Glossary term
Photodiode
A semiconductor light sensor that converts incident optical power into electrical current for detection and measurement.
Definition
deviceA photodiode is a semiconductor device that converts incident photons into electrical current through the photoelectric effect.
A photodiode is a semiconductor p-n or p-i-n junction designed to generate current when photons create electron-hole pairs in or near the depletion region. It is used in optical receivers, light meters, encoders, safety sensors, medical instruments, and precision measurement systems.
A photodiode converts optical power into electrical current. When photons with sufficient energy enter the semiconductor, they generate carriers that are separated by the junction field. The resulting photocurrent is approximately proportional to incident optical power over the linear operating range:
where R_\lambda is responsivity at wavelength \lambda and P_{opt} is received optical power. Responsivity depends on semiconductor material, wavelength, quantum efficiency, package optics, and bias condition.
Operating modes
In photovoltaic mode, the photodiode operates with zero or small bias and can produce a voltage or current from light. This is useful for low-noise or low-power sensing. In photoconductive mode, reverse bias widens the depletion region, lowers capacitance, and improves speed, but increases dark current and noise. Avalanche photodiodes add internal gain through impact ionization, improving sensitivity at the cost of bias complexity and noise.
The surrounding circuit is often as important as the diode. A transimpedance amplifier converts photodiode current into voltage, and its feedback resistor, capacitance, op-amp bandwidth, input noise, layout leakage, and stability set much of the measurement performance.
Engineering limits
Photodiode performance is limited by shot noise, thermal noise, dark current, junction capacitance, saturation, package window transmission, ambient light, temperature drift, and bandwidth. In optical communication, the link budget and signal-to-noise ratio determine receiver sensitivity. In instrumentation, calibration and linearity may dominate.
Common mistakes
A common mistake is to select a photodiode only by active area. Larger area collects more light but raises capacitance and can reduce bandwidth. Another is to ignore wavelength response or assume dark current is negligible at elevated temperature. A good design review checks wavelength, optical power range, responsivity, bias, capacitance, amplifier noise, bandwidth, ambient-light rejection, and calibration method.