Glossary entry

Responsivity (R)

The ratio of photocurrent generated by a photodetector to incident optical power. Measured in A/W; sets the fundamental signal level for any optical receiver.

Photodetector responsivity is defined as

\mathcal{R} \;=\; \frac{I_\text{ph}}{P_\text{opt}} \quad [\text{A/W}],

where $I_\text{ph}$ is the photocurrent and $P_\text{opt}$ is the optical power incident on the active area.

Responsivity is fundamentally bounded by the photon energy. The quantum efficiency $\eta$ is the fraction of incident photons that produce collected carriers; for unity quantum efficiency, every photon of energy $h\nu = hc/\lambda$ produces one electron of charge $e$ :

\mathcal{R}_\text{max} \;=\; \frac{\eta \, e \, \lambda}{h c} \;\approx\; \frac{\eta \, \lambda \, [\mu\text{m}]}{1.24} \quad [\text{A/W}].

At $\eta = 1$ : $\mathcal{R}_\text{max} = 0.68$ A/W at 850 nm, $1.06$ A/W at 1310 nm, $1.25$ A/W at 1550 nm.

Typical values for telecom and datacom detectors:

Detector	Material	Wavelength	Responsivity
Si PIN	Si	850 nm	0.5 – 0.6 A/W
InGaAs PIN	InGaAs/InP	1310 nm	0.85 – 0.95 A/W
InGaAs PIN	InGaAs/InP	1550 nm	0.95 – 1.05 A/W
Ge PIN	Ge	1310 / 1550 nm	0.6 – 0.85 A/W
Ge-on-Si waveguide PD	Ge	1310 / 1550 nm	0.6 – 1.0 A/W
InGaAs APD ( $M = 10$ )	InGaAs/InP	1550 nm	8 – 10 A/W
Avalanche photodiode (general)	varies	varies	$M \times$ unity-gain $\mathcal{R}$

Responsivity is wavelength-dependent — high-bandgap materials cut off at short wavelengths (Si: $\sim 1100$ nm cutoff), and any material has reduced responsivity beyond its band edge due to vanishing absorption.

APDs report responsivity at the operating gain $M$ : $\mathcal{R}_\text{APD} = M \cdot \mathcal{R}_0$ where $\mathcal{R}_0$ is the unity-gain responsivity. APD datasheets typically also specify the operating bias voltage corresponding to the rated gain.