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 paper 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 air atmosphere from 25°C to 750°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°C. In contrast, the LGS-based resonator maintains exceptional stability and repeatability over multiple thermal cycles up to 750°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.

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.