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Photodetectors, based on photon absorption or thermal emission, are made from semiconductor materials that are very sensitive in the spectral range from ultraviolet to far infrared. Detector figures of merit such as responsivity, noise, noise equivalent power and detectivity, and ideal performance and cooling requirements are given. Modes of operation include charge‐coupled device, photovoltaic diode, photoconductor, and bolometer. Most semiconductor detectors operate as photon detectors where the minimum photon energy detected is defined by a discrete activation energy of the semiconductor. However micromachined bolometers are in development for thermal imaging and chemical spectroscopy. Materials and detector types are discussed. Detector material preparation technologies are presented for intrinsic detectors where the photon absorption coefficient is high, and for the low absorption coefficient detectors such as impurity‐doped Ge or Si. Fabrication technology making possible large photodetector focal planes that detect photons and perform signal processing are integrated structures either in monolithic form or in hybrid form. The monolithic arrays range up to 10 6 detectors. The hybrid focal planes, mostly for infrared detection, contain up to 10 5 . Photodetector arrays have been developed for imaging across the spectrum and are used in CCD cameras, chemical spectroscopy, missile seekers, and night vision equipment.
Photodetectors, based on photon absorption or thermal emission, are made from semiconductor materials that are very sensitive in the spectral range from ultraviolet to far infrared. Detector figures of merit such as responsivity, noise, noise equivalent power and detectivity, and ideal performance and cooling requirements are given. Modes of operation include charge‐coupled device, photovoltaic diode, photoconductor, and bolometer. Most semiconductor detectors operate as photon detectors where the minimum photon energy detected is defined by a discrete activation energy of the semiconductor. However micromachined bolometers are in development for thermal imaging and chemical spectroscopy. Materials and detector types are discussed. Detector material preparation technologies are presented for intrinsic detectors where the photon absorption coefficient is high, and for the low absorption coefficient detectors such as impurity‐doped Ge or Si. Fabrication technology making possible large photodetector focal planes that detect photons and perform signal processing are integrated structures either in monolithic form or in hybrid form. The monolithic arrays range up to 10 6 detectors. The hybrid focal planes, mostly for infrared detection, contain up to 10 5 . Photodetector arrays have been developed for imaging across the spectrum and are used in CCD cameras, chemical spectroscopy, missile seekers, and night vision equipment.
Photons impinging upon matter interact in a manner determined by the nature of the chemical bonds in the material and the energy of the incident photons. Interacting photons may be reflected, refracted, diffracted, transmitted, or absorbed. Each of these phenomena can be used to measure some parameter of interest in chemical analysis. Photo‐assisted chemical analytical techniques require an accurate, sensitive method of photon detection and quantification. Photographic film proved to be the first useful method for such evaluations. Later, photomultiplier tubes provided the means to improve many photo‐analytical techniques. With the advent of semiconductor technology, numerous single‐element, broad‐spectrum photodetecting devices were developed and applied to the task. Most recently, photodetector technology has entered the age of high density integration. Large‐area photon detectors are becoming commonplace in most analytical equipment. The advances afforded by these devices have allowed for significant improvements in the performance of systems designed for photo‐analytical chemical analysis in general, and specifically for spectroscopic‐based equipment. A working knowledge of the operation and limitations of photodetectors is therefore necessary for the modern chemist. Photodetector devices convert electromagnetic radiation or photons to electric signals which can be processed to obtain the spectral, spatial, and temporal information inherent in the radiation. Photodetectors, may be operated in many modes. The more popular ones are photoconductors, photodiodes, charge‐transfer devices, and pyroelectrics. The detectors may be used as single elements such as in street light controls, film camera exposure control, or motion detectors for security. Photodetectors also find application in the form of linear arrays used in analytical spectrometers, night‐vision equipment, in small, low cost spectrometers for the control of building ventilation and environmental pollution monitoring or configured as large matrix arrays in video cameras. This chapter includes the following topics: Principles; figures of Merit; photodetector modes of operation; detector fabrication and performance; and health and safety factors.
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