Semiconductor p-n junctions formed in a cylindrical geometry as concentric cladding layers surrounding a wire or fiber ‘substrate’ could have significant advantages for optoelectronic devices such as LEDs and solar cells, especially with regard to optical coupling efficiency and high-throughput manufacturing. Fiber-based semiconductor device components may also prove useful in conformable electronics or electrotextiles, and for giant -area flexible circuits. We describe techniques and results for chemical vapor deposition and melt coating to form 2- to 50-micron thick cladding layers of silicon or germanium on various types of fibers and refractory metals. These Ge or Si cladding layers can be recrystallized to achieve large (several millimeters or greater) grains oriented along the axis of the fiber. Additional GaAs cladding layers are grown on the recrystallized Ge or Si by vapor-phase epitaxy or metallic solution growth. p-n junctions are formed by diffusion or epitaxy. Light-sensitive diodes have been fabricated in these structures.