Photodetector: devices that sense and convert light into electrical signals

Common types

• Photoresistors (Light-dependent resistors, LDRs) — passive components whose resistance decreases as illumination increases; historically made with materials such as selenium and now often with semiconductor compounds.
• Photodiodes and phototransistors — semiconductor junction devices that generate photocurrent when photons create charge carriers; phototransistors add internal gain.
• Image sensors (CCD and CMOS) — arrays of light-sensitive elements that convert spatially distributed light into digital images, used in cameras and scientific detectors.
• Photomultiplier tubes (PMTs) and avalanche photodiodes (APDs) — specialized detectors that amplify weak light signals for low-light and single-photon applications.

How photodetectors work

Most photodetectors rely on the interaction between photons and electrons in a material. When photons with sufficient energy are absorbed, they can free electrons or create electron–hole pairs in a semiconductor; these charges are collected to form a measurable current or to change a device's conductivity. Performance depends on factors such as spectral sensitivity (which wavelengths are detected), quantum efficiency (photons converted to charge), response time, and noise.

Materials and spectral response

Different materials determine the wavelength range and sensitivity. Silicon covers visible to near-infrared wavelengths and is common for photodiodes and image sensors. Other compound semiconductors such as indium gallium arsenide (InGaAs) or germanium extend sensitivity into longer infrared regions. Early detectors used selenium and vacuum photoemission in tubes.

Performance metrics

When selecting a detector consider sensitivity, speed, dynamic range, noise-equivalent power, and linearity. Trade-offs are typical: simple resistive sensors are inexpensive but slow; photodiodes offer fast and linear responses; image sensors provide spatial resolution; PMTs and APDs offer high gain and low-light performance at higher cost and complexity.

Applications

Photodetectors are used in photography and machine vision, fiber-optic communications, environmental and industrial sensing, scientific instruments (spectroscopy, astronomy), medical imaging, barcode scanners, and safety systems. Component guides and application notes provide design advice and component comparisons for specific tasks; see manufacturer and tutorial resources for practical guidance.

Design considerations

Practical system design addresses illumination levels, required wavelength range, desired speed, noise tolerance, and interfacing electronics. Optical filters, lenses, and signal conditioning often accompany detectors. For detailed electrical characteristics and selection rules, consult datasheets and design guides, or introductory references such as educational overviews and industry application notes available from suppliers.

History and further reading

Development ranges from early photoelectric and selenium cells to modern semiconductor imagers and single-photon detectors. Advances in materials and microelectronics have miniaturized imaging systems and improved sensitivity and speed. For broader context and component selection help see technical reviews and supplier notes; introductory and advanced references are available at specialised resources.