This document discusses different types of photodetectors. It describes photoconductive detectors like light dependent resistors (LDRs) and junction photodetectors including p-n photodiodes, PIN photodiodes, avalanche photodiodes, and Schottky photodiodes. PIN photodiodes are presented as an improvement over p-n photodiodes by having a larger depletion region for higher quantum efficiency. Avalanche photodiodes provide internal gain through impact ionization. Schottky photodiodes have a high speed due to being majority carrier devices. Phototransistors are also discussed as providing gain. Applications mentioned include fiber optics, cameras, medical devices, barcodes, and security systems