High-current large-bandwidth photosensor on standard CMOS processes

Antoine Dupret; Eric Belhaire; Jean-Claude Rodier

doi:10.1117/12.262540

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19 December 1996 High-current large-bandwidth photosensor on standard CMOS processes

Antoine Dupret, Eric Belhaire, Jean-Claude Rodier

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Proceedings Volume 2950, Advanced Focal Plane Arrays and Electronic Cameras; (1996) https://doi.org/10.1117/12.262540
Event: Advanced Imaging and Network Technologies, 1996, Berlin, Germany

Abstract

On standard CMOS processes, basically two photosensors may be designed: photodiodes or vertical bipolar phototransistors. A trade-off must be found between the area of the sensor, its sensitivity and its bandwidth. In most designs, the high sensitivity of the sensor is a key point and led to choosing a phototransistor based solution. However this choice is made at the expense of the bandwidth of the sensor. For small currents, an analysis shows that it is mainly proportional to the base-emitter capacitance C_be and to the collector current. Hence, in the case of a floating base bipolar and for a given current, the only way of reducing C_be is to decrease the emitter area. On the other hand, the sensitivity is to be preserved. We have proposed and tested an original sensor based on the splitting of phototransistors. The basic idea is to use minimum size emitter bipolar transistors and to increase their collector-base junction perimeter. Thanks to this design, for a given sensor area, the bandwidth has been improved by a factor of 3 and the sensitivity has been preserved. This solution has been successfully used on an operational retina performing stochastic computations at video rates. In particular, thanks to our design, we have been able to successfully implement a 150 by 50 micrometer² optoelectronic random generator providing up to 100,000 random variables per second.

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Antoine Dupret, Eric Belhaire, and Jean-Claude Rodier "High-current large-bandwidth photosensor on standard CMOS processes", Proc. SPIE 2950, Advanced Focal Plane Arrays and Electronic Cameras, (19 December 1996); https://doi.org/10.1117/12.262540

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