This book provides accessible and authoritative coverage of semiconductor-based photodetectors for various optical wavelengths. It begins by explaining the main characteristics of all photodetectors, before moving on to cover the classical PIN photodiode, the avalanche photodiode, phototransistors, the metal-semiconductor-metal photodiode and photodetectors designed for ultraviolet wavelengths. Coverage concludes with a discussion of the issues surrounding the very important question of noise in these types of sensors.