This book brings together key concepts and case studies that showcase the evolving role of semiconductors in optoelectronic devices and clean energy technologies. It concisely explores materials and methods relevant to photodiodes, photodetectors, and photocatalytic systems. Topics include a simulation-based understanding of GaAs PIN photodiodes, the fundamentals and historical context of semiconductors, and insights into advanced photovoltaic detectors-including avalanche structures for enhanced sensitivity. The book also highlights recent approaches in the photodeposition of co-catalysts over 2D heterostructures to improve hydrogen production and solar water-splitting performance. With a balance of foundational knowledge and current research trends, this volume offers practical insights into device design, light-matter interactions, and nanomaterial integration. It is intended for researchers and engineers in materials science, electronics, and renewable energy. The content is accessible, well-organized, and suitable as a reference or learning resource for those interested in the intersection of semiconductors, photonics, and sustainable technologies.