N-p-n AlGaN/GaN heterojunction bipolar transistors (HBTs) on sapphire substrates with high current gain $\beta $ of 129, low specific ON-resistance ( $\textit{R}_{\biosc{on},\text{sp}}\text{)}$ of 0.28 m $\sf\Omega \cdot$ cm $^{\text{2}}$ and high current density $\textit{J}_{\textit{C}}$ of $\sim$ 15 kA/cm $^{\text{2}}$ (if normalized to emitter area, $\textit{R}_{\biosc{on},\text{sp}} \sim$ 0.15 m $\sf\Omega \cdot$ m $^{\text{2}}$ and $\textit{J}_{\textit{C}} \sim$ 28.4 kA/cm $^{\text{2}}\text{)}$ have been demonstrated. The analysis of component of $\textit{R}_{\biosc{on},\text{sp}}$ yields the collector resistance is $\sim$ 5.2% of the total $\textit{R}_{\biosc{on},\text{sp}}$ , showing the low ON-resistance advantage of GaN HBTs. The open-base avalanche breakdown voltage (BV $_{\text{CEO}}\text{)}$ is $\sim$ 160 V. High-temperature performance of GaN HBT is also evaluated. The $\textit{R}_{\biosc{on},\text{sp}}$ is reduced as temperature increasing in the range of 25 $^{\circ}$ C–100 $^{\circ}$ C due to enhanced hole concentration in the base layer. The cutoff frequency ( $\textit{f}_{\textit{T}}\text{)}$ of greater than 4 GHz is determined at $\textit{V}_{\text{CE}} =$ 9 V. These results dominate that GaN HBTs have been anticipated to become a new technology for next-generation power switches and RF power amplifier circuit.

A laser-diode-based white lighting module is fabricated via spectral component optimization, which can achieve both high luminous flux and high color rendering index (CRI). In this work, the laser module is constituted by blue laser diodes (LDs) which excite YAG:Ce-Al₂O₃ and red LDs that can compensate for the lack of red spectrum to improve the CRI of the light source. To fulfill the requirements of flexibility and compactness of light source, the blue and red LDs beams are combined by optical fiber coupling. A simulation framework is employed to optimize the dominant wavelength of red LDs and the power ratio of red to blue LDs. According to the results of the integrating sphere, high luminous flux of 1102 lm and high CRI of 77.8 are achieved simultaneously, which is consistent with the simulation results. The tunable correlated color temperature (CCT) varying from 4000 K to 2800 K and high angular color uniformity (ACU) can be obtained.

The high-temperature operation of the AlGaN/GaN high electron mobility transistors (HEMTs) and direct-coupled FET logic (DCFL) inverters, NOR gates, and NAND gates are demonstrated. The ${p}$ -GaN-gate HEMTs which are used as drivers in DCFL circuits exhibit proper E-mode operation with a relatively stable threshold voltage ${V}_{\text {TH}}$ (drift −0.31 V) and high electron mobility $\mu $ of 587 cm2/ $\text{V}\cdot \text{S}$ at 300 °C. The fabricated logic gates have a small switching time at room temperature (RT). As the temperature increases to 300 °C, these logic gates exhibit a slight increase in switching time, wherein the fall time and rise time of the inverter from room temperature to 300 °C increases from 16 ns to 129 ns and 128 ns to $0.864~\mu \text{s}$ , respectively. The logic gates herein exhibit accurate and stable operation at 300 °C. Furthermore, the reason for the switching time increase at high temperature is revealed from the perspective of devices.

The challenge for improving the internal quantum efficiency (IQE) of InGaN-based light emitting diodes (LED) in the green light range is referred to as the ‘green gap’. However the IQE of InGaN-based LEDs often drops when the emission peak wavelength is adjusted through reducing the growth temperature. Although hydrogen (H ₂ ) can improve surface morphology, it reduces the indium incorporation significantly. Here, a novel usage of H ₂ treatment on the GaN barrier before the InGaN quantum well is demonstrated to enhance indium incorporation efficiency and improve the IQE simultaneously for the first time. The mechanism behind it is systematically investigated and explained in detail. The possible reason for this phenomenon is the strain relieving function by the undulant GaN barrier surface after H ₂ treatment. Test measurements show that applying 0.2 min H ₂ treatment on the barrier would reduce defects and enhance indium incorporation, which would improve the localization effect and finally lead to a higher IQE. Although further increasing the treatment time to 0.4 min incorporates more indium atoms, the IQE decreases at the expense of more defects and a larger polarization field than the 0.2 min sample.

The impact of device parameters, including AlN film thickness (h_AlN), number of interdigital transducers (N_IDT), and acoustic propagation direction, on the performance of c-plane AlN/sapphire-based SAW temperature sensors with an acoustic wavelength (λ) of 8 μm, was investigated. The results showed that resonant frequency (f_r) decreased linearly, the quality factor (Q) decreased and the electromechanical coupling coefficient $\left(K_t^2\right)$ increased for all the sensors with temperature increasing from −50 to 250 °C. The temperature coefficients of frequency (TCFs) of sensors on AlN films with thicknesses of 0.8 and 1.2 μm were −65.57 and −62.49 ppm/°C, respectively, indicating that a reduction in h_AlN/λ favored the improvement of TCF. The acoustic propagation direction and N_IDT did not obviously impact the TCF of sensors, but they significantly influenced the Q and $K_t^2$ of the sensors. At all temperatures measured, sensors along the a-direction exhibited higher f_r, Q and $K_t^2$ than those along the m-direction, and sensors with N_IDT of 300 showed higher Q and $K_t^2$ values than those with N_IDT of 100 and 180. Moreover, the elastic stiffness of AlN was extracted by fitting coupling of modes (COM) model simulation to the experimental results of sensors along different directions considering Euler transformation of material parameter-tensors. The higher f_r of the sensor along the a-direction than that along the m-direction can be attributed to its larger elastic stiffness c₁₁, c₂₂, c₄₄, and c₅₅ values.

Laser-based illuminant has been a topic of increasing interest in recent years and both of color quality and blue light hazards are the key issues that need to be addressed. In this paper, the performances of color quality and blue light hazard were investigated by comparing 465 nm laser-excited phosphor materials and traditional 452 nm laser-excited phosphor materials respectively. Phosphor-converted laser-based illuminant (pc-LBI) consisting of 465-nm and 635-nm LDs (LBI-465) has a color rendering index (CRI) of 88.7, which was 8.2% higher than that of pc-LBI consisting of 452-nm and 635-nm LDs (LBI-452). The correlated color temperature (CCT) of white light of both pc-LBIs ranges from 2200 K to 5000 K with the adjustment of red lasers. The blue light hazard efficiency of radiation (BLHER) of LBI-465 is below 0.15, which is 19% lower than that of LBI-452. The results showed that high color quality and low blue light hazard were simultaneously achieved by LBI-465. In addition, the above experimental results are in good agreement with the simulated results. Based on these above results, pc-LBI is believed to render laser-based white light sources with good color performance and boost the growth of solid-state lighting.