Avalanche Photodiode

A reverse-biased silicon junction so hungry for current that a single photon sparks an avalanche of thousands of electron collisions — turning light into an electrical shout.

Converts photons into electrical current with built-in gain, making it sensitive enough for detecting weak light in laser rangefinders, autonomous-vehicle LIDAR, fiber-optic receivers, and quantum cryptography systems. Unlike a standard photodiode that produces one electron per absorbed photon, an APD uses a high reverse-bias voltage to accelerate the initial carrier into a high-field multiplication region, triggering a controlled avalanche of impact ionizations that delivers 50–500 electrons per photon in linear mode. Silicon APDs cover 400–1000 nm at gains up to several hundred, while InGaAs/InP APDs extend coverage to 1300–1550 nm telecom wavelengths at lower gains of 10–30.

The reach-through APD structure (RAPD) is built as a p⁺ / π / p / n⁺ vertical stack. Light enters through the p⁺ contact and is absorbed in the wide, lightly doped π region where the electric field is low (~3×10⁴ V/cm) — just strong enough to sweep carriers apart without triggering multiplication prematurely. Photo-generated electrons drift down into the narrow p multiplication layer where the field exceeds 2×10⁵ V/cm, surpassing the impact-ionization threshold. There, a drifting electron collides with the lattice, generating a new electron-hole pair; each daughter carrier also ionizes, producing a cascade. The gain M is described by McIntyre's 1966 formula M = 1/(1−⟨α(E)⟩·W), where α is the electron ionization coefficient and W is the multiplication-layer width. Because silicon has α_e ≫ α_h (electrons ionize far more readily than holes), the avalanche is efficiently initiated by electrons alone, giving a lower excess noise factor F(M) ≈ 2 + k_eff·M with k_eff < 0.1. InGaAs/InP APDs use a separate absorption–charge–multiplication (SACM) structure to confine the high field to the InP multiplication region while absorbing photons in the lower-gap InGaAs layer.

In plain terms

Imagine rolling a marble down a hill covered in sticks. The marble knocks one stick loose, that stick knocks two more, those knock four, and so on — suddenly you have a whole avalanche from one little marble. An APD does the same thing with electrons: one photon starts the chain reaction, and thousands of electrons come tumbling out the other end.

Also called: APD, linear-mode APD, reach-through APD, RAPD, InGaAs APD, Si APD.

Why it matters

Provides 10–100× higher sensitivity than a standard PIN photodiode without the complexity of a photomultiplier tube, enabling compact, low-noise LIDAR receivers in autonomous vehicles.
InGaAs APDs are the dominant detector in long-haul fiber-optic receivers at 1310 and 1550 nm, enabling coherent DWDM systems carrying tens of terabits per second on transoceanic cables.
Linear-mode APD arrays in laser rangefinders resolve centimetre-level target distances in military and survey applications where a single photon pulse must be reliably detected against bright backgrounds.
In quantum-key-distribution (BB84) receivers, InGaAs APDs operated just below the Geiger threshold detect single-photon-level signals while maintaining the room-temperature operation needed for fielded systems.

Where it lives

Hamamatsu S12053 series — Si APD for laser rangefinders and bar-code scanners
Excelitas C30902EH — near-single-photon-class Si APD for LIDAR and spectroscopy
Lumentum / II-VI InGaAs APD receivers — 10/25/100 Gbit/s fiber-optic transceivers
Velodyne HDL-64E — 64-channel rotating LIDAR using Si APD arrays for autonomous vehicles
ID Quantique ID230 — InGaAs APD for quantum-key-distribution receivers at 1550 nm
Princeton Lightwave (now Argo AI) Geiger-mode APD arrays — 3D flash LIDAR

Key numbers

Gain M (Si, linear mode): 50–500 (Higher gains approach Geiger mode (SPAD); linear mode retains proportional response)
Spectral range (Si APD): 400–1000 nm (Peak QE ~700–900 nm; response falls beyond Si bandgap at ~1.1 µm)
Spectral range (InGaAs APD): 900–1700 nm (Optimised for telecom 1310/1550 nm; gain M = 10–30 at these wavelengths)
Impact ionization threshold (Si): >2×10⁵ V/cm (Electric field required in multiplication layer; achieved at 100–400 V reverse bias)
k_eff (Si): 0.02–0.05 (Ratio of hole-to-electron ionization rates; low k_eff gives low excess noise)
Dark current (Si APD at M=100): ~nA (Bulk dark current; surface leakage also present at room temperature)
NEP (Si APD): ~10⁻¹⁵ W/√Hz (Typical noise-equivalent power at optimal gain for 800 nm illumination)

Surprising facts

A single photon deposits about 0.3 eV of energy into silicon — roughly 10⁻¹⁹ joules. A reach-through APD converts this into a current pulse with 50–500 electrons arriving in nanoseconds, without any cooling or vacuum hardware.
McIntyre's 1966 noise formula shows that if only electrons ionize (k = 0), the excess noise factor approaches 2 at high gain — essentially the theoretical minimum for any avalanche detector. Silicon's k_eff ≈ 0.02–0.05 is among the lowest of any semiconductor, which is why Si APDs remain the detector of choice for high-sensitivity visible-wavelength systems sixty years after McIntyre's paper.
The impact-ionization threshold field in silicon (~2×10⁵ V/cm) is reached across a multiplication layer only 1–3 µm thick, meaning the entire gain process happens in a region thinner than a human red blood cell.
At gain M = 100, a Si APD with k_eff = 0.03 achieves a noise-equivalent power (NEP) around 10⁻¹⁵ W/√Hz — sensitive enough to detect the photon flux from a candle burning 25 km away.

Open questions

Gain-bandwidth product ceiling: Every APD faces a hard trade-off: higher gain means more time spent in the multiplication region, which widens the current pulse and limits bandwidth. The gain-bandwidth product (GBP) is approximately constant for a given device — a Si APD with M = 100 and bandwidth 200 MHz gives GBP ≈ 20 GHz. Attempts to break this limit require engineered multiplication layers using quantum-well staircase structures or dead-space engineering, but the improvement is modest. For InGaAs SACM APDs, GBP of ~100–150 GHz is typical; beyond this, engineers switch to coherent receivers or photomultiplier tubes.

A short history

The theoretical framework for avalanche multiplication noise was established by R. J. McIntyre at RCA Laboratories in 1966, deriving the excess noise factor F(M) that still governs every APD design today. The first practical silicon reach-through APD was developed at RCA in the early 1970s, capitalising on the low hole-ionization rate in silicon identified by McIntyre. Webb, McIntyre, and Conradi published the foundational properties paper in RCA Review in 1974. InGaAs/InP APDs for the 1550 nm telecom band were pioneered in the 1980s by Bell Labs and Japanese groups as single-mode fiber networks expanded, leading to the separated-absorption-charge-multiplication (SACM) structure that isolates the high-field multiplication region from the absorbing InGaAs layer. By the 2000s, InGaAs APD receivers were standard in all long-haul fiber systems. The 2010s saw APD arrays integrated into solid-state LIDAR modules for autonomous vehicles — transitioning the APD from a telecommunications component to a key sensor in self-driving car perception stacks. John Campbell (University of Virginia) summarised the state of the art in his 2007 Journal of Lightwave Technology review.

Sources

McIntyre, R. J. 'Multiplication noise in uniform avalanche diodes,' IEEE Trans. Electron Devices 13, 164 (1966) — foundational APD noise theory (https://www.semanticscholar.org/paper/Multiplication-noise-in-uniform-avalanche-diodes-Mcintyre/5d9ed77925163f439d039fe8a0d2ff233e8ec083)
Campbell, J. C. 'Recent advances in telecommunications avalanche photodiodes,' J. Lightwave Technol. 25, 109 (2007) (https://doi.org/10.1109/JLT.2006.888481)
Laser Components — 'Silicon Reach-Through Avalanche Photodiodes' application note (https://www.lasercomponents.com/us/photonics-portal/knowledge-center/application-notes/silicon-reach-through-avalanche-photodiodes/)
Laser Components — APD Selection Guide: Si or InGaAs (https://www.lasercomponents.com/us/photonics-portal/knowledge-center/selection-guides/apd-selection-guide-si-or-ingaas/)
Hamamatsu Photonics — InGaAs APD product family (LIDAR sensors) (https://www.hamamatsu.com/jp/en/product/optical-sensors/distance-position-sensor/lidar-sensor/ingaas-apd.html)