Introduction
With the rapid development of science and technology, gas sensors are finding increasing applications in numerous areas of human life such as environmental monitoring, health care, automotive safety, and explosive detection. Consequently, tremendous efforts have been conducted in exploring new materials and technologies for sensing gases. Among various types of gas sensors, field-effect transistor (FET) sensors based on AlGaN have gained considerable research interest due to their exceptional properties such as high conductivity, high electron mobility, and high thermal stability. This article aims to provide a comprehensive review of AlGaN-based FET sensors, with a specific focus on their gas-sensing properties, the fabrication methods, and the potential applications.
Overview of AlGaN-based FET Sensors
Aluminum gallium nitride (AlGaN) is a compound semiconductor, which has a wide bandgap and high electron affinity, making it an excellent material platform for FET devices. The AlGaN-based FET sensors possess several advantages over the metal oxide semiconductor sensors and other types of gas sensors, including high sensitivity, selectivity, and stability. Moreover, they offer the potential for detecting various gases such as nitrogen oxides, hydrogen, carbon dioxide, ammonia, methane, and volatile organic compounds (VOCs).
Fabrication Methods
Several methods have been reported for the fabrication of AlGaN-based FET sensors, including molecular beam epitaxy, metalorganic chemical vapor deposition, and pulsed laser deposition. Among them, molecular beam epitaxy is considered as the most common technique for producing high-quality AlGaN thin films and heterostructures. The fabrication process of AlGaN-based FET sensors involves several steps such as substrate cleaning, buffer layer deposition, AlGaN deposition, and device etching. In addition, different methods of making the gate electrode, including metal evaporation, electron beam lithography, and lift-off process, have been reported to improve the device performance.
Gas-sensing Properties
The gas-sensing properties of AlGaN-based FET sensors depend on several factors such as the thickness of the sensing layer, the type and concentration of the target gas, and the operational temperature. The sensing mechanism is based on the modulation of the drain current, which occurs due to the changes in the surface potential caused by the presence of gas molecules. The electrical conductivity of the AlGaN-based FET sensors increases or decreases in response to oxidizing or reducing gases, respectively, due to the chemisorption or physisorption of the gas molecules onto the surface of the sensing layer.
Potential Applications
AlGaN-based FET sensors possess a vast potential for various applications such as environmental monitoring, industrial control, and medical diagnosis. For instance, these sensors have been used for the detection of nitrogen oxides in automotive exhausts, carbon dioxide in greenhouse gases, and ammonia in livestock farming. In addition, they have been applied for the detection of volatile organic compounds (VOCs), which are often considered as significant environmental pollutants and have been linked to several health issues. Moreover, the AlGaN-based FET sensors have been integrated into micro-electromechanical systems (MEMS) devices for real-time, in-situ monitoring of gas concentrations, indicating their potential use in personalized health care and early disease diagnosis.
Conclusion
In summary, AlGaN-based FET sensors can be considered as a promising candidate for gas sensing applications with their excellent properties such as high sensitivity, selectivity, and stability. Several fabrication techniques have been reported for their preparation, and different sensing mechanisms have been studied for improving their performance towards various gases. Moreover, their potential applications in different fields such as environmental monitoring, health care, and industrial control highlight the need for the continuous exploration of new methods to further enhance their sensing properties and reduce their fabrication cost.
AFR传感器：实现精准化汽车控制的利器
随着汽车技术的快速发展，汽车已不再是一种简单工具，而是一种集高科技、高智能于一身的交通工具，那么这种高科技的设备就需要得到高度的监控和控制，以防止故障的发生，从而确保汽车行驶的安全和舒适。随着对汽车发动机控制精度的要求不断提高，AFR传感器成为实现精准化汽车控制的利器。
AFR传感器是一种广泛应用于汽车发动机控制系统中的传感器。它测量发动机燃烧室内的空气与燃料的比例，确保发动机正常运转，并防止发动机因为过度的油气混合物使油耗和废气排放过高，同时也避免油气过于稀薄使发动机出现缺氧状况，因此AFR传感器的工作是至关重要的。
AFR传感器的工作原理是通过测量进入发动机的空气和燃料的比例来判断油气混合物的稀薄程度，并发送这些信息到汽车的控制系统，以便对发动机进行精准的控制。如果油气混合物过稠，则会导致油耗量和污染物排放量的上升，而如果过稀则可能导致发动机爆缸或燃烧不完全。
AFR传感器通常由一个氧气传感器和一个空气流量计组成。氧气传感器测量发动机燃烧室内氧气浓度，而空气流量计测量进入发动机的空气流量，两者都发挥着相互协作的作用。
AFR传感器的优点不仅仅是可以提高汽车的安全性和舒适度，还可以实现更加智能化的控制。利用AFR传感器的信息，控制系统可以更加精准地控制发动机的燃油喷射量，使油耗降低，同时也减少了尾气排放和烟雾的产生。
AFR传感器的应用已经非常广泛，尤其在目前的三元催化器控制系统中更是不可或缺的一部分。AFR传感器确保发动机油气混合物的精准程度，从而让三元催化器可以正确地处理废气，使废气排放物质得到有效的净化，降低环境的污染。
然而，由于传感器的精度要求非常高，所以AFR传感器的价值和性能往往也是各种汽车制造商研发竞争的焦点，不同品牌在传感器的精度以及稳定性上的竞争也在不断升级。
同时，由于AFR传感器与发动机的控制系统紧密相连，在使用过程中需要定期的检测和维护。传感器出现故障会导致发动机的性能下降、油耗量的上升，甚至可能因为由于燃油的过度排放导致环境的污染问题。
在对AFR传感器的使用中，应该定期的对传感器的进行检测和维护，保证传感器的正常运行和精准度，从而使汽车的性能和安全性都能得到更好的保障。
总的来说，AFR传感器在汽车领域的应用还将持续不断扩大，很可能成为未来汽车发动机控制最为重要的配套设备之一。同时，由于环境保护和油耗的要求也日益见长，传感器精准性的要求也会随之增加，因此对AFR传感器的研发和改进也是汽车行业未来的一个重要课题。
