Merged canonical page (2026-06-17): absorbed the former duplicate
avalanche-photodiodes(plural). One slug for the APD/SPAD concept.
An avalanche photodiode (APD) is a reverse-biased p-n junction that exploits impact ionisation to amplify photocurrent internally: a single absorbed photon generates an electron–hole pair that is accelerated by a high reverse-bias field (typically 20–200 V) until it creates secondary carriers, yielding gains of 10–100× before readout electronics. Operated above breakdown it becomes a single-photon avalanche diode (SPAD), producing a binary Geiger-mode pulse per absorbed photon with timing jitter in the tens-to-hundreds of picoseconds.
Physics / mechanism
The defining material parameters are the impact-ionisation coefficients (α for electrons, β for holes), the excess noise factor F(M), the gain-bandwidth product (GBP), and dark current. A favourable α/β ratio (low k) means low excess noise. Separate absorption, charge and multiplication (SACM) structures decouple the absorption and gain regions and are now standard. InGaAs/InP APDs dominate telecom (1310/1550 nm) with GBP ~160 GHz commercially; silicon APDs cover visible/NIR but cut off near 1 µm.
Competitive landscape
| Detector | Wavelength | Noise (k) | CMOS-compatible |
|---|---|---|---|
| Si APD | 400–900 nm | Low (k~0.02) | Yes |
| InGaAs/InP APD | 900–1650 nm | Medium (k~0.4) | No |
| Ge-on-Si APD | 800–1600 nm | Medium-high (k~0.3) | Yes |
- Si APD — cheap, band-limited to ~1 µm.
- InGaAs/InP APD — telecom wavelengths, expensive, III-V fab-constrained, temperature-hostile for LiDAR.
- Ge-on-Si APD — the primary challenger: CMOS-compatible, wafer-scale, lower cost, but a less favourable k-ratio (higher noise).
- HgCdTe APD — MWIR/LWIR with near-unity k (near-noiseless gain), but cryogenic and defence-niche (see Mercury Cadmium Telluride).
- AlInAsSb, Al₀.₈Ga₀.₂As — research-stage low-noise candidates.
- SPADs in SiPh platforms are displacing linear InGaAs APDs in quantum and ranging applications.
Applications
SPADs are the key detector in time-of-flight 3D imaging, single-photon LiDAR, quantum-key-distribution receivers, and fluorescence-lifetime microscopy. Market sizing for the single-photon family (SPAD/SiPM ~$2.1B→$9.4B 2024–2031; SNSPD a smaller cryo-niche) is in 2026 06 17 Single Photon Detector Market Spad Snspd — but SPAD/SiPM volume is incumbent-captured (Sony, STMicro, Onsemi On Semiconductor).
Investment view (vehicle-agnostic)
The discrete high-speed / high-sensitivity detector is a single-component layer with a strategic-acquisition ceiling: value integrates into the silicon-photonics receiver or is bought by incumbents (the Photonic Photodetection Layer non-area). The larger, faster-growing pool is the single-photon / SiPM family for LiDAR, 3D sensing and quantum (2026 06 17 Single Photon Detector Market Spad Snspd: SPAD/SiPM ~$2.1B→$9.4B 2024–2031), but that volume is incumbent-captured (Sony, STMicro, Onsemi On Semiconductor) and SNSPD is a small cryogenic niche held by specialists.
Routes: public via the captured incumbents and SiPh primes; track a SiPh-native SPAD displacement play (monolithic Ge APD/SPAD on a CMOS SiPh platform, defensible IP in quench circuitry and pixel architecture) — a foundry-process-advantaged angle (GF 45CLO / 22FDX) that a European team could pursue. Worked single-component pass-case: Moon Photonics (HgCdTe e-APD).
Companies using
Connected ideas
Sources
Frontier (open questions)
- Can SPAD arrays on 300 mm CMOS reach single-photon timing jitter below 50 ps at wafer-level yield?
- Will InGaAs/InP APDs be displaced by Si-SPAD arrays for 1550 nm LiDAR as nodes shrink, or does the NIR absorption edge remain a hard limit?
- What gating/quench co-integration best amortises per-pixel readout overhead in megapixel SPAD imagers?
- Does a single-photon detector cross from niche into a system play big enough to escape the commodity-detector ceiling, outside incumbent capture? (Photonic Photodetection Layer)