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SPAD

1996ActivePublished: 28 August 2026Updated: 28 August 2026Published
Key innovation
Enables detection of a single photon with picosecond-scale timing resolution by operating in Geiger mode, where one charge carrier triggers a self-sustaining avalanche that produces a digital output pulse.
Category
Robotics
Abstraction level
Primitive
Operation level
DataRobot control
Use cases
Digital / solid-state LiDAR (direct time-of-flight, dToF)3D imaging and depth sensingSPAD-SoC automotive LiDAR (e.g. RoboSense)Single-photon counting (TCSPC)Fluorescence lifetime imaging microscopy (FLIM)Positron emission tomography (PET)Quantum key distribution (QKD) and quantum opticsExtremely low-light imaging / night vision

How it works

1) Bias: the diode is reverse-biased above breakdown by an excess voltage. 2) Absorption: a photon generates an electron-hole pair in the depletion region. 3) Avalanche: the strong electric field (>3×10^5 V/cm) drives impact ionization and a self-sustaining avalanche, yielding a fast current pulse. 4) Timestamp: the pulse leading edge, read by a discriminator and a time-to-digital converter (TDC), marks the photon arrival time. 5) Quench: a quenching circuit (passive/active) lowers the bias below breakdown to stop the avalanche. 6) Reset and dead time: the diode returns to its operating bias; during the dead time it cannot register photons. In dToF LiDAR, repeated time-of-flight measurements are accumulated into a histogram (TCSPC) from which distance is derived; SPAD arrays build a 3D depth image.

Problem solved

Conventional photodetectors (PIN photodiodes, and even APDs in linear mode) cannot detect a single photon or time its arrival with picosecond precision at chip scale. SPAD addresses detection at extremely low light levels and precise time-of-flight measurement, which is a prerequisite for digital dToF LiDAR, low-light 3D imaging, and single-photon counting techniques.

Components

Multiplication region (avalanche junction)Converts a single carrier into a macroscopic current pulse

The depleted p-n junction where a strong electric field drives impact ionization and a self-sustaining avalanche after photon absorption.

Guard ringStabilizes and localizes avalanche breakdown

A doped structure around the active area that prevents premature edge breakdown and confines the avalanche to the defined region.

Quenching circuitStops the avalanche and resets the detector

A passive (ballast resistor) or active (discriminator that lowers the bias) circuit that stops the avalanche and resets the diode; it sets the dead time.

Passive quenchingA ballast resistor (~100 kΩ+) self-quenches the avalanche via the developed voltage drop.
Active quenchingA fast discriminator detects the avalanche and actively lowers the bias below breakdown, reducing dead time.

Official

Time-to-digital converter (TDC) / readoutPrecise measurement and digitization of photon arrival time

Readout electronics that record the avalanche timing; in dToF LiDAR it accumulates repeated measurements into a time-of-flight histogram (TCSPC).

Official

Implementation

Implementation pitfalls
AfterpulsingHigh

Carriers trapped during the avalanche are released during recovery and trigger secondary avalanches, producing non-Poissonian statistics.

Fix:Increase dead time, lower excess bias, improve material quality.
High dark count rate (DCR)High

Thermally generated carriers cause false avalanches, limiting sensitivity, especially at room temperature and in InGaAs.

Fix:Cooling, material selection, limiting excess bias, temporal gating.
Dead-time saturationMedium

At high light levels the detector saturates because it registers no photons during the dead time, distorting counts.

Fix:Active quenching (shorter dead time), SPAD arrays, statistical histogram correction.

Evolution

Original paper · 1996 · Applied Optics, Vol. 35, No. 12 · Sergio Cova
Avalanche photodiodes and quenching circuits for single-photon detection
Sergio Cova, Massimo Ghioni, Andrea Lacaita, Carlo Samori, Franco Zappa
1961
Microplasma and avalanche gain distribution theory

R.J. McIntyre's work on microplasma instability and avalanche gain statistics, foundational for avalanche detectors.

1996
Comprehensive framework for SPADs and quenching circuits
Inflection point

Cova et al. in Applied Optics systematized single-photon detection with avalanche photodiodes and quenching circuits.

2003
SPAD integration in standard CMOS
Inflection point

Demonstration of SPADs in standard CMOS technology opened the way to low-cost integrated arrays and on-chip readout.

2021
Commercial SPAD dToF sensors for automotive

Introduction of stacked SPAD dToF sensors (e.g. Sony IMX459) for automotive LiDAR; growth of megapixel arrays.

Hyperparameters (configurable axes)

Excess bias (overvoltage)Critical

Voltage above breakdown; higher improves PDE but increases DCR and afterpulsing.

Dead timeHigh

Recovery time after an avalanche; sets the maximum count rate (active quenching: ~1–2 ns).

Photon detection efficiency (PDE/PDP)High

Probability that an incident photon triggers an avalanche; depends on wavelength and excess bias.

Dark count rate (DCR)High

False counts from thermally generated carriers; CMOS SPADs can reach <10 Hz.

Timing jitterHigh

Uncertainty of the timestamp; modern CMOS SPADs ~50 ps.

Material / wavelength rangeMedium

Si (visible/NIR), Ge-on-Si and InGaAs/InP (telecom infrared).

Array size / fill factorMedium

Pixel count and active-area fraction; modern arrays >1 Mpx with >70% fill factor.