Photodiode

Photons with energy above the band gap create electron–hole pairs in the depletion region; the field sweeps them apart, producing a photocurrent proportional to light intensity.

• Photovoltaic mode (zero bias) — low noise, slower; this is how solar cells work.
• Photoconductive mode (reverse bias) — wider depletion, lower capacitance, faster response.

A transimpedance amplifier (op-amp with a feedback resistor) converts the tiny current into a voltage: $V_{out} = -I_{ph} R_f$. PIN photodiodes add an intrinsic layer for speed; avalanche photodiodes add internal gain.