The book will cover the two most important applications of semiconductor diodes – solar cells and LEDs - together with quantitative coverage of the physics of the PN junction at the senior undergraduate level. The purpose of this book, therefore, is to introduce the physical concepts required for a thorough understanding of p-n junctions starting with semiconductor fundamentals and extending this to the practical implementation of semiconductors in both PV and LED devices. The treatment of a range of important semiconductor materials and device structures is also presented.The book is aimed at senior undergraduate levels (years three and four). An introductory background in quantum mechanics is assumed, together with general knowledge of junior mathematics, physics and chemistry however, no background in electronic materials is required. As such this book is designed to be relevant to all engineering students with an interest in semiconductor devices and not specifically to electrical or engineering physics/engineering science students only. This is intentional since solar cells and LEDs involve a wide range of engineering disciplines and should not be regarded as belonging to only one branch of engineering.