From Photons to Electrons
A photodetector converts light into electrical current through the photoelectric effect in a semiconductor. When a photon with energy exceeding the bandgap is absorbed, it creates an electron-hole pair that is swept out by the junction's electric field, producing photocurrent. The efficiency of this conversion — quantum efficiency — determines how many carriers are generated per incident photon and sets the fundamental limit on responsivity.
Responsivity and Wavelength
Responsivity increases linearly with wavelength (up to the cutoff) because longer-wavelength photons carry less energy, so more photons arrive per watt of optical power — each still generating one electron-hole pair. A silicon detector at 850 nm achieves roughly 0.54 A/W with 80% quantum efficiency, while the same detector at 500 nm yields only 0.32 A/W. Beyond the bandgap cutoff wavelength, responsivity drops to zero.
Noise Sources
Three noise mechanisms compete in photodetectors: shot noise from the random arrival of photons and dark-current carriers, thermal (Johnson) noise from the load resistance, and 1/f noise at low frequencies. The noise-equivalent power (NEP) combines these into a single sensitivity figure. Cooling the detector reduces dark current exponentially, which is why infrared detectors are often cryogenically cooled.
High-Speed Detection
For fiber-optic communications operating at 10–100+ Gbps, photodetector bandwidth is critical. PIN diodes achieve tens of gigahertz by minimizing the depletion-region transit time and junction capacitance. Avalanche photodiodes (APDs) trade bandwidth for internal gain, amplifying the photocurrent before it reaches the noisy electronic amplifier, improving receiver sensitivity by 5–10 dB in telecom systems.