A new A4-size contact-type image sensor with 8 elements per millimeter (1,728 elements) was developed by using amorphous silicon alloy pin photodiodes as the light receiving elements. The matrix switching method is employed for signal reading. This method can read the signals from all of the 1,728 photodiodes using a 54 (54-channel input) x 32 (32.channel output) matrix switching scheme. It is advantageous in that the required input and output external circuits can be simplified. To prevent signal crosstalk between the diodes in the matrix, amorphous silicon alloy pin diodes, which are made in the same process as the photodiodes for maintaining a low cost, are connected in series to each photodiode in reverse polarity as blocking diodes.
To achieve superior signal-to-noise ratio and response at the same time, amorphous silicon alloy pin diodes are optimized to satisfy both a high photo-to-dark ratio and a high rectification ratio. The image sensor can operate at up to 2 MHz signal reading clock, maintaining a signal-to-noise ratio of over 24 dB under 0. 1–1x sec exposure.