This document outlines the principles, characteristics, types, noise, response time, merits, demerits, and applications of photodiodes. It discusses p-i-n photodiodes, avalanche photodiodes (APDs), and InGaAs APDs. P-i-n photodiodes operate based on the photoelectric effect and separation of photogenerated carriers by the reverse bias. APDs multiply the primary photocurrent through impact ionization. Both device types are used in optical communication systems and detectors, while APDs see additional use in applications requiring gain like laser range finders. Noise sources include quantum and dark current noise, and response time depends on carrier transit and diffusion times.