SPADdiodit

SPADdiodit, or SPAD diodes, are a class of semiconductor photodetectors capable of detecting individual photons. They are avalanche photodiodes operated in Geiger mode, biased above their breakdown voltage. When a photon is absorbed, a carrier triggers a self-sustaining avalanche, producing a large current pulse that is easily detected. To stop the avalanche, the device requires quenching, either passive (resistor) or active (circuit-based quenching). After quenching, a short dead time prevents immediate re-triggering.

Key performance parameters include photon detection efficiency (PDE), dark count rate (DCR), timing jitter, afterpulsing probability, and dynamic range. PDE depends on wavelength, overvoltage, and device design; DCR increases with temperature and material defects; afterpulsing stems from trap states within the detector; jitter ranges from tens of picoseconds to a few hundred picoseconds in silicon devices; dead time is set by the quenching method and device recovery.

SPADs are produced in silicon for visible to near-infrared light (roughly up to 700–900 nm); materials such as InGaAs/InP extend sensitivity to 1.1–1.7 μm, suitable for telecom wavelengths. Monolithic SPAD arrays and cameras integrate many SPADs with readout electronics in CMOS processes, enabling high-throughput photon counting and imaging.

Applications span time-correlated single-photon counting (TCSPC) for fluorescence lifetime measurements, time-of-flight and LIDAR, quantum information experiments and quantum key distribution, and single-photon imaging including fluorescence lifetime imaging (FLIM).

Compared with conventional photodiodes, SPADdiodit offer single-photon sensitivity and fast timing but exhibit higher dark counts and afterpulsing, especially at elevated temperatures or in larger area devices. Ongoing research targets lower DCR, reduced afterpulsing, and tighter integration with CMOS electronics.