Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical communication to scientific research and industrial automation. Photodetectors can be classified by their mechanism of detection, such as the photoelectric effect, photochemical reactions, or thermal effects, or by performance metrics like spectral response. Common types include photodiodes, phototransistors, and photomultiplier tubes, each suited to specific uses. Solar cells, which convert light into electricity, are also a type of photodetector.
History The development of photodetectors began with the discovery of the photoelectric effect by Heinrich Hertz in 1887, later explained by Albert Einstein in 1905. Early photodetectors, such as selenium cells invented in the late 19th century, were used in light meters and telegraph systems. The 1930s saw the invention of photomultiplier tubes, enabling the detection of faint light signals, which revolutionized fields like nuclear physics and astronomy. The mid-20th century brought semiconductor-based photodetectors, such as photodiodes and phototransistors, which transformed industries like telecommunications and computing. Today, advancements continue with high-speed detectors and quantum technologies.
Classification Photodetectors can be classified based on their mechanism of operation and device structure. Here are the common classifications:
Based on mechanism of operation
Photodetectors may be classified by their mechanism for detection:
Photoconductive effect: These detectors work by changing their electrical conductivity when exposed to light. The incident light generates electron-hole pairs in the material, altering its conductivity. Photoconductive detectors are typically made of semiconductors. Photoemission or photoelectric effect: Photons cause electrons to transition from the conduction band of a material to free electrons in a vacuum or gas. Thermal: Photons cause electrons to transition to mid-gap states then decay back to lower bands, inducing phonon generation and thus heat. Polarization: Photons induce changes in polarization states of suitable materials, which may lead to change in index of refraction or other polarization effects. Photochemical: Photons induce a chemical change in a material. Weak interaction effects: photons induce secondary effects such as in photon drag detectors or gas pressure changes in Golay cells. Photodetectors may be used in different configurations. Single sensors may detect overall light levels. A 1-D array of photodetectors, as in a spectrophotometer or a Line scanner, may be used to measure the distribution of light along a line. A 2-D array of photodetectors may be used as an image sensor to form images from the pattern of light before it. A photodetector or array is typically covered by an illumination window, sometimes having an anti-reflective coating.
Based on device structure Based on device structure, photodetectors can be classified into the following categories:
MSM Photodetector: A metal-semiconductor-metal (MSM) photodetector consists of a semiconductor layer sandwiched between two metal electrodes. The metal electrodes are interdigitated, forming a series of alternating fingers or grids. The semiconductor layer is typically made of materials such as silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or antimony selenide (Sb2Se3). Various methods are employed together to improve its characteristics, such as manipulating the vertical structure, etching, changing the substrate, and utilizing plasmonics. The best achievable efficiency is shown by Antimony Selenide photodetectors. Photodiodes: Photodiodes are the most common type of photodetectors. They are semiconductor devices with a PN junction. Incident light generates electron-hole pairs in the depletion region of the junction, producing a photocurrent. Photodiodes can be further categorized into: a. PIN Photodiodes: These photodiodes have an additional intrinsic (I) region between the P and N regions, which extends the depletion region and improves the device's performance. b. Schottky Photodiodes: In Schottky photodiodes, a metal-semiconductor junction is used instead of a PN junction. They offer high-speed response and are commonly used in high-frequency applications. Avalanche Photodiodes (APDs): APDs are specialized photodiodes that incorporate avalanche multiplication. They have a high electric field region near the PN junction, which causes impact ionization and produces additional electron-hole pairs. This internal amplification improves the detection sensitivity. APDs are widely used in applications requiring high sensitivity, such as low-light imaging and long-distance optical communication. Phototransistors: Phototransistors are transistors with a light-sensitive base region. Incident light causes a change in the base current, which controls the transistor's collector current. Phototransistors offer amplification and can be used in applications that require both detection and signal amplification. Charge-Coupled Devices (CCDs): CCDs are imaging sensors composed of an array of tiny capacitors. Incident light generates charge in the capacitors, which is sequentially read and processed to form an image. CCDs are commonly used in digital cameras and scientific imaging applications. CMOS Image Sensors (CIS): CMOS image sensors are based on complementary metal-oxide-semiconductor (CMOS) technology. They integrate photodetectors and signal processing circuitry on a single chip. CMOS image sensors have gained popularity due to their low power consumption, high integration, and compatibility with standard CMOS fabrication processes. Photomultiplier Tubes (PMTs): PMTs are vacuum tube-based photodetectors. They consist of a photocathode that emits electrons when illuminated, followed by a series of dynodes that multiply the electron current through secondary emission. PMTs offer high sensitivity and are used in applications that require low-light detection, such as particle physics experiments and scintillation detectors. These are some of the common photodetectors based on device structure. Each type has its own characteristics, advantages, and applications in various fields, including imaging, communication, sensing, and scientific research.
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