Application to a Long Linear Image Sensor

A contact linear image sensor can make the equipment compact, because it does not require a high-magnification lens system. The sensor unit consists of an illuminator, a compact optical guide, and the long linear photosensor array, which is, for example, 210 mm long for the iso-A4 

A photoconductive array or a photodiode array can be used for this kind of sensor. 

In the photoconductive sensor, the electrode is ohmic where carrier replenishment occurs. Electron photoconductivity is the dominant mode. In this case, the photocurrent is a secondary current. Photocurrent and photoconductive gain are given by (Bube, 1961) 

The photocurrent for a unit element under 100-lux illumination was 0.1 A at 10 V applied voltage when undoped a-Si H was used (Motosugi el al., 1981). This value is large enough for practical use as the linear sensor, while it is one order of magnitude smaller than the CdS-CdSe photoconductive sensor, which has a simple gap geometry. (Komiya et al., 1981). 

In the photodiode array, photosensitive a-Si H is sandwiched between a transparent electrode and a metal electrode. Indium tin oxide (ITO) is commonly used for the transparent electrode. 

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