photodetection

Photodetection is the conversion of incident light into an electrical signal. Photodetectors rely on the generation and/or modulation of charge carriers by photons in a semiconductor or other photosensitive material. The resulting output—current, voltage, or capacitance—depends on light intensity, wavelength, and the device’s spectral response. Performance is described by responsivity, bandwidth, noise, and dynamic range.

Common device families include photodiodes (PN, PIN) that generate photocurrent under reverse bias, and avalanche photodiodes that provide internal gain. Photoconductors change their electrical conductivity with illumination, while photomultiplier tubes use a dynode chain to amplify weak signals. In imaging and sensing, arrays of photodetectors are integrated with readout electronics in CCD or CMOS sensors to form cameras and spectrometers.

Materials span a wide spectral range. Silicon detectors cover visible to near-infrared, while III-V semiconductors such as GaAs and InGaAs extend sensitivity into the near-IR. Mercury cadmium telluride and related compounds enable mid-IR detection, often with cooling. Two-dimensional materials and organic photodetectors offer flexible or specialized functionality. The choice of material is guided by the desired spectral range, operating temperature, speed, and noise performance.

Key performance metrics include responsivity (A/W), quantum efficiency (external and internal), specific detectivity (D*), and noise-equivalent power (NEP). Time response (rise/fall time) and bandwidth determine speed, while dark current and various noise sources limit sensitivity. Device design—biasing, impedance matching, optical coupling, and filtering—affects overall performance and suitability for imaging, communications, spectroscopy, and safety applications.