Transistor-based sensors and fabrication methods for a transistor-based sensor. A semiconductor layer is arranged over a substrate, and an interconnect structure is arranged over the semiconductor layer and the substrate. The semiconductor layer includes first sections composed of a semiconductor material, second sections composed of the semiconductor material, and cavities. The first sections have an alternating arrangement with the second sections in a lateral direction. The semiconductor material of the first sections is polycrystalline, and the semiconductor material of the second sections is single-crystal. First and second openings each extend in a vertical direction through the metallization levels of the interconnect structure to the semiconductor layer or through the substrate to the semiconductor layer. The first opening defines a first fluid inlet coupled to the cavities, and the second opening defines a first fluid outlet coupled to the cavities.