Enhanced Ammonia Sensing Using p+-Doped Channel MOSFET Sensors

Air pollution caused by industrial emissions remains a critical challenge globally, requiring the development of precise, reliable, and cost-effective gas sensors. This study presents a novel gas sensing technology based on a metal-oxide-semiconductor field-effect transistor (MOSFET) featuring a p+-doped channel. The sensor's design is optimized for ammonia detection, and it is fabricated using the TSMC D35 process. A custom-built automated measurement system and a gas concentration control system were used to evaluate the sensor's performance. The results show that the sensor exhibits high sensitivity and stability, making it a promising solution for real-time environmental monitoring. I. Introduction The rapid industrial growth in recent decades has led to significant increases in air pollution, with gases like ammonia posing severe environmental and health risks. Traditional air quality sensors typically measure particulate matter (PM2.5) but are not capable of detecting specific gases in real-time. There is a growing need for low-cost, accurate, and scalable gas sensors capable of measuring multiple gaseous pollutants. This research focuses on developing a silicon-based MOSFET sensor with a p+-doped channel layer for ammonia detection, aiming to address the limitations of existing sensor technologies. By combining sensitivity, simplicity, and low production cost, the proposed sensor could significantly improve air quality monitoring. II. Methods A. Sensor Design The sensor is designed around a metal-oxide-semiconductor field-effect transistor (MOSFET) structure, which offers stability, scalability, and ease of fabrication. The main components include an N-well back-gate, a p+-doped channel layer for ammonia detection, and a Metal 1 layer used for signal input. The MOSFET design provides reliable performance over time, and the p+-channel enables efficient interaction with target gases. This structure ensures that the sensor is both compact and suitable for mass production. B. Post-Processing Once the MOSFET chip is fabricated using the TSMC D35 process, additional post-processing steps are required to expose the p+-doped channel for gas interaction. These steps include metal etching with Piranha solution at 120°C and silicon dioxide removal using buffered oxide etch (BOE). These processes ensure that the sensing layer is fully exposed to gases such as ammonia, allowing for accurate measurements. C. Measurement and Gas Control Systems To measure the sensor’s performance, a customized automated measurement system was integrated with Keithley 2634B and 2450 parameter analyzers. This system allows for precise voltage control and real-time current monitoring through a unified interface. For precise gas concentration control, a closed-loop system with mass flow controllers (MFCs) was built. This system accurately adjusts gas concentrations to ensure reliable sensor testing and performance analysis. III. Results and Discussion A. Electrical Characteristics The sensor’s electrical characteristics were initially tested by applying a drain-source voltage (Vds) of 200 mV. The resulting current-voltage (I-V) curve demonstrated a typical FET behavior, with the current decreasing as the gate voltage increased. This behavior is due to the expansion of the depletion region, which reduces current flow. The sensor's maximum operational voltage was determined to be 10 V to avoid device breakdown. B. Ammonia Detection In gas detection experiments, the sensor was exposed to ammonia at controlled concentrations. The results indicated that the sensor's current decreased as the ammonia concentration increased. This response is attributed to the interaction between ammonia molecules and the p+-doped channel, leading to electron trapping and a decrease in channel conductivity. These findings highlight the sensor's sensitivity and confirm its potential for ammonia detection in environmental monitoring applications. IV. Conclusion This research successfully demonstrates the development of a novel MOSFET-based gas sensor with a p+-doped channel for ammonia detection. The sensor’s design, which is based on the TSMC D35 process, ensures both low-cost production and high sensitivity. Experimental results confirm the sensor’s effectiveness in detecting ammonia, with promising potential for real-time environmental monitoring. The combination of low cost, scalability, and high sensitivity makes this sensor a valuable tool for future applications in air quality monitoring. Figure 1