Electronics Guide

Photodiodes and Photodetectors

Convert light into electrical signals through semiconductor physics. Topics include PIN photodiodes, avalanche photodiodes (APDs), silicon photomultipliers (SiPMs), metal-semiconductor-metal detectors, quantum well infrared photodetectors, photoconductors and photoconductive cells, position-sensitive detectors, quadrant photodiodes, linear array detectors, spectral response characteristics, noise sources and signal-to-noise ratio, bandwidth and response time, temperature compensation, bias circuit design, and transimpedance amplifiers.

Image Sensors

Capture two-dimensional optical information for imaging applications. Coverage includes CMOS image sensor architectures, CCD sensor technologies, back-illuminated sensors, stacked sensor designs, global and rolling shutter methods, pixel architectures and sizes, color filter arrays (Bayer, X-Trans), high dynamic range techniques, low-light performance optimization, infrared and thermal imaging, time-of-flight sensors, event-based vision sensors, hyperspectral imaging arrays, scientific imaging sensors, and radiation-hardened imagers.

Optical Sensors and Transducers

Measure physical parameters using light as the sensing medium. This section addresses fiber optic sensors, interferometric sensors, intensity-based sensors, fluorescence sensors, Raman sensors, surface plasmon resonance, photoacoustic sensors, optical encoders, laser distance sensors, optical proximity sensors, color sensors, ambient light sensors, UV index sensors, smoke and particle detectors, and chemical optical sensors.

Photomultipliers and Intensifiers

Amplify weak optical signals for low-light detection applications. Topics encompass photomultiplier tube designs, microchannel plates, image intensifier tubes, streak cameras, single-photon counting modules, gated intensifiers, electron-multiplying CCDs, intensified CCDs, photon counting arrays, dark current reduction, quantum efficiency optimization, timing resolution, spatial resolution, dynamic range considerations, and cooling requirements.

Fundamentals of Photodetection

Photodetection relies on the interaction between photons and semiconductor materials. When a photon with energy exceeding the bandgap strikes a semiconductor, it can promote an electron from the valence band to the conduction band, creating an electron-hole pair. These charge carriers can be collected and measured as electrical current, converting optical signals into electrical form.

Key performance parameters include quantum efficiency (the fraction of incident photons that generate collected charge carriers), responsivity (electrical output per unit optical input), spectral response (sensitivity versus wavelength), dark current (output in the absence of light), and noise equivalent power (the optical power that produces a signal equal to the noise level).

Photodiode Technologies

Photodiodes are the most common photodetectors, available in various configurations optimized for different applications. PN junction photodiodes offer simple construction and reliable operation. PIN photodiodes insert an intrinsic layer between p and n regions to increase the depletion width, improving speed and sensitivity. Avalanche photodiodes operate under high reverse bias where impact ionization provides internal gain, enabling detection of weak signals.

Phototransistors and Photomultipliers

Phototransistors combine photodetection with transistor amplification, providing higher sensitivity than photodiodes at the cost of reduced speed. They are commonly used in optocouplers and simple light sensing applications. Photomultiplier tubes use vacuum tube technology with dynodes to achieve extreme sensitivity through electron multiplication, remaining important for low-light scientific applications despite their size and high voltage requirements.

Spectral Range and Materials

Different semiconductor materials enable detection across various wavelength ranges. Silicon photodetectors respond from approximately 200 nm to 1100 nm, covering ultraviolet, visible, and near-infrared. Germanium and indium gallium arsenide extend sensitivity into the short-wave infrared. Mercury cadmium telluride and quantum well structures detect mid-wave and long-wave infrared for thermal imaging applications.

Image Sensor Arrays

Image sensors contain millions of photodetector elements (pixels) arranged in two-dimensional arrays, enabling complete scene capture. CCD (charge-coupled device) sensors use charge transfer to read out accumulated signals, offering excellent image quality. CMOS (complementary metal-oxide-semiconductor) image sensors integrate amplification and processing with each pixel, enabling higher speeds and lower power consumption. Modern image sensors achieve remarkable performance with pixel sizes below 1 micrometer and total resolutions exceeding 100 megapixels.