Large area hybrid photodiodes

Hybrid Photodiodes (HPD) represent one of the most promising options for high granularity single photon detection. HPD's are photodetectors consisting of a photocathode, which is deposited on the inner side of the entrance window, and a solid state sensor encapsulated in a vacuum envelope. HPD's combine the high sensitivity of photomultiplier tubes with the excellent space and energy resolution of solid state detectors. After reviewing the physical principles of HPD's the article gives a short overview of the history of this detector. A number of commercially available devices will be discussed. Current and future applications in high energy physics lie in the fields of scintillator readout (calorimetry, fibre tracking) and Cherenkov light detection. It is for the latter application that various large area Hybrid Photodiodes are currently under development.