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In this work, a p-GaN/AlGaN/GaN hybrid anode diode (HAD) temperature sensor was fabricated by using hydrogen plasma treatment. The zero-temperature coefficient (ZTC) is about 0.7 V from the ${I}$ – ${V}$ characteristics measured at various temperatures. At the forward region below the ZTC bias point, the anode voltage ( ${V}_{{\mathrm {A}}}$ ) at a specific anode current density ( ${I}_{{\mathrm {A}}}$ ) decreases linearly with the increase of temperature, resulting in a sensitivity ranging from 1.64 mV/K (10−5 mA/mm) to 0.69 mV/K (10−2 mA/mm). The leakage current at the reverse bias also presents a temperature-dependent behavior with a sensitivity ranging from 16.76 mA/K (−100 V) to 27.94 mA/K (−5 V). The experimental results can be interpreted well by the two-dimensional variable range hopping model.

In this article, the normally- OFF etching-free p-GaN stripe array gate AlGaN/GaN high-electron-mobility-transistors (PSAG-HEMTs) are designed and experimentally demonstrated through hydrogen plasma treatment. The unique threshold voltage ( ${V}_{TH}$ ) modulation technique based on the PSAG structure is proposed and simulated. Using this method, the ${V}_{TH}$ can be continuously shifted from −0.14 to +1.03 V by just tuning the widths of p-GaN ( ${W}_{p}$ ) and hydrogenated p-GaN (HR-GaN) stripes. To improve breakdown voltage (BV), the PSAG is extended to the drain side based on the 3-D simulation results so that a new electric-field peak is introduced at the edge of the PSAG in the drain side and then the surface electric field is optimized. Besides this, an enhanced conductivity effect is observed and results in a low ON-resistance. The fabricated PSAG-HEMT with $0.75~\mu \text{m}~{W}_{p}$ and 4- $\mu \text{m}$ extension length ( ${L}_{E}$ ) exhibits a positive ${V}_{TH}$ of +0.8 V, a low specific ON-resistance ( ${R}_{\text {ON,sp}}$ ) of 2.73 $\text{m}\Omega \cdot cm^{2}$ , a high BV of 852 V (1449 V) at ${I}_{D} = 1 ~\mu \text{A}$ /mm, and BFOM of 266 MW/cm2 (769 MW/cm2) with substrate grounded (floating), which demonstrate significant improvements over Reference single-gate E-mode and D-mode devices. This work provides a promising architecture for future high voltage normally- OFF p-GaN HEMT devices.

This letter reports the phenomenon of current drops in an AlGaN/GaN heterojunction with CF4 plasma treated in a polar gas ambient. Ungated AlGaN/GaN HEMT with CF4 plasma treatment was tested in ethanol, acetonitrile, and an acetic acid gas ambient, and the current dropped by 52%, 51% and 61%, respectively, which are much higher than the drops (6%, 7%, and 8%) in normal ungated HEMT. On the one hand, based on “ambient doping”, a possible model was proposed to explain this phenomenon. On the other hand, the lower initial current was very important to increase the sensitivity. This means that the initial current should be elaborately designed to achieve maximum sensitivity.

A dependable and robust technique for nanomachining is ion implantation. In this work, hydrogen (H) ion implantation was used, for the first time, to passivate p-GaN, except for the gate area, in order to create a normally off p-GaN/AlGaN/GaN high-electron-mobility transistor (HEMT). Ion implantation passivation reduces H ion diffusion in p-GaN, allowing it to withstand temperatures above 350 °C. Through experiments and analyses, the H ion implantation energy and dosage required to passivate p-GaN, by generating Mg-H neutral complexes, were determined to be 20 keV and 1.5 × 1013 cm−2, respectively. After conducting annealing procedures at various temperatures, we discovered that 400 °C was the ideal temperature to effectively obtain a normally off p-GaN HEMT. A threshold voltage of 0.8 V was achievable. The p-GaN HEMT also had a breakdown voltage of 642 V at a gate voltage of 0 V, maximum transconductance of 57.7 mS/mm, an on/off current ratio of 108, an on-resistance of 8.4 mm, and a maximum drain current of 240.0 mA/mm at a gate voltage of 6 V after being annealed at 400 °C.

Electrical characterizations of AlGaN/GaN heterojunctions isolated by N implantation at elevated temperatures were investigated. Three-terminal measurements were carried out to characterize leakage paths, and crystal lattice damage due to implantation was monitored by high-resolution x-ray diffraction. Compared with room temperature implantation, the current leakage was reduced by ∼103 times by the implantation at 300 °C. The low leakage was attributed to low acceptor-like energy levels due to low crystal lattice damage by the “dynamic annealing” effect at high-temperature implantation. The post-annealing process increased the current leakage by two orders of magnitude. This indicates that the implantation isolation process should be conducted after higher temperature processes (>450 °C) in the fabrication of GaN devices. These results can provide valuable information for the fabrication, reliability, and mass production of various GaN-based photonics and electronics.

Herein, a monolithic micro-pixelated light-emitting diode ( μLED) display realized using ion-surface-treatment-based planar-geometry pixelation is reported. A high-resolution blue-emitting μLED array is fabricated by optimizing ion species of a near-surface selectively positioned isolation process. The μLED array exhibiting inhibited surface recombination and higher output power is achieved by using heavy ions, such as fluorine (F−), due to more stable opto-electrical isolation and better process reliability. Furthermore, the low forward voltage of F−-IST-based μLED pixels can effectively improve the wall-plug efficiency for the μLED array with a pixel diameter of 6 μm. Conversely, the poor thermal stability of light hydrogen (H+) ions limits its device applications. Analysis based on a rate equation model reveals that the luminescence mechanism of F−-IST-based μLEDs is dominated by radiative recombination, which will benefit the planar-geometry pixelation for realizing highly efficient full-color microdisplays.

A bulk acoustic wave (BAW) driven magnetoelectric (ME) antenna has narrow operating bandwidth due to its high Q factor, and an effective mechanism for bandwidth enhancement is yet to be explored. This article presents a bandwidth-enhanced magnetoelectric (BWE-ME) antenna made of a Mo/AlN/FeGa sandwich stack, which is composed of three different resonant regions. These resonant regions in the discrete device can be equated as a parallel connection of dual high-overtone bulk acoustic resonators (HBARs) and single film bulk acoustic resonators (FBARs) with tiny frequency shift among the three resonators resulting in bandwidth broadening of the BWE-ME antenna. The resonant mode and return loss curves (S11) are simulated in a two-dimensional finite element method and fitted with the Mason equivalent circuit model. The frequency domain analysis shows that the magnetic flux density bandwidth generated by the multi-resonant mode interaction is 18 MHz, which matches the bandwidth of the measured reference gain S21 curve of the BWE-ME antenna, and the far-field radiated power characterization also shows the corresponding effective bandwidth distributed. The fabricated microelectromechanical systems antenna achieves a fractional bandwidth of 2.7% while maintaining the advantage of small size (0.49 mm2). Discrete composite BAW resonators that effectively combine the multi-resonant regions of HBARs and FBARs have the potential to realize compact and broadband BAW-ME antennas in the future.

A thin SiN x film was deposited on the p-GaN layer before H plasma implantation, which cause four orders of magnitude reduction in gate reverse leakage current. The OFF-state breakdown voltage increases by 89%. It displays a more stable dynamic performance. It is believed that the decrease of gate leakage current is attributed to the increase of trap activation energies in the p-GaN layer, It is concluded that improved dynamic characteristics are owing to the fact that the SiN x film blocks the excessive H plasma into the AlGaN barrier layer and reduces the implantation damage in AlGaN barrier layer.

Vertical GaN trench-gate MOSFETs with ∼130 nm stepped sidewalls in the p-GaN channel layer are studied and two significant influences have been observed compared to the devices with smooth sidewalls. The first effect is the degraded channel mobility of 14.6 cm2 (V·s)−1 to 4.5 cm2 (V·s)−1 which can be attributed to the increased probability of surface acoustic phonons scattering under inversion conditions. Another impact is that only when a drain voltage (V DS) is applied over 30 V can the devices switch on. It can be speculated that the stepped sidewall has a horizontal channel and the transverse electric field is inadequate to drive the electrons at low V DS. According to the investigation, when devices need to be switched at a high V DS, the stepped sidewall morphology should be taken into consideration.

E-mode P-GaN/AlGaN/GaN heterostructure field-effect transistors (HFETs) of various sizes were fabricated, and their parasitic source resistance (RS) was measured. The measurement results showed that RS varied greatly with changing gate bias, and the degree of RS change also differed with the gate bias of different-sized device samples. Through theoretical analysis, it is found that polarization Coulomb field (PCF) scattering caused by the device process and gate bias can affect electron mobility ( $$\mu_{GS}$$ ) in the gate-source region, which causes the variations in $$\mu_{GS}$$ for different-sized devices and same-sized devices under different gate biases. When $$\mu_{GS}$$ changes with the device size and gate bias, the RS will change accordingly. Our study is the first to discover the gate bias dependency of RS for E-mode P-GaN/AlGaN/GaN HFETs due to PCF scattering, which provides a new idea for further in-depth studies on the RS of E-mode P-GaN/AlGaN/GaN HFETs and device performance optimization.

This study has demonstrated AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) on Si substrates with a SiNx/SiON composite gate dielectric. The threshold voltage shift in the devices was investigated. The MIS-HEMTs with the SiNx/SiON composite gate dielectric exhibited superior threshold voltage uniformity and small threshold voltage hysteresis than the reference device with SiNx only gate dielectric. The variation of the device threshold voltage was mainly related to trapping process by the interface states, as confirmed by band diagrams of MIS-HEMTs at different gate biases. Based on frequency-dependent capacitance measurements, interface state densities of the devices with the composite and single gate dielectrics were extracted, where the former showed much smaller interface state density. These results indicate that the SiNx/SiON composite gate dielectric can effectively improve the device performance of GaN-based MIS-HEMTs and contribute to the development of high-performance GaN electronic devices.

We present a multiple instance learning class activation map (MIL-CAM) approach for pixel-level minirhizotron image segmentation given weak image-level labels. Minirhizotrons are used to image plant roots in situ. Minirhizotron imagery is often composed of soil containing a few long and thin root objects of small diameter. The roots prove to be challenging for existing semantic image segmentation methods to discriminate. In addition to learning from weak labels, our proposed MIL-CAM approach re-weights the root versus soil pixels during analysis for improved performance due to the heavy imbalance between soil and root pixels. The proposed approach outperforms other attention map and multiple instance learning methods for localization of root objects in minirhizotron imagery.

In this paper, a solar-blind ultraviolet photodetector (PD) based on the graphene/vertical Ga2O3 nanowire array heterojunction was proposed and demonstrated. To the best of our knowledge, it is the first time that vertical Ga2O3 nanowire arrays have been realized. Ga2O3 nanowires were obtained by thermally oxidizing GaN nanowires grown by molecular beam epitaxy on n-doped Si substrate. Then, a monolayer graphene film was transferred to Ga2O3 nanowires to form the graphene/vertical Ga2O3 nanowire array heterojunction and transparent electrodes. The fabricated device exhibited a responsivity (R) of 0.185 A/W and rejection ratio (R258 nm/R365 nm) of 3×104 at the bias of −5 V. Moreover, the fast response times of this PD were 9 and 8 ms for the rise and decay times under 254 nm illumination, respectively, which are attributed to the unique properties of nanowire arrays and the graphene/vertical Ga2O3 nanowire array heterojunction structure.

In this paper, we systematically investigate the effect of hydrogen plasma treatment radio frequency (RF) power, treatment time, and rapid thermal annealing (RTA) on the performance of high-resistivity-cap-layer high-electron-mobility-transistor (HRCL-HEMT). It is found that high RF power contributes to obtain high drain current, but induces more damage. An appropriate RTA process not only can repair the plasma damage but also improves the static characteristics of HRCL-HEMT. Subsequently, a large hydrogen plasma treatment time window of 4 min is demonstrated with an optimized RF power of 2 W and RTA at 350 °C for 5 min. Furthermore, the thermal stability of the optimized HRCL-HEMT is investigated by high temperature test and long-term thermal stress test. No obvious degradation of ${V}_{\text {TH}}$ , ${I}_{G}$ , ${R}_{\text {SH}}$ , and dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ is observed in both before and after passivation devices after 1000-h thermal stress. Finally, gate reliability of the HRCL-HEMT is also analyzed by time-to-failure test. The maximum gate voltage is determined to be 6.28 V for a ten-year lifetime with a failure rate of 1% at 25 °C.

Spin logics provide a shortcut toward logic-in-memory architectures. Here, using a T-type magnetic stack with a perpendicular magnetic layer, an in-plane layer, and a spacing layer in between, we can switch both the in-plane and perpendicular layers and independently control their magnetization without an external magnetic field. By initializing magnetization of the in-plane layer, we can further control chirality (clockwise and counterclockwise) of the current-dependence of perpendicular magnetization. Based on these properties and the majority gate theory, we experimentally construct five Boolean logic gates in a single device in the desired field-free condition, which steps forward to practical spin–orbit torque logics.

In this letter, a device structure of high-resistivity-cap-layer HEMT (HRCL-HEMT) is developed for normally-off p-GaN gate HEMT toward high breakdown voltage and low current collapse. It demonstrates that the breakdown capability and current collapse of the device were effectively improved due to the introduction of a thick HR-GaN cap layer. The fabricated HRCL-HEMT exhibits a high breakdown voltage of 1020 V at ${I}_{\text {DS}} = 10~\mu \text{A}$ /mm with the substrate grounded. Meanwhile, the dynamic ${R}_{\text {on}}$ is only 2.4 times the static ${R}_{\text {on}}$ after off-state ${V}_{\text {DS}}$ stress of 1000 V with the substrate grounded (the OFF to ON switching time interval is set to $200~\mu \text{s}$ ).

The nature of charge transport in GaN-based light-emitting diodes (LEDs) under reverse biases still remains elusive as the bias- and temperature-dependent characteristics of the current can hardly be formulated using a single transport mechanism. In this letter, based on numerical fitting, we develop a combined tunneling-hopping transport model to describe the complete electrical characteristics of the reverse leakage current in InGaN/GaN blue LEDs. This model depicts that the current behaviors are majorly limited by the charge transport process through the depletion region near the neutral n-side, where electrons at the valance band are ready to tunnel into the unoccupied localized gap states in neighborhood near the electron quasi-Fermi level ( ${E}_{\mathrm {fn}}$ ), followed by variable-range hopping or nearest-neighbor hopping via these localized states along a relatively constant ${E}_{\mathrm {fn}}$ , depending on temperature.