High-temperature-resistant surface acoustic wave (SAW) devices commonly employ a structure where Pt electrodes are grown on langasite (LGS) substrates. This paper compares the resonance performance of LGS/Pt SAW devices before and after annealing, revealing that devices of this structure require annealing activation to become functional. Comparative experiments with Au electrodes confirm that the annealing process primarily affects the Pt electrodes. Analysis of Pt microstructures before and after annealing reveals the formation of larger, irregular grains in the Pt films, which would lead to changes in their mechanical properties. Nanoindentation testing comparing load-displacement curves suggests that the annealing process may improve performance of SAW devices by relieving residual stresses within the Pt films.

Surface acoustic wave (SAW) resonators demonstrate significant potential in passive wireless high-temperature sensing applications. However, their performance and reliability under extreme temperatures are fundamentally constrained by the thermal stability of the piezoelectric substrate. This article presents the design, characterization, and sensing device of single-port SAW resonators based on aluminum nitride (AlN) and langasite (LGS) substrates. High-temperature experiments were conducted in an air atmosphere from 25 degrees C to 750 degrees C using a muffle furnace, enabling real-time monitoring of the resonant frequency evolution. The AlN-based resonator, benefiting from a high phase velocity, achieved a resonant frequency of 2.1 GHz and exhibited superior temperature sensitivity, making it advantageous for developing compact, highly sensitive wireless sensors. However, it showed irreversible sensing characteristic drift during thermal cycling, limiting its stable operating temperature to below 600 degrees C. In contrast, the LGS-based resonator maintains exceptional stability and repeatability over multiple thermal cycles up to 750 degrees C. Furthermore, we successfully demonstrate a passive wireless sensing system using the LGS resonator, which enables precise temperature detection by monitoring shifts in the resonant frequency.

In this work, a novel method for the theoretical derivation of general single nonuniform transmission lines is proposed by reasonably splitting complex propagation coefficient and characteristic impedance. The split quantities can at least be obtained as functions of line position by numerically solving corresponding Riccati ordinary differential equations. As a result, the voltage and current wave distributions along nonuniform transmission lines are derived in an explicit analytical form. Further on, the corresponding two-port network parameter expressions are formulated also in an analytical closed form. Both the voltage/current and network parameter expressions can be effectively simplified by adopting appropriate boundary conditions. Finally, the proposed method is applied to analyze a lossy linearly-tapered coaxial line and a lossy irregular coaxial line as two examples of nonuniform transmission lines. As a conclusion, the correctness, effectiveness, universality, and high efficiency of our theory and method are confirmed by the good agreement with the corresponding electromagnetic field simulation provided by the ANSYS high frequency structure simulator and the significantly shorter computational time compared to the conventional cascade method.

This paper systematically investigates the DC characteristics of SiGe heterojunction bipolar transistors (HBTs) over a wide temperature range from 10 K to 300 K. Measurements of Gummel curves and output characteristics reveal pronounced non-ideal behavior in base current at low temperatures, primarily due to enhanced trap-assisted tunneling mechanisms. Moreover, the devices maintain good output characteristics under cryogenic conditions, indicating their potential for low-temperature amplifier applications. This study provides experimental support for modeling and application of SiGe HBTs in ultra-low-temperature environments.

In this study, a GMA based approach to predict proton exchange membrane fuel cell (PEMFC) stack voltage and remaining useful life (RUL) was proposed, and how different combinations of input and output sizes affect model performance was analyzed. The results show that the GMA model effectively captures the voltage degradation trend of the PEMFC, accurately reproducing the early rapid voltage drop and the later smooth degradation. Model performance is strongly influenced by the input and output configurations. Smaller input sizes lead to larger fluctuations in performance metrics (e.g., RMSE and score), whereas larger input sizes provide more informative features and improve predictive accuracy. In particular, with an input size of 300 and an output size of 40, the model achieves its best performance, yielding the lowest RMSE and a near optimal Score. Overall, the GMA model offers clear advantages for improving the accuracy and reliability of PEMFC prediction, and its predictive effectiveness and stability can be further enhanced through careful selection of input and output sizes. This study provides a practical reference for PEMFC RUL prediction and supports maintenance planning, performance evaluation and life cycle management of fuel cells.

Surface acoustic wave (SAW) devices exhibit extensive application potential in temperature sensing under extreme environments. Aluminum nitride (AlN) has long been considered a promising candidate for high-temperature SAW device fabrication. However, the temperature coefficient of frequency (TCF) behavior of AlN/sapphire-structured SAW devices over wide temperature ranges remains insufficiently explored, particularly in low-temperature regions. This study fabricated a SAW temperature sensor based on c-plane AlN thin films and systematically investigated its TCF characteristics across a broad temperature spectrum from 14 K to 500 K.

This paper proposes a new Thru-based de-embedding technique by combining Thru-only method with electromagnetic (EM) simulation. Based on theoretical derivation, half de-embedding of the other dummy than Thru dummy can set an effective criterion for calibrating the EM simulation. The calibrated EM simulation can in turn provide additional necessary condition for completing de-embedding. The proposed method is applied to the designed on-wafer coplanar waveguide (CPW) test structures for de-embedding a nonuniform transmission line at frequencies up to 110 GHz. The validity and accuracy of our method are verified by using HFSS EM simulation to mimic practical on-wafer measurements in comparison with the traditional OS method.

This paper investigates the cryogenic electrical characteristics of SiGe HBTs with particular emphasis on heterojunction behavior and impurity ionization effects. The SiGe base layer was epitaxially grown using reduced-pressure chemical vapor deposition (RP-CVD), and the devices were experimentally characterized down to 4.8 K. Elemental distribution analysis through TEM confirmed high-quality epitaxial growth. To analyze the temperature-dependent electrical behavior, we developed a physics-based turn-on voltage (Von-T) analytical model and incorporated the Altermatt incomplete ionization model to account for Mott-transition-level dopant freeze-out. Simulation and experimental results show that while the Von-T model works well for highly doped regions, significant deviations occur at low temperatures in the collector due to freeze-out effects. These results provide insight into the physical limits of SiGe HBT performance under cryogenic operation and highlight the importance of modeling incomplete ionization in moderately doped regions.

This paper presents a thermal gas flow sensing system, from surface acoustic wave (SAW) temperature sensor to oscillation circuit and multi-module miniaturization integration. A single-port GaN/Si SAW resonator with single resonant mode and excellent characteristics was fabricated. Combined with an in-house-developed SiGe HBT, a temperature-sensitive high-frequency oscillator was constructed. Under constant temperature control, system-level flow measurement was achieved through dual-oscillation configuration and modular integration. The fabricated SAW device shows a temperature coefficient of frequency (TCF) −28.29 ppm/K and temperature linearity 0.998. The oscillator operates at 1.91 GHz with phase noise of −97.72/−118.62 dBc/Hz at 10/100 kHz offsets. The system demonstrates excellent dynamic response and repeatability, directly measuring 0–50 sccm flows. For higher flows (>50 sccm), a shunt technique extends the test range based on the 0–10 sccm linear region, where response time is <1 s with error <0.9%. Non-contact operation ensures high stability and long lifespan. The sensor shows outstanding performance and broad application prospects in flow measurement.

This work is focusing on the advanced process module development of SiGe HBT, including highly doped collector wells, silicon epitaxial collector region, in-situ highly doped polysilicon, emitter-base self-aligned structure and SiGe base epitaxy. Innovative explorations are conducted on the annealing temperature and the epitaxial conditions of the collector region. Test results achieved the anticipated targets, providing a sound basis for future large-scale production.

This paper presents the temperature-dependent DC characteristics of Silicon-Germanium Heterojunction Bipolar Transistors (SiGe HBTs) using a modified and extended Mextram model, covering 80 K to 400 K. An improved method for extracting Gummel characteristics, with a focus on the ideality factor and saturation current, is proposed. The method incorporates a temperature-dependent saturation current model and a correction factor, improving accuracy. Simulation results match experimental data with a maximum deviation of 15%. This approach enables accurate performance predictions for SiGe HBT circuit simulation and design.

This work analyzes the temperature characteristics of RF small-signal parameters for SiGe HBTs over a range of 80 K to 400 K. By extracting small-signal model parameters, we investigate how temperature variations affect device performance. The findings reveal significant enhancements in cut-off frequency from 189 GHz at 400 K to 406 GHz at 80 K, highlighting the suitability of SiGe HBTs for cryogenic applications and advanced RF circuit designs.

This study investigates the effects of Co-60 gamma-ray irradiation on the DC and RF characteristics of the SiGe HBTs, with a total dose of up to 4000 krad(Si). The degradation of the forward base current is primarily attributed to surface recombination due to the induced interface traps. The ideality factor of the forward excess base current is affected by the positive oxide-trapped charges at the interface of the emitter-base spacer oxide. TCAD simulation results indicate that the effective integral region of the surface recombination rate is associated with the positive oxide-trapped charge density. The accumulation of positive oxide-trapped charges in the shallow trench isolation oxide has an impact on the potentials of the interface and epi-collector region, subsequently affecting the base diffusion current. Therefore, the ideality factor of the reverse excess base current depends on the device geometry. The RF characterization suggests that the depletion capacitance of the base-emitter junction is more susceptible to gamma-ray irradiation compared to the base-collector junction. And the cut-off frequency experiences a slight degradation as the total dose increases.

We experimentally demonstrate that the dominant mechanism of single-event transients in silicon-germanium heterojunction bipolar transistors (SiGe HBTs) can change with decreasing temperature from +20 °C to −180 °C. This is accomplished by using a new well-designed cryogenic experimental system suitable for a pulsed-laser platform. Firstly, when the temperature drops from +20 °C to −140 °C, the increased carrier mobility drives a slight increase in transient amplitude. However, as the temperature decreases further below −140 °C, the carrier freeze-out brings about an inflection point, which means the transient amplitude will decrease at cryogenic temperatures. To better understand this result, we analytically calculate the ionization rates of various dopants at different temperatures based on Altermatt’s new incomplete ionization model. The parasitic resistivities with temperature on the charge-collection pathway are extracted by a two-dimensional (2D) TCAD process simulation. In addition, we investigate the impact of temperature on the novel electron-injection process from emitter to base under different bias conditions. The increase of the emitter–base junction’s barrier height at low temperatures could suppress this electron-injection phenomenon. We have also optimized the built-in voltage equations of a high current compact model (HICUM) by introducing the impact of incomplete ionization. The present results and methods could provide a new reference for effective evaluation of single-event effects in bipolar transistors and circuits at cryogenic temperatures, and could provide a new evidence of the potential of SiGe technology in applications in extreme cryogenic environments.

This work presents a dummy finger structure for eliminating the transverse spurious mode on the GaN/Si surface acoustic wave (SAW) resonators. The fabricated resonators have a high quality factor, and the transverse spurious mode is effectively suppressed. A maximum quality factor of 8177 at a resonant frequency (f(r)) of 1.9173 GHz is obtained. Moreover, it is shown that the transverse spurious mode is independent of the propagation directions. A delay line with the dummy finger shows a minimum insertion loss of 16.44 dB. These results could pave the way for future intelligent lab-on-chip sensor applications.

Surface acoustic wave (SAW) resonators with an ultrahigh Q-factor are designed and fabricated on silicon-based gallium nitride (GaN/Si). The temperature-dependent performance is characterized over a wide range, from 10 to 500 K. Finite element analysis is employed to guide the design of the SAW resonator from indications of the Rayleigh mode and weak propagation direction dependence of SAW in the c -plane of GaN/Si. The SAW resonator with 100 pairs of interdigital transducers (IDT), 100 pairs of grating reflectors (GR) for each side, aperture size of 80 µm, metallization ratio of 0.5, and electrode width of 500 nm resonates at 1.9133 GHz accordingly with an ultrahigh Q-factor of 7622 at room temperature, which contributes the f r × Q r , up to 14.583×10 12 Hz. A resonator operating over 10 to 500 K indicates an approximately linear decreasing temperature dependence above 280 K while being approximately constant below 40 K. The fitting to resonator characteristics using the modified Butterworth Van Dyke (mBVD) model reveals a reduction in both the electrode and mechanical losses while worsening the dielectric loss with cooling down.

We present several calculations for the DC inductance of planar polygonal nonuniform spiral inductors, including common layout shapes of square, hexagon, octagon, and circle. The term “nonuniform” is a novel classification proposed in this paper to characterize the turn-to-turn variable metal width inductors. We take three extreme cases into consideration, according to the spiral coil from inside to outside: type 1 with the increasing width, type 2 with the fixed width and space (conventional inductors), and type 3 with the decreasing width. Five unified calculations are first proposed for the conventional inductors (type 2 inductors), which are only layout dependent with different coefficients, and then we successfully further three of the five calculations, with a simple modification on some calculation parameters, for all nonuniform types. Fractional inductors are treated as “integer plus fraction” parts, both of which can be easily determined by derived calculations. Automatic scripts are employed to generate over a million inductor layouts for electromagnetic (EM) simulation. Our calculations match the EM inductance values typically within around 3%. Comparison with both measured and published experimental results also shows good agreement.