Investigation of the Performance of a Thin-Film Transistor-Driven Linear Image Sensor
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INVESTIGATION OF THE PERFORMANCE OF A THIN-FILM TRANSISTOR-DRIVEN LINEAR IMAGE SENSOR G. BRUNST, S. GRIEP, H. HARMS, K. ROHLEDER, and K. ROSAN Siemens AG, Corporate Research and Development, Otto-Hahn-Ring 6, D-8000 Minchen 83, West Germany
ABSTRACT The performance of a thin-film transistor (TFT) driven linear image sensor is analyzed. By experiment and computer simulation the charge transfer is investigated and the results are applied to interprete the performance of the conventional readout circuit. Its main disadvantage are the large number of crossovers between signal lines. Therefore, an alternative readout circuit with reduced number of crossovers is proposed.
INTRODUCTION Recent progress in computer technology and software initiated a breakthrough of office automation and desktop publishing. Image processing calls for small size and cheap scanners and telecopiers. Here 1:1 imaging and therefore page wide linear image sensors are needed. In the field of largearea electronics amorphous silicon (a-Si:H) proved to be a very promising material. Linear image sensors made in a-Si:H technology are already in production. However, in the state of the art of hybrid readout each individual sensor element must be connected to a readout circuit made in c-Si technology. Thus for an A4 wide image sensor of e.g. 300dpi resolution, 2688 wire bonds to 21 readout chips are needed. In order to increase fabrication yield and reduce cost the integration of sensor elements and switching devices like thin-film transistors (TFTs) in a-Si:H technology is highly requested. In a TFT-driven linear image sensor the number of wire bonds is favourably decreased to about 150, because only one readout chip remains. On the other hand due to the integration a high number of crossovers of signal lines and driving lines are introduced. Ito et al. /I/ found that these crossovers cause severe problems: signal reduces, crosstalk increases, and switching noise arises due to the increased capacitive coupling of lines. Therefore, a special LSI chip with signal amplification and noise cancellation is necessary /2/. But, the performance of a TFT driven linear image sensor is still limited. A detailed investigation helps to optimize sensor performance by finding the best TFT geometry and readout circuit. In this paper the signal transfer from a photodiode through a TFT to the readout circuit is investigated by experiment and computer simulation and a new readout circuit is proposed.
THIN-FILM TRANSISTOR The investigated TFT is of the inverted staggered structure /3/. aSiNx :H, a-Si:H and n +-a-Si:H are successively plasma deposited in one vacuum pumpdown. Channel length 1 of the TFT is 10pm, channel width w is 100pm. In Fig. 1 the transfer characteristics and in Fig. 2 the output characteristics are shown. The threshold voltage is typically 5V, field effect 2 mobility 0.5cm /Vs, and off-current about pA. The on-resistance of the TFT at a gate voltage of 15V is 4MOhm, which determines the time constant of the signal transfer. The internal switching time
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