PD Responsivity Calculator (QE ↔ A/W, Power → Photocurrent)

A · Quantum efficiency ↔ responsivity

Wavelength λ (nm) Quantum efficiency QE (%)

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Responsivity R (A/W)

B · Optical power → photocurrent

Incident power Responsivity R (A/W)

—Photocurrent I

R = QE · λ(nm)1239.84 (A/W) I = R · P QE as a fraction; 1239.84 = h·c/e conversion constant

InGaAs responsivity (typical); orange dot = calculator A operating point

Typical responsivity by material (typical values)
Material	Wavelength	Typical R	Note
Si	850 nm	~0.5 A/W	Short-wave, cutoff ~1.1 µm
InGaAs	1310 nm	~0.9 A/W	Telecom-band workhorse
InGaAs	1550 nm	~1.0 A/W	Near 80% of quantum limit

Why longer wavelengths give higher responsivity

Responsivity is the photocurrent produced per watt of light. At equal quantum efficiency, longer wavelengths give higher R: long-wave photons carry less energy, so one watt contains more photons, and each absorbed photon contributes one electron. The upper bound (quantum limit, dashed line) therefore rises linearly with wavelength — until the bandgap cutoff: once photon energy drops below the bandgap (InGaAs ≈0.73 eV ↔ 1.7 µm), no carriers are excited and response collapses.

Avalanche photodiodes (APDs) exceed the quantum limit through internal gain — this page's formula does not apply to them. Besides responsivity, check dark current (weak-light floor) and capacitance / bandwidth (speed) — our MINI-PD factory reports state all three measured per unit.

PD ResponsivityPhotodiode Responsivity

Why longer wavelengths give higher responsivity