Light sensors

Light sensors made from a-Si H are either p-i-n or Schottky barrier structures. Unlike crystalline silicon, a p-n jimction is ineffective without the undoped layer, because of the high defect density in doped a-Si H. Illumination creates photoexcited carriers which move to the junction by diffusion or drift in the built-in potential of the depletion layer and are collected by the junction. A photovoltaic sensor (solar cell) operates without an externally applied voltage and collection of the carriers results from the internal field of the junction. When the sensor is operated with a reverse bias, the charge collection generally increases and the main role of the doped layers is to suppress the dark current. A Schottky device replaces the p-type layer with a metal which provides the built-in potential. 

The relative importance of drift and diffusion is different in a-Si H compared to the crystalline semiconductors. The ratio of the depletion width, fV, and the diffusion length, L, is from Eqs. (9.6) and (8.56)-(8.58) 

The structure of a p-4-n device is shown in Fig. 10.1. The depletion layer width of low defect density undoped a-Si H at zero bias is about 1 pm, but is less than 100 A in heavily doped material (see Fig. 9.9). The p and n layers provide the built-in potential of the junction but contribute virtually nothing to the collection of carriers. Therefore the doped layers need be no more than the width of the depletion layer to establish the junction-any additional thickness unnecessarily reduces the charge collection by absorbing incident light. An efficient sensor usually requires that the undoped layer be as thick as possible to absorb the maximum flux of photons, but it cannot be thicker than 

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A selection of applications is presented in the following subsections solar cells, TFTs, light sensors (visible, IR, X-ray), and chemical sensors. Also, light-emitting devices, in particular utilizing erbium incorporation in a-Si H, are presented. Finally, electrostatic loudspeakers in which an a-Si H film is incorporated are described. Details of various applications described here, as well as many other applications, can be found in the excellent edited books [4, 5, 11,13, 574]. 

The prototype family A receptor, rhodopsin itself, clearly functions as a monomer despite its occurrence in very high density in the light-sensitive membranes however, it could be argued that rhodopsin should not be used as an example for receptors in general as it has very special requirements in respect to signaling due to its function as an ultra-rapid light sensor. 

Halavaty, A. S. and K. Moffat (2007). N- and C-terminal flanking regions modulate light-induced signal transduction in the LOV2 domain of the blue-light sensor photo tropin 1 from Avena sativa. Biochemistry 46 14001-14009. 

Schlain L., Spar S., Continuous arterial blood gas monitoring with transmitted light sensors and LED light sources, Proc. SPIE 2131 452 (1994). 

Photomultiplier tubes or photodiodes (light sensors) are used as detectors in UV-VIS spectrophotometers, while thermcouples (heat sensors) are used as detectors for infrared (IR) spectrometry. This is the reason UV-VIS instruments are called spectrophotometers while IR instrument are called spectrometers. 

A photomultiplier tube is a light sensor combined with a signal amplifier. The light emerging from the sample compartment strikes the photosensitive surface and the resulting electrical signal is amplified. 

The sample compartment must consist of a light-tight box so that no stray light—only light from the instrument s light source—reaches the light sensor. 

The basic instrumentation in the present work is a Royco Model 225/518 High Concentration Particle Counter. The location of the air inlet and light sensing unit of the instrument in the card room has been described previously (2). The inlet was fitted with a vertical elutriator preseparator designed to prevent particles >15 vin aerodynamic diameter from entering the light sensor. Thus the collection efficiency of this instrumentation as a function of particle size should be similar to that of the Vertical Elutriator Cotton Dust Sampler. 

Illumination creates excess electrons and holes which populate the extended and localized states at the band edges and give rise to photoconductivity. The ability to sustain a large excess mobile carrier concentration is crucial for efficient solar cells and light sensors and depends on the carriers having a long recombination lifetime. The carrier lifetime is a sensitive function of the density and distribution of localized gap states, so that the study of recombination in a-Si H gives much information about the nature of the gap states as well as about the recombination mechanisms. 

The light sensor rhodopsin is a heptahelical receptor with a built-in chromophore. Thanks to spectral changes in rhodopsin on illuminadon, discrete, sequential steps in the activation process could be distinguished. Since the relation of different activity states to their structural and molecular conformers, and finally to their functional states, is an issue that applies to all receptors, the information obtained with rhodopsin is considered in some detail (see ref. 57). 

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