# Thermal Gas Flow Sensor Using SiGe HBT Oscillators Based on GaN/Si SAW Resonators

## TL;DR

This paper introduces a thermal gas flow sensor using GaN/Si SAW resonators and SiGe HBT oscillators for high-precision and stable flow measurements.

## Contribution

A novel thermal gas flow sensing system using GaN/Si SAW resonators and SiGe HBT oscillators with non-contact operation and extended measurement range.

## Key findings

- The SAW device has a TCF of −28.29 ppm/K and temperature linearity of 0.998.
- The oscillator operates at 1.91 GHz with phase noise of −97.72/−118.62 dBc/Hz.
- The system measures 0–50 sccm flows with <0.9% error and <1 s response time.

## Abstract

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.