Photodetectors

Abstract

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.