With clinical installations now more than five years old and the number of installations rapidly approaching 1000, amorphous-silicon x-ray imagers can finally claim to have entered the diagnostic imaging mainstream. However, the trip to the mainstream has not been easy and it is surely not yet complete. Amorphous silicon was proposed as a useful electronic material in the 1960s, originally as a basis for low-cost solar cells. Over the following 20 years, various researchers learned how to make photodiodes, photoconductors and, finally, thin-film transistors (TFTs) with performance sufficient to support commercial electronic applications. As the materials technology progressed, so did the fabrication capabilities, producing in the late 1970s several varieties of two-dimensional electronic arrays including the TFT array that underpins the active-matrix LCD now found in almost every laptop computer. Amorphous-silicon arrays became interesting as imagers because of their potentially low manufacturing cost, compatibility with deposition over large areas, and appropriate electronic performance. Imaging large areas with conventional crystalline silicon devices requires either bulky coupling optics or the very high cost of tiling numerous small imagers. Thus, applications requiring large imaging areas such as contact document scanning or x-ray imaging immediately emerged as potential markets for large amorphous-silicon sensor arrays.
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