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.

AlGaN/GaN E/D-mode GaN inverters are successfully fabricated on a 150-mm Si wafer. P-GaN gate technology is applied to be compatible with the commercial E-mode GaN power device technology platform and a systematic study of E/D-mode GaN inverters has been conducted with detail. The key electrical characters have been analyzed from room temperature (RT) to 200 °C. Small variations of the inverters are observed at different temperatures. The logic swing voltage of 2.91 V and 2.89 V are observed at RT and 200 °C at a supply voltage of 3 V. Correspondingly, low/high input noise margins of 0.78 V/1.67 V and 0.68 V/1.72 V are observed at RT and 200 °C. The inverters also demonstrate small rising edge time of the output signal. The results show great potential for GaN smart power integrated circuit (IC) application.

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 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.

Half-bridge modules with integrated GaN high electron mobility transistors (HEMTs) and driver dies were designed and fabricated in this research. Our design uses flip-chip technology for fabrication, instead of more generally applied wire bonding, to reduce parasitic inductance in both the driver-gate and drain-source loops. Modules were prepared using both methods and the double-pulse test was applied to evaluate and compare their switching characteristics. The gate voltage ($V_{gs}$) waveform of the flip-chip module showed no overshoot during the turn-on period, and a small oscillation during the turn-off period. The probabilities of gate damage and false turn-on were greatly reduced. The inductance in the drain-source loop of the module was measured to be 3.4 nH. The rise and fall times of the drain voltage ($V_{ds}$) were 12.9 and 5.8 ns, respectively, with an overshoot of only 4.8 V during the turn-off period under $V_{dc}$ = 100 V. These results indicate that the use of flip-chip technology along with the integration of GaN HEMTs with driver dies can effectively reduce the parasitic inductance and improve the switching performance of GaN half-bridge modules compared to wire bonding.

AlGaN/GaN lateral Schottky barrier diodes (SBDs) with three different anode geometries (stripe, circular, and the conventional plane one) and different rows of anode trenches are fabricated and electrically characterized to study the influence of anode trench geometries. The SBDs with anode trenches exhibit the lower on-state resistance (R_ON) than that with the conventional plane one. It can be explained that the anode trenches made the Schottky metal directly contact to the 2DEG at the sidewall of the AlGaN/GaN interface, removing the AlGaN barrier layer in the conventional plane anode. In addition, the R_ON of the SBDs with circular trenches is smaller than that of the SBDs with stripe ones. Furthermore, the R_ON decreases with the increasing rows of anode trenches, which can be attributed to the increased contact area between the Schottky metal and the 2DEG. For the reverse characteristics, the anode trenches do not lead to performance degradation. The fabricated devices exhibit the low reverse current (I_R, I_R < 1 μA/mm), and the breakdown voltage (V_BK) remains unchanged with different anode geometries.