Photon effects photoconductive

There are a number of excellent references which will enable the reader to obtain an overview of infrared and optical detector research and development. Among the books published in the 1960s which contain much basic information still of value are those by Smith et al. [2.1], Holier et al. [2.2], Kruse et al. [2.3], Jamieson et al. [2.4], Wolfe [2.5], Conn and Avery [2.6], and Hudson [2.7], The theoretical basis for photoconductivity and the other photon effects can be found in Ryvkin [2.8], Bube [2.9], Pell [2.10], and an issue of Applied Optics devoted to photon effects [2.11]. Imaging devices are well covered in the two volumes by Biherman and Nudelman [2.12,13] and in six volumes in the series Advances in Electronics and Electron Physics [2.14]. Of more recent interest is Moss et al. [2.15], Willardson and Beer [2.16], Materials for Radiation Detection [2.17], Dimmock [2.18], Seib and Aukerman [2.19], Emmons et al. [2.20], Anderson and McMurtry [2.21], Melchior et al. [2.22], an issue of the Proceedings of the IEEE devoted to remote sensing [2.23], and a symposium on unconventional infrared detectors [2.24]. 

Although the list of photon effects in Table 2.1 is extensive, only the photoconductive, photovoltaic, and photoemissive ones have been widely... 

The second photon effect of general utility is the photovoltaic effect. Unlike the photoconductive effect, it requires an internal potential barrier with a built-in electric field to separate a photoexcited hole-electron pair. Although it is possible to have an extrinsic photovoltaic effect, see Ryvkin [2.32], almost all practical photovoltaic detectors employ the intrinsic photoeffect. Usually this occurs at a simple p — n junction. However, other structures employed include those of an avalanche, p—i — n, Schottky barrier and heterojunction photodiode. There is also a photovoltaic effect occuring in the bulk. Each will be discussed, with emphasis on the p—n junction photoeffect. 

The lead compounds PbS, PbSe, PbTe are narrow-gap semiconductors that have been widely investigated for infrared detectors, diode lasers, and thermo-photovoltaic energy converters. Their photoconductive effect has been utilized in photoelectric cells, e.g., PbS in photographic exposure meters. Integrated photonic devices have been fabricated by their heteroepitaxial growth on Si or III-V semiconductors. 

The difference between a photoconductive detector and a photodiode detector lies in the presence of a thin p-doped layer at the surface of the detector element, above the bulk n-type semiconductor. Holes accumulate in the p-layer, and electrons in the n-type bulk, so between the two there is a region with a reduced number density of carriers, known as the depletion layer. The important effect of this is that electron-hole pairs, generated by photon absorption within this depletion layer, are subjected to an internal electric field (without the application of an external bias voltage) and are automatically swept to the p and n regions, and... 

QUANTUM EFFICIENCY. A measure of the efficiency of conversion or utilization of light or other energy, being in general the ratio of the number of distinct events produced in a radiation sensitized process to the number of quanta absorbed (the intensity-distribution of the radiation in frequency or wavelength should be specified). In the photoelectric and photoconductive effects, the quantum efficiency is the number of electronic charges released for each photon absorbed. For a phototube, the quantum... 

All photoeffects involve the absorption of photons to produce an excited state in the absorber or liberate electrons directly. With the direct release of electrons, photoemission may occur from the surface of solids. While the excited state may revert to the ground state, it may proceed further to a photochemical reaction to provide an electron-hole pair (exciton) as the primary photoproduct. The exciton may dissociate into at least one free carrier, the other generally remaining localized. In an externally applied electric field, photoconduction occurs. Photomagnetic effects arise in a magnetic field. Absorption of photons yield photoelectric action spectra which resemble optical absorption spectra. Photoeffects are involved in many biological systems in which charge transfer takes place (e.g., as observed in the chlorophylls and carotenoids) [14]. 

The phenomenon of excitonlc energy transport in polymer films has been studied actively for the past decade (2 ). This photophysl-cal process is relevant to photodegradation and photoconductivity in polymers, but in this contribution we wish to emphasize the potential application of polymer films as photon "harvesters" with subsequent transfer of energy to a reaction center, analogous to the so-called "antenna effect" in chloroplasts. 

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