Glossary entry

Avalanche photodiode (APD)

A photodetector with internal carrier-multiplication gain via impact ionization in a high-field region. Provides higher sensitivity than PIN at the cost of multiplication noise and bandwidth-gain tradeoff.

An avalanche photodiode separates absorption and multiplication regions. Photons are absorbed in a low-field region (similar to a PIN), and the resulting photocarriers drift into a high-field multiplication region where they trigger impact ionization, producing additional carrier pairs. The avalanche multiplication factor $M$ — typically 10 to 100 — amplifies the photocurrent before any external electronics:

I_\text{photo,APD} \;=\; M \cdot \mathcal{R}_0 \cdot P_\text{opt},

where $\mathcal{R}_0$ is the unity-gain responsivity (effectively the PIN responsivity at the operating wavelength).

Carrier multiplication is intrinsically noisy because each photogenerated carrier triggers a statistically variable number of secondary carriers. The excess noise factor is

F(M) \;=\; k M + (1 - k)(2 - 1/M),

where $k$ is the ratio of hole to electron ionization coefficients. Lower $k$ produces lower excess noise. Telecom InGaAs/InP APDs typically have $k \approx 0.3$ – $0.5$ ; silicon APDs have $k \approx 0.02$ – $0.04$ .

Operating bias is just below the breakdown voltage where the field is strongest. Bias is highly temperature-sensitive — the breakdown voltage shifts by 50–200 mV/K depending on material — so APD operation requires active temperature control (see thermoelectric cooler).

Typical APD characteristics at 1550 nm:

Parameter	Typical value
Operating bias	25 – 70 V
Multiplication factor $M$	10 – 30
Effective responsivity ( $M \cdot \mathcal{R}_0$ )	5 – 20 A/W
Bandwidth	1 – 10 GHz (lower at high $M$ )
Noise-equivalent power	0.1 – 1 pW/√Hz

APDs outperform PINs when shot-noise from the photocurrent exceeds the thermal noise of the receiver electronics — high signal levels favor PINs (simpler, more linear), low signal levels favor APDs (gain provides advantage). For single-photon counting, single-photon avalanche diodes (SPADs) operate in Geiger mode beyond the breakdown voltage, producing a single current pulse per absorbed photon.