a-Si:H TFT Driven Linear Image Sensor

Amorphous Silicon Image Sensor Arrays

We have developed a technology for 2D matrix-addressed image sensors using amorphous silicon photodiodes and thin film transistors. We have built a small prototype, having 192×192 pixels with a 20μm pixel pitch, and assessed its performance. The nip photodiodes can have dark current densities of less than 1011 A.cm-2 (up to 5V reverse bias) and peak quantum efficiencies of 88% (at 580nm). We operated the sensor in real time mode at high speed (50 Hz frame rate and 64μS line time). The image sensor has a low noise performance giving a dynamic range in excess of 104. The maximum crosstalk is about 2%, which allows at least 50 grey levels. The bottom contact of the photodiode acts as a light shield from light through the substrate, which enables the sensor to be operated as an intimate contact image sensor to image a document placed directly on top of the array. In this mode, the CTF was 75% at 2 lp.mm1. Good quality images are demonstrated in both front projection and intimate contact imaging modes.

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

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.