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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →A single-photon avalanche diode (SPAD) is a photodiode biased above its breakdown voltage so that one absorbed photon can trigger a self-sustaining avalanche and produce a digital electrical pulse. To judge whether a SPAD sensor will work for your instrument, read three specifications together: pixel pitch and fill factor (how much active area you get and how finely you sample space), photon-detection efficiency (how often incident photons become counted events), and readout architecture (whether the device gives pulses, counts, gates, or timestamps). A headline efficiency number without its wavelength, temperature, bias, and array context is not enough for a meaningful comparison.
What a SPAD sensor actually detects
A SPAD is operated in Geiger mode, meaning the reverse bias is set above the photodiode’s breakdown voltage. A photon that creates an electron–hole pair can start an avalanche. Quenching circuitry then stops the avalanche and resets the diode so it can detect another event. The immediate output is a short electrical pulse, not an analog signal proportional to the photon’s energy.
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That binary event model makes SPADs useful for photon counting, fluorescence lifetime measurements, time-of-flight ranging, quantum-optics experiments, and low-light imaging. The sensor can preserve the arrival time of each event, accumulate events over an exposure, or simply pass pulses to external electronics. The useful output therefore depends as much on the pixel’s readout as on the diode itself.
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Pixel pitch is the spacing, not the active detector
Pixel pitch is the center-to-center distance between neighboring pixels. It is often called “pixel size” in product literature, but it does not mean that the entire square or rectangular pixel is light-sensitive. A pixel also needs isolation structures, a guard ring to control edge breakdown, metal routing, quenching and recharge devices, and possibly counters or memory.
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The photosensitive area is the portion of that pitch where incoming light can efficiently reach the SPAD junction. Fill factor is the ratio of photosensitive area to total pixel area. Two sensors with the same 10 µm pitch can therefore have very different usable light-collection areas and different array sensitivity.
Why smaller pixels are not automatically better
A tighter pitch samples a scene more finely and can support a higher spatial resolution for a given die size. It can also place more timing elements in the same field of view. The trade-off is that guard rings and electronics occupy a larger fraction of each pixel as the pitch shrinks. The diode may become smaller, and the available area for shielding, memory, or counters may be reduced.
Conversely, a larger pixel can devote more area to the avalanche diode and on-pixel circuitry, potentially improving fill factor or adding storage, but it samples space more coarsely and may limit array resolution. There is no universal best pitch: select the smallest pitch that meets the spatial requirement while retaining enough active area and the functions your readout needs.
How fill factor enters sensitivity
For an array, fill factor directly scales the chance that an incident photon reaches a sensitive diode. The 2019 Light: Science & Applications review summarizes the relationship by noting that fill factor is multiplied by photon-detection probability to obtain overall photon-detection efficiency. A pixel with excellent diode physics can still produce modest array sensitivity if most of its pitch is occupied by circuitry.
As a scope descriptor rather than a universal limit, a 2016 Sensors review discussed compact SPAD imagers targeting pixel sizes below 25 µm and fill factors above 20%. Those figures describe the class of architectures reviewed; they are not minimum requirements or guarantees for every current device.
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PDP versus PDE: the efficiency terms that are often confused
Photon-detection probability (PDP)
PDP is the probability that a photon absorbed by the SPAD’s active region actually triggers an avalanche. It is wavelength-dependent because silicon absorption, optical coatings, and the avalanche-junction response vary across the spectrum. PDP is a property of the diode under specified operating conditions, including excess bias.
An optimized individual CMOS SPAD reached a peak visible-region PDP of 70% in the 2019 review by Bruschini and colleagues. That is a single-diode result, not an assertion that a complete imaging array delivers 70% PDE.
Photon-detection efficiency (PDE)
PDE is the probability that an incident photon on the pixel produces a registered output event. For an array, it includes the diode’s PDP and the fraction of the pixel that is photosensitive. A simplified relationship is:
array PDE ≈ fill factor × PDP
The approximation assumes the relevant optical and electrical conditions are otherwise comparable. In a specification sheet, always check whether “PDE” refers to an individual device, a pixel, or a full array, and at what wavelength it was measured. A visible-wavelength peak cannot be used to predict near-infrared performance, and a single-diode PDP cannot be substituted for an array PDE.
Questions to ask about any PDE number
- Wavelength: Is the value measured at your laser or LED wavelength, or only at the sensor’s peak?
- Bias: What excess bias produced the result? Higher bias can change detection probability, timing, dark counts, and afterpulsing.
- Temperature: Was the measurement made at a controlled temperature or in conditions similar to your instrument?
- Scope: Is it PDP for one optimized diode, PDE for one pixel, or average PDE across an array?
- Operating mode: Was the pixel free-running, gated, or operated with a particular hold-off time?
What “readout” means in a SPAD
Readout is the path from an avalanche event to the information delivered outside the sensor. The same SPAD diode can be paired with very different electronics, so readout specifications should be treated as part of the sensor architecture rather than as an accessory detail.
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Basic pulse-output pixels
A basic pixel exposes an avalanche pulse, often after quenching and level conversion. External electronics count those pulses, measure their intervals, or build a time-correlated histogram. This approach keeps the pixel relatively simple and can preserve flexibility, but it requires high-bandwidth routing and processing outside the array. The system designer must handle pulse discrimination, synchronization, storage, and data transfer.
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Pixels with counters and memory
A counting pixel accumulates events during an exposure or gate and returns a digital count. On-pixel counting reduces the volume of raw pulse traffic and can make a large array easier to read. The cost is circuit area: counters, memory, and control lines reduce fill factor or force a larger pitch. Counter depth and overflow behavior matter when illumination or exposure time varies widely.
Time-correlated pixels
Time-correlated architectures preserve timing information rather than only an event total. Depending on the design, a pixel may output a gate or detection flag, measure a time interval, or store timestamps for individual photons. Timestamp storage can support time-of-flight and fluorescence-lifetime measurements, but memory capacity, timestamp resolution, arbitration, and data bandwidth become limiting factors.
The 2016 Sensors review reported examples including 0.75 ns time gating and 368 ps timestamping jitter. These are results achieved by particular reviewed designs, not general SPAD guarantees. A sensor claiming a similar number should be checked for its wavelength, bias, temperature, event rate, and measurement method.
Readout choices by application
| Information needed | Suitable readout style | Main benefit | Main trade-off |
|---|---|---|---|
| Integrated intensity over an exposure | On-pixel counter and memory | Low external pulse bandwidth; direct per-pixel counts | Counter and memory consume pixel area and can overflow at high event rates |
| Photon arrival times or time-of-flight | Timestamping or time-to-digital/time-correlated circuitry | Retains temporal information for each accepted event | More circuit complexity, storage, arbitration, and data bandwidth |
| Flexible laboratory processing | External pulse output | Simple pixel and adaptable downstream timing or counting | External electronics must handle signal integrity, synchronization, and throughput |
| Short, precisely defined observation intervals | Gated output or detection flag | Rejects events outside the selected window | Gate control and timing accuracy can limit duty cycle and usable signal |
The noise and timing metrics that belong beside PDE
Dark count rate
Dark counts are avalanche events that occur without a photon. Compare the rate per pixel or per unit area, and note the temperature, excess bias, and hold-off used for the measurement. A high PDE at the expense of a much higher dark count rate may reduce the useful signal-to-noise ratio in a low-light experiment.
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Afterpulsing
Afterpulsing occurs when charge carriers trapped during one avalanche are released later and trigger a spurious event. It can distort photon statistics and create false temporal correlations. Longer hold-off times can reduce afterpulsing, but they also make the pixel unavailable for longer after each detection.
Dead time and hold-off
Dead time is the interval after an avalanche during which the pixel cannot register another event. Hold-off is the intentional reset interval used to allow the diode and quenching circuit to recover. These values set the maximum sustainable count rate and can cause pile-up or nonlinearity when photons arrive rapidly. Ask whether the stated dead time is per pixel, whether it is fixed or programmable, and whether the quoted count rate is raw or corrected.
Timing jitter
Timing jitter is the spread in reported arrival time for otherwise simultaneous photons. Lower jitter improves time-of-flight distance precision and fluorescence-lifetime resolution. It should be compared at the wavelength and event conditions relevant to your experiment; a timestamp resolution number alone does not describe total system timing uncertainty.
Crosstalk
Optical or electrical crosstalk causes an avalanche in one pixel to produce a correlated event in a neighbor. It matters in imaging, photon-correlation measurements, and dense arrays. Check whether a crosstalk figure is quoted for adjacent pixels, for a specified event rate, and with any shielding or correction enabled.
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An array may generate more events than its interface can export. Evaluate per-pixel storage, the number of events that can be retained, row or column arbitration, output bandwidth, and behavior when buffers fill. A sensor that meets PDE and jitter targets can still lose data if the readout saturates under the expected photon flux.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare two SPAD sensors fairly
- Define the primary objective. Decide whether your experiment values photon counts, precise timing, spatial sampling, or a balance of these.
- Match the wavelength. Compare PDE or PDP at the wavelength you will use, not each device’s most favorable wavelength.
- Record array geometry. Put pixel pitch, fill factor, active-array size, and any stated guard-ring or circuitry constraints in the same table.
- Normalize operating conditions. Compare temperature, excess bias, gating, hold-off, and event rate whenever the specifications provide them.
- Check noise and recovery. Review dark count rate, afterpulsing, dead time, and crosstalk alongside efficiency.
- Follow the data path. Determine whether the output is pulses, accumulated counts, gates, or timestamps; then verify storage depth, interface bandwidth, and overflow behavior.
- Map the numbers to the measurement. For an imager, estimate spatial sampling and usable sensitivity. For a timing instrument, prioritize jitter, gate width, timestamp capacity, and synchronization.
Do not rank sensors by one headline figure. A high-fill-factor counting array may be preferable for intensity imaging, while a lower-fill-factor design with timestamp memory may be the better choice for time-resolved work.
Common specification mistakes and how to avoid them
- “The pixel is 10 µm, so 100% of it detects light.” Pixel pitch includes nonphotosensitive circuitry and guard structures; find the fill factor.
- “The sensor has 70% efficiency.” Ask whether that is the 70% peak visible PDP reported for an optimized individual CMOS SPAD or a measured array PDE at your wavelength.
- “More pixels always means more sensitivity.” Increasing spatial sampling can shrink each diode and lower fill factor unless the architecture compensates.
- “A timestamp resolution is the same as timing accuracy.” Timestamp quantization, detector jitter, clock uncertainty, and optical-system variation all contribute to the final result.
- “A count-rate limit is just an interface specification.” Per-pixel dead time, afterpulsing, arbitration, and buffer overflow can limit usable counts before the external link is full.
What to request from a manufacturer or paper
For a credible comparison, request a complete operating-condition table rather than an isolated maximum. It should identify wavelength, temperature, excess bias, gate or free-running mode, hold-off, dark-count definition, afterpulse measurement method, timing-jitter method, crosstalk geometry, and whether values are typical, maximum, or measured on a particular sample. For arrays, ask for the distribution or average across pixels and the usable active area, not only a best pixel.
Also request a block diagram of the readout. It should show whether counters, memory, timestamp units, or time-to-analog conversion are inside each pixel, shared by a column, or located off-chip. This determines the trade between pitch, fill factor, event capacity, power, and interface bandwidth.
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Bottom line for choosing a SPAD
Start with the information your experiment must preserve. Choose pitch and fill factor together for spatial work; choose wavelength-specific PDE together with dark counts and afterpulsing for photon counting; and choose readout, jitter, dead time, storage, and bandwidth together for time-resolved measurements. Treat every efficiency and timing number as conditional on its test setup. The most useful SPAD is not the one with the largest isolated specification, but the one whose diode, pixel geometry, noise behavior, and readout deliver the required measurement without creating a new bottleneck.
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