Single-photon avalanche diodes (SPADs) are highly sensitive photodetectors capable of detecting individual photons by exploiting the Geiger mode operation of avalanche photodiodes. Unlike conventional avalanche photodiodes operating in linear mode, SPADs are biased above their breakdown voltage, enabling them to detect single photons with high timing resolution and low noise. Their unique characteristics make them indispensable in applications requiring ultra-sensitive detection, such as quantum cryptography, LiDAR, and fluorescence microscopy.

### Geiger Mode Operation
SPADs operate in Geiger mode, where a reverse bias voltage exceeding the breakdown voltage is applied. This creates a high electric field across the depletion region, allowing a single photogenerated carrier to trigger a self-sustaining avalanche multiplication process. The resulting current spike indicates photon detection. However, the avalanche must be quickly quenched to prevent damage and allow the device to reset for subsequent detections.

The Geiger mode operation is characterized by two key parameters: photon detection efficiency (PDE) and dark count rate (DCR). PDE quantifies the probability that an incident photon triggers an avalanche, while DCR measures false counts due to thermal or tunneling-generated carriers. Silicon SPADs typically achieve PDE values between 20% and 50% in the visible to near-infrared range, while InGaAs SPADs, optimized for longer wavelengths, exhibit PDEs of 10% to 30% in the 900 nm to 1700 nm range. DCR is highly temperature-dependent, with cooling significantly reducing noise in both material systems.

### Quenching Circuits
Quenching circuits are essential for SPAD operation, as they terminate the avalanche and reset the diode. Two primary quenching methods exist: passive and active.

Passive quenching employs a high-impedance resistor in series with the SPAD. When an avalanche occurs, the voltage drop across the resistor reduces the bias below breakdown, extinguishing the current. While simple, passive quenching suffers from slow recovery times due to the RC time constant of the resistor and SPAD capacitance.

Active quenching uses fast electronics to detect the avalanche and rapidly lower the bias voltage. This method offers faster reset times and better control over dead time—the period during which the SPAD cannot detect another photon. Advanced active quenching circuits incorporate hold-off circuits to further minimize afterpulsing, a phenomenon where trapped carriers release and trigger secondary avalanches.

Integrated CMOS SPADs often feature on-chip quenching circuits, enabling compact arrays with high timing resolution. Custom-designed ASICs further optimize performance by reducing parasitic capacitance and improving signal processing.

### Silicon vs. InGaAs SPADs
Silicon SPADs are the most widely used due to their high PDE in the visible spectrum and relatively low DCR. They are fabricated using planar or trench isolation techniques in CMOS-compatible processes, allowing for large-scale integration in imaging arrays. Silicon SPADs excel in applications like fluorescence lifetime imaging (FLIM) and time-correlated single-photon counting (TCSPC), where timing resolution down to tens of picoseconds is required.

InGaAs SPADs are tailored for near-infrared wavelengths, making them suitable for telecommunications and quantum key distribution (QKD). However, they exhibit higher DCR and afterpulsing compared to silicon devices. Cooling to temperatures around -40°C mitigates these issues. InGaAs SPADs often employ gated quenching, where the detector is briefly biased above breakdown only during expected photon arrival times, reducing noise.

### Applications in Quantum Cryptography
Quantum cryptography, particularly QKD, relies on SPADs for detecting single photons encoding quantum information. InGaAs SPADs are commonly used in fiber-based QKD systems due to their compatibility with telecom wavelengths. Time-bin and polarization-encoded protocols demand high timing resolution and low noise, achievable with gated InGaAs detectors.

Silicon SPADs are employed in free-space QKD systems operating at shorter wavelengths. Their high PDE and fast response enable secure key distribution over satellite links. Recent advances in superconducting nanowire single-photon detectors (SNSPDs) challenge SPADs in QKD, but SPADs remain widely used due to their practicality and room-temperature operation.

### LiDAR Systems
SPADs are integral to time-of-flight (ToF) LiDAR systems for automotive, robotics, and topographic mapping. Their single-photon sensitivity enables long-range detection even with weak return signals. Silicon SPAD arrays, fabricated in CMOS, allow high-resolution 3D imaging with millimeter precision.

Flash LiDAR systems illuminate a scene with a broad laser pulse and use a SPAD array to capture the entire depth map in a single shot. This contrasts with scanning LiDAR, which relies on mechanical beam steering. SPAD-based LiDAR benefits from robustness and scalability, though sunlight interference and multi-path reflections pose challenges mitigated through advanced filtering algorithms.

### Fluorescence Microscopy
In life sciences, SPADs enable fluorescence microscopy techniques like FLIM and super-resolution imaging. TCSPC with SPADs measures the arrival time of photons emitted by fluorophores, revealing molecular interactions and environmental properties. Silicon SPAD arrays provide parallel detection, accelerating data acquisition while maintaining picosecond timing precision.

Single-molecule localization microscopy (SMLM) leverages SPADs to achieve nanometer-scale resolution by precisely tracking photon emission from individual molecules. The high sensitivity and low noise of SPADs are critical for detecting faint signals in densely labeled biological samples.

### Future Developments
Ongoing research focuses on improving SPAD performance through novel materials, such as germanium-on-silicon and quantum dot-enhanced structures, to extend spectral range and reduce DCR. Integration with nanophotonics aims to enhance light coupling efficiency, while machine learning algorithms optimize signal processing in real-time applications.

SPADs continue to push the boundaries of single-photon detection, enabling breakthroughs in secure communication, autonomous systems, and biomedical imaging. Their versatility and evolving capabilities ensure their prominence in both established and emerging technologies.