Fully integrated and portable semiconductor-type multi-gas sensing module for IoT applications

This paper suggests a portable multi-gas sensing module with chemiresistive gas sensors on a micro-electro-mechanical-systems (MEMS)-based heating platform. MEMS gas sensors are composed of four microheaters mounting heterogeneous sensing nanomaterials (SnO₂ nanotubes and ZnO nanowires with/without a Pt catalyst) and operating power is only 6 mW per each heater. A comprehensive approach to control heaters and readout multiple resistive sensors is implemented on a printed circuit board through the combination of analog/digital signal processing and a highly reconfigurable circuit structure. A wide range of sensor resistance (1 kΩ-100 MΩ) can be read with a relative error less than 1% by adaptively adjusting the gain of a trans-impedance amplifier. Heater controller circuits are operated in a way to give optimized power for each heater, and the target power is maintained at a constant value to keep a stable temperature condition. The module's multiple gas sensing capability enables selectivity enhancement by principle component analysis (PCA). Operation mode is readily reprogrammed to give repetitive sleep mode, and it is demonstrated that fabricated sensors show decent response and recovery to frequent heater on/off repetition. For both of portable and stationary applications, Bluetooth Low Energy and Wi-Fi communication is realized through a RF-microcontroller combination system-on-chip (SoC).