistances without substrate thinning were 9 GW.cm⁻². The devices showed an almost temperature-independent breakdown to 600 K. These results show the remarkable potential of NiO/Ga₂O₃ rectifiers for performance beyond the limits of both SiC and GaN. The important points to achieve the excellent performance were: (1) low drift doping concentration, (2) low power during the NiO deposition and (3) formation of a guard ring.

The characteristics of sputtered NiO for use in pn heterojunctions with Ga2O3 were investigated as a function of sputtering parameters and postdeposition annealing temperature. The oxygen/ nickel and Ni2O3/NiO ratios, as well as the bandgap and resistivity, increased as a function of O2/Ar gas flow ratio. For example, the bandgap increased from 3.7 to 3.9 eV and the resistivity increased from 0.1 to 2.9 Ω cm for the O2/Ar ratio increasing from 1/30 to 1/3. By sharp contrast, the bandgap and Ni2O3/NiO ratio decreased monotonically with postdeposition annealing temperatures up to 600 °C, but the density of films increased due to a higher fraction of NiO being present. Hydrogen is readily incorporated into NiO during exposure to plasmas, as delineated by secondary ion mass spectrometry measurements on deuterated films. The band alignments of NiO films were type II-staggered gaps with both α- and β-Ga2O3. The breakdown voltage of NiO/β-Ga2O3 heterojunction rectifiers was also a strong function of the O2/Ar flow ratio during deposition, with values of 1350 V for 1/3 and 830 V for 1/30.

Vertical geometry NiO/β n-Ga2O/n+ Ga2O3 heterojunction rectifiers with contact sizes from 50 to 200 μm diameter showed breakdown voltages (VB) up to 7.5 kV for drift region carrier concentration of 8 × 1015 cm−3. This exceeds the unipolar 1D limit for SiC and was achieved without substrate thinning or annealing of the epi layer structure. The power figure-of-merit, VB2/RON, was 6.2 GW cm−2, where RON is the on-state resistance (9.3–14.7 mΩ cm2). The average electric field strength was 7.56 MV/cm, approaching the maximum for β-Ga2O3. The on–off ratio switching from 5 to 0 V was 2 × 1013, while it was 3 × 1010–2 × 1011 switching to 100 V. The turn-on voltage was in the range 1.9–2.1 V for the different contact diameters, while the reverse current density was in the range 2 × 10−8–2 × 10−9 A cm−2 at −100 V. The reverse recovery time was 21 ns, while the forward current density was >100 A/cm2 at 5 V.

The prevalence of breast cancer in women underscores the urgent need for innovative and efficient detection methods. This study addresses this imperative by harnessing salivary biomarkers, offering a noninvasive and accessible means of identifying breast cancer. In this study, commercially available disposable based strips similar to the commonly used glucose detection strips were utilized and functionalized to detect breast cancer with biomarkers of HER2 and CA15-3. The results demonstrated limits of detection for these two biomarkers reached as low as 1 fg/ml much lower than those of conventional enzyme-linked immunosorbent assay in the range of 1∼4 ng/ml. By employing a synchronized double-pulse method to apply 10 of 1.2 ms voltage pulses to the electrode of sensing strip and drain electrode of the transistor for amplifying the detected signal, and the detected signal was the average of 10 digital output readings corresponding to those 10 voltage pulses. The sensor sensitivities were achieved approximately 70/dec and 30/dec for HER2 and CA15-3, respectively. Moreover, the efficiency of this novel technique is underscored by its swift testing time of less than 15 ms and its minimal sample requirement of only 3 μl of saliva. The simplicity of operation and the potential for widespread public use in the future position this approach as a transformative tool in the early detection of breast cancer. This research not only provides a crucial advancement in diagnostic methodologies but also holds the promise of revolutionizing public health practices.