Abstarct: High-sensitivity gas and optical sensors with desirable response/recovery times, low operating temperatures, low cost, simple structure, good stability, repeatability, selectivity, and low detection limits are highly demanded in various applications such as industrial, healthcare, agricultural, environmental monitoring, and mining applications, and are therefore of great interest to researchers. mextal oxide semiconductor (MOS)-baxsed gas/optical sensors, due to their properties, are considered excellent candidates for commercialization in this field. Another prominent feature of these sensors is their capability to be fabricated at micro/nano scales and their compatibility with integrated electronic circuits, which promises their growing utilization in the internet of things (IOT) and wearable devices. Despite the outstanding characteristics of MOS-baxsed gas sensors, they suffer from several drawbacks such as high operating temperatures, low selectivity, and, in some cases, even weak responses and long response/recovery times, which pose serious challenges to their commercialization and practical application. To overcome these challenges, several strategies have been proposed, including doping, using the nanostructured and/or heterostructured materials, nanocomposites, Schottky junction, and the substitution of light activation instead of thermal activation. Due to the importance of this topic, extensive research is being conducted worldwide on semiconductor-baxsed gas/optical sensors. The subject of this thesis is the experimental investigation of semiconductor gas/optical sensors baxsed on tin oxide (SnO₂) nano- and heterostructures. In this research, SnO₂ nanostructures, nanocomposites, and heterostructures were synthesized and used in sensor devices to address some of the aforementioned challenges and to significantly improve the performance of MOS gas/optical sensors. The results showed that this research has successfully overcome some of the challenges. In the first step of this study, SnO₂ nanoparticles with an average size of approximately 10 nm were synthesized via the hydrothermal method, and the fabricated gas sensor demonstrated successful room-temperature selectivity toward ammonia and ethanol over several VOC gases. However, this sensor exhibited poor long-term stability. In the second step, to improve the sensor’s selectivity and stability, the ZnO-SnO₂ sensing material was synthesized by chemical precipitation by varying the composition ratios of the precursors.Two gas/optical sensors with changes in the morphology of the sensing materials were fabricated and tested. The first sample, composed of spherical ZnO-SnO₂ nanoparticles, exhibited a 4 s response time and a relative response of 219% to acetone, showing high selectivity with the operating temperature less than 100 °C under UV light activation, and the second sample, baxsed on Zn₂SnO₄ nanocube structures, exhibited good long-time stability. In other words, the stability issue was resolved, but the operating temperature slightly increased. In the next step of this project, to reduce the operating temperature to room temperature, a ternary rGO-SnO₂-ZnO nanocomposite was synthesized via the hydrothermal method. The rGO-SnO₂-ZnO nanocomposite gas characterization tests showed an ultra-fast photoresponse time (below one second) and good selectivity toward ammonia gas at room temperature compared to other gases, although its stability was limited to about two weeks. In the last step of this project, a binary rGO-SnO₂ nanocomposite was synthesized via the hydrothermal method, which achieved two-month stability and demonstrated self-powered high selectivity toward ammonia gas at room temperature with a response nearly 10 times higher than that of other gases, along with a fast response time. In conclusion, the results indicate that SnO₂-baxsed nanocomposite sensors with enhanced performance are promising candidates for real-world applications in medical and smart wearable devices.