Recent advancements in fiber technology have enabled the integration of functional materials with specific geometries, leading to diverse applications such as sensors, actuators, energy harvesting, and healthcare devices. However, the selection, control, and engineering of semiconductors within fibers are crucial for achieving high-performance functional fibers. Stress development and capillary instability during the thermal drawing process can lead to cracks and deformations in the semiconductor cores, affecting fiber performance. This study presents a mechanical design to achieve ultralong, fracture-free, and perturbation-free semiconductor fibers by understanding stress development and capillary instability at three stages of fiber formation: viscous flow, core crystallization, and cooling. The exposed semiconductor wires are integrated into a single flexible fiber with well-defined interfaces with metal electrodes, enabling optoelectronic fibers and large-scale optoelectronic fabrics. The use of crystalline semiconductors, such as silicon (Si) and germanium (Ge), is more favorable than glassy semiconductors due to their superior electrical properties. Various crystal growth techniques, including the molten-core method, have been developed to produce continuous long crystalline semiconductor fibers. The study identifies two stages of stress formation: core solidification and subsequent cooling, and proposes a mechanical design to avoid cracks and fractures. The selection of cladding materials with appropriate annealing points and thermal expansion coefficients is crucial for suppressing cracking. The optoelectronic fibers exhibit high responsivity, low noise equivalent power, and mechanical robustness, making them suitable for various applications, including healthcare, robotics, wearable communications, and assistive technology. The fibers can be woven into large-scale functional fabrics, enabling flexible and conformal sensing and communication systems.Recent advancements in fiber technology have enabled the integration of functional materials with specific geometries, leading to diverse applications such as sensors, actuators, energy harvesting, and healthcare devices. However, the selection, control, and engineering of semiconductors within fibers are crucial for achieving high-performance functional fibers. Stress development and capillary instability during the thermal drawing process can lead to cracks and deformations in the semiconductor cores, affecting fiber performance. This study presents a mechanical design to achieve ultralong, fracture-free, and perturbation-free semiconductor fibers by understanding stress development and capillary instability at three stages of fiber formation: viscous flow, core crystallization, and cooling. The exposed semiconductor wires are integrated into a single flexible fiber with well-defined interfaces with metal electrodes, enabling optoelectronic fibers and large-scale optoelectronic fabrics. The use of crystalline semiconductors, such as silicon (Si) and germanium (Ge), is more favorable than glassy semiconductors due to their superior electrical properties. Various crystal growth techniques, including the molten-core method, have been developed to produce continuous long crystalline semiconductor fibers. The study identifies two stages of stress formation: core solidification and subsequent cooling, and proposes a mechanical design to avoid cracks and fractures. The selection of cladding materials with appropriate annealing points and thermal expansion coefficients is crucial for suppressing cracking. The optoelectronic fibers exhibit high responsivity, low noise equivalent power, and mechanical robustness, making them suitable for various applications, including healthcare, robotics, wearable communications, and assistive technology. The fibers can be woven into large-scale functional fabrics, enabling flexible and conformal sensing and communication systems.