Photodetector

Photodetectors are devices that convert light into an electrical signal. They operate by absorbing photons in a photosensitive material, generating charge carriers that can be collected as current or translated into a voltage. The response depends on the device structure: photovoltaic (no external bias) or photoconductive (biased). In photovoltaic devices such as photodiodes, a built-in or applied field sweeps carriers, producing a current proportional to light intensity. In photoconductive devices, illumination increases the material’s conductivity, changing the current under bias.

Common types include photodiodes (pn junction and PIN), avalanche photodiodes with internal gain, phototransistors, and photomultiplier tubes. Imaging sensors use arrays of photosensitive elements in CCD or CMOS technology. Detectors are built from materials selected for wavelength: silicon for visible light; InGaAs for near infrared; GaAs, PbS, PbSe, and HgCdTe for longer wavelengths. Some detectors are designed as photoconductors or as light-activated transistors.

Key performance metrics include responsivity (A/W), quantum efficiency, spectral response, noise, dark current, and dynamic range. Detectivity D* combines sensitivity and noise. Speed is described by rise and fall time or bandwidth. Detectors are operated at different temperatures to reduce noise and extend sensitivity.

Applications span optical communications, imaging, spectroscopy, astronomy, and environmental or biomedical sensing. Design choices balance speed, sensitivity, wavelength range, and integration with readout electronics. The term photodetector is used for devices that generate electrical signals from light, whether for single-point measurements or imaging arrays.