High Performance Input Scanning Arrays Using Amorphous Silicon Photodiodesand Thin-Film Transistors

Abstract

The use of large area hydrogenated amorphous silicon (a-Si:H) technology has enabled compact, full page width scanners to be built inexpensively, and is now the dominant method for fabricating low-end facsimile machines. This technology has now been extended to scanners with considerably higher levels of performance. High speed, high resolution, full-width input scanning arrays have been developed using a-Si:H photodiodes and thin-film transistors (TFTs). A 12″ long array has been designed to scan 3 colors at 400 spots per inch, and operates at speeds of up to 40 pages per minute, achieving a signal/noise ratio of 400:1 at intensities of 30 μWcm-₂. The color scan array is made using 3 rows of a-Si:H photodiodes, one per color, addressed by TFTs which share sets of common data lines. The data lines are arranged in a low capacitance non-crossing configuration which allows the scanner to achieve high responsivity with low crosstalk. The data lines are connected to a number of readout chips, each of which amplifies and multiplexes the photosignals onto a single video output line. Optoelectronic test results and images obtained from this device will be presented. These results indicate that high quality color images can be obtained from a-Si:H scanners, and that the present scanner is more limited by the speed of the readout chips than by the a-Si:H devices themselves.