This book includes proceedings of the 21st International Workshop on Physics of Semiconductor Devices. The workshop is jointly organized by the Indian Institute of Technology, Delhi, and Solid State Physics Laboratory, Delhi, in collaboration with the Society for Semiconductor Devices and Semiconductor Society of India. This book disseminates the current knowledge of semiconductor physics and its applications across the scientific community. It is based on a biennial workshop that provides the participating research groups with a stimulating platform for interaction and collaboration with colleagues from the same scientific community. The book discusses the latest developments in III-nitrides; materials and devices, compound semiconductors, VLSI technology, optoelectronics, sensors, photovoltaics, crystal growth, epitaxy, and characterization, graphene, and other 2D materials and organic semiconductors. The research articles included in this book are contributed by various eminent scientists from all over the world. The book serves as a reference resource for researchers and practitioners in academia and industry.