What Are Single-Photon Avalanche Diodes?

Single-photon avalanche diodes (SPADs) are semiconductor photodetectors capable of registering individual photons by exploiting an internally amplified avalanche multiplication process. They are p-n junction devices biased above their breakdown voltage, a condition called Geiger mode, in which the absorption of a single photon initiates a self-sustaining avalanche of carriers that produces a measurable macroscopic current pulse. Because the avalanche grows exponentially once triggered, no external amplification is needed to detect the arrival of a single photon, making SPADs among the most sensitive optical detectors available. The field draws on semiconductor physics, high-speed circuit design, and photonics, and SPADs have been fabricated in silicon, InGaAs/InP, germanium-on-silicon, and III-V compound semiconductor platforms to cover wavelength ranges from the ultraviolet through the short-wave infrared.

Operating Principles and Geiger Mode

Biasing a p-n junction above breakdown creates an electric field strong enough that a single photoelectron or thermally generated carrier can trigger a diverging avalanche chain. The resulting current pulse is macroscopic, typically tens of microamperes to milliamperes, and represents a binary detection event: either a photon arrived and triggered the device, or thermal generation produced a false dark-count event. The probability that a photon actually initiates an avalanche, rather than being absorbed without triggering one, is the photon detection efficiency (PDE), which is the product of quantum efficiency and avalanche trigger probability.

Key performance metrics include dark count rate (DCR), timing jitter, afterpulsing probability, and dead time. Timing jitter, the statistical spread in delay between photon absorption and the electrical output pulse, is limited by carrier transit time and avalanche buildup statistics; back-illuminated silicon SPADs developed for solid-state LiDAR and reported in APL Photonics achieve timing jitter below 100 picoseconds full-width-at-half-maximum, a figure that directly sets depth resolution in time-of-flight ranging.

Quenching Circuits and Timing

After each detected event, the SPAD must be quenched by reducing its bias below breakdown before reset, otherwise the sustained avalanche current heats the junction and generates additional carriers that trigger spurious afterpulse events. Passive quenching uses a series resistor to allow the avalanche current to drop the bias voltage; the reset time is set by the RC time constant and is typically on the order of tens of nanoseconds to microseconds. Active quenching circuits use fast sensing and feedback to force the bias low within nanoseconds of avalanche onset, enabling shorter dead times and higher count rates.

Research on electronic interface design for SPADs published via PMC describes integrated active quenching and time-to-digital converter circuits that combine the quenching function with sub-100-picosecond timestamp generation in a single application-specific integrated circuit, enabling dense SPAD arrays for time-resolved imaging.

Materials and Array Integration

Silicon SPADs are the most mature platform, benefiting from CMOS compatibility and the ability to integrate quenching and readout electronics in the same die. Standard silicon covers wavelengths from roughly 400 to 1000 nanometers, limited by the bandgap. InGaAs/InP SPADs extend coverage to the 900 to 1650 nanometer range used in telecommunications and eye-safe LiDAR, at the cost of higher DCR and more complex fabrication. Germanium-on-silicon SPADs offer a CMOS-compatible route to near-infrared detection and have been demonstrated in waveguide-coupled configurations, as shown in work on room-temperature waveguide-coupled silicon SPADs published in npj Nanophotonics.

SPAD arrays, sometimes called digital silicon photomultipliers (dSiPMs), sum the outputs of thousands of SPAD pixels on a single substrate to measure photon number and spatial distribution simultaneously, enabling large-format single-photon imaging for scientific and consumer applications.

Applications

Single-photon avalanche diodes have applications across a range of fields, including:

• LiDAR systems for autonomous vehicles, robotics, and airborne terrain mapping, exploiting picosecond timing to resolve depth
• Quantum key distribution and quantum communication, where single-photon detection is required to register quantum states
• Positron emission tomography detector modules, where SPAD arrays replace photomultiplier tubes as silicon photomultipliers
• Fluorescence lifetime imaging microscopy (FLIM) in biology and materials characterization
• Astronomical single-photon counting and optical communications through free space