β-Ga ₂ O ₃ Schottky barrier diodes were fabricated in a vertical geometry structure consisting of Ni/Au rectifying contacts without edge termination on Si-doped epitaxial layers (10 μm, n~4×10 ¹⁵ cm ^-3 ) on Sn-doped bulk Ga ₂ O ₃ substrates with full-area Ti/Au back Ohmic contacts. The reverse breakdown voltage, VBR, was a function of rectifying contact area, ranging from 1600 Vat 3.1×10 ^-6 cm ² (20-μm diameter) to ~250 Vat 2.2×10 ^-3 cm ^-2 (0.53-mm diameter). The current density near breakdown was not strongly dependent on contact circumference but did scale with contact area, indicating that the bulk current contribution was dominant. The lowest ON-state resistance, Ron, was 1.6 mQ· cm ² for the largest diode and 25 mQ·cm ² for the 1600-V rectifier, leading to a Baliga figure-of-merit (V2BR/Ron) for the latter of approximately 102.4 MW·cm ^-2 . The ON-OFF ratio was measured at a forward voltage of 1.3 V and ranged from 3×10 ⁷ to 2.5×10 ⁶ for reverse biases from -5 to -40 V and showed only a small dependence on temperature in the range from 25 °C to 100 °C.

One of the biggest issues with GaN-based hydrogen sensors is their sensitivity to humidity in the ambient. We demonstrate that encapsulation of Pt-AlGaN/GaN Schottky diode with poly(methyl methacrylate) (PMMA) provides effective mitigation of the effects of water. Without PMMA encapsulation, the absolute current signal for detection of 500 ppm of H ₂ was decreased by a factor of 8 in the presence of water. By sharp contrast, encapsulated diodes show no decrease in response in the presence of water. The relative current changes are of the order 2.8 × 10 ⁵ % when 500 ppm H ₂ is introduced to the surface of bare or PMMA encapsulated diodes in the absence of water or to encapsulated diode in the presence of water. Detection limits of ~100 ppm H ₂ (0.01% by volume) were obtained with standard forward bias detection mode at 1.3 V.

We investigated the hydrogen sensing characteristics of Pt Schottky diodes on and (010) β-Ga₂O₃ bulk crystals. The Pt Schottky diodes on β-Ga₂O₃ wafer exhibited the fast, reversible, and cyclic response upon hydrogen exposure. The maximum value of the relative current change of the Ga₂O₃ diode sensor was as high as 7.86 × 10⁷ (%) at 0.8 V, which is slightly higher than that of the (010) Ga₂O₃ diode. The hydrogen responses of both β-Ga₂O₃ diode sensors are believed to result from oxygen and gallium atomic configurations of Ga₂O₃ surfaces for hydrogen adsorption. The Pt Schottky diodes of Ga₂O₃ wafers did not show any clear response to other gases, such as N₂, CO, CO₂, O₂, CH₄, NO₂, and NH₃. Our finding suggests that the Pt Schottky diodes on β-Ga₂O₃ hold great potential for the applications of hydrogen gas sensors with high sensitivity and selectivity.

InGaN/GaN multiquantum well light-emitting diodes (LED) with emission wavelength of 450 nm were irradiated with average energy of 9.8 MeV and dose of 5.5×1011 cm−2 neutrons. Right after irradiation, the forward current of the irradiated LEDs was decreased as a result of the creation of deep levels by the neutron-induced lattice displacement. However, unstable lattice damages resulting from the collisions with the incoming neutrons were removed at room temperature 6 days after the irradiation. The diode turn-on voltage, ideality factor, and optical emission intensity were recovered to preirradiated state by self-annealing process at room temperature.

The extraction efficiency of nonpolar a-plane (11-20) GaN LEDs on sapphire substrates has been enhanced by selectively etching the mesa sidewall faces and the n-type GaN surfaces with photoenhanced chemical wet etching. Submicron-sized trigonal prisms having prismatic planes of {1-100} were clearly displayed on the n-type GaN surfaces as well as the sidewall face after 5 min etching at 60°C. The radiation patterns have shown that more light is extracted in all directions and the output powers of surface textured a-plane GaN LEDs have increased by 25% compared with control samples. PEC wet etching produced unique feature of etching morphology on the mesa sidewall faces and the n-type GaN surface.

Enhanced hydrogen sensing performance of Pt Schottky diodes on ZnO single crystal wafers in humid ambient conditions is reported using a polymethylmethacrylate (PMMA) membrane layer. ZnO diode sensors showed little change in forward current when switching to wet ambient H₂ conditions with 100% relative humidity. This sensitivity drop in the presence of water vapor can be attributed to surface coverage of hydroxyl groups on the Pt surface in humid ambient conditions. The hydrogen sensitivity of PMMA-coated diode sensors recovered up to 805% in wet H₂ ambient conditions at room temperature. The PMMA layer can selectively filter water vapor and allow H₂ molecules to pass through the membrane layer. It is clear that the PMMA layer can effectively serve as a moisture barrier because of low water vapor permeability and its hydrophobicity. In both dry and wet conditions, ZnO diodes exhibited relatively fast and stable on/off switching in each cycle with good repeatability.