Photodiode Current

Formula

Description

A photodiode generates current proportional to the incident optical power. The responsivity S relates the photocurrent to the optical power at a specific wavelength, typically peaking at 0.4-0.6 A/W for silicon photodiodes in the 800-900 nm range. The total optical power on the detector is the irradiance (power per unit area) multiplied by the active area of the photodiode. This relationship is fundamental to fiber optic receivers, light meters, optical encoders, and any system that converts light to an electrical signal.

Variables

• Iph — Photocurrent generated by the photodiode (A)
• S — Spectral responsivity at the wavelength of interest (A/W)
• E — Irradiance (optical power density) at the detector surface (W/m²)
• A — Active (photosensitive) area of the detector (m²)

Practical Notes

Responsivity varies with wavelength: silicon photodiodes work from 200-1100 nm, InGaAs from 800-1700 nm. Dark current (leakage with no light) sets the minimum detectable signal. Transimpedance amplifiers convert the photocurrent to voltage for processing. For precision measurements, operate the photodiode in photovoltaic mode (zero bias) for lowest noise, or photoconductive mode (reverse bias) for fastest speed.