A photodiode is described as a semiconductor diode which generates a potential difference or changes its electrical resistance when it is exposed to light. This book reflects current development and expansion of photodiodes, inclusive of the primary reviews and the precise applications developed by the writers themselves. The key thought behind this book is to enable authors to deal with a broad variety of background and highlight the progresses in photodiode-related areas. This book discusses new problems and connected solutions in various areas of primary physics, design and tool and circuit applications. We intend to help students, and even experts, in understanding the concept in a simpler way. This book will be a good source of reference to anyone who holds interest in optoelectronic devices.