The Pad HPD: A highly segmented hybrid photodiode

Hybrid Photodiodes (HPD, also referred to as Hybrid Photo Detectors) represent one of the most promising options for high granularity single photon detection. We report about the development of the Pad HPD, a 5-inch device with a bialkali photocathode and encapsulated analog electronics for the readout of the 2048 channels. The design of the Pad HPD, which was optimised for the use in the RICH detectors of LHCb, comprises many features required for medical imaging applications. Several HPDs have been fabricated in a dedicated UHV facility at CERN. We briefly summarise the fabrication process and the excellent performance figures obtained in a lab set-up and with Cherenkov radiators in test beams. Emphasis is put on the performance increase which resulted from recent minor design changes.