Modeling and Optimization of a Silicon Photosensor for a Reading Aid

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Department of Electrical Engineering, Stanford University., 1967 - Blind, Apparatus for the - 182 pages
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In the development of a reading aid for the blind, experimental evidence pointed out that the sensing device used should have a spectral response that matches that of the human eye. The conclusions presented in this paper are the result of a research effort to achieve these spectral objectives in silicon planar photodiode and phototransistor structures. It is demonstrated theoretically that for a given set of base-region recombinative properties, there is an optimum base-collector junction depth for achieving the desired visible region sensitivity. In addition, by truncating the collector region with an epitaxial collector-substrate junction, the desired near-infrared insensitivity can be achieved. For the recombinative properties typical of the experimental planar devices fabricated in the Integrated Circuits Laboratory at Stanford University, it is theoretically shown and experimentally verified that the optimum base-collector and epitaxial junction depths are nominally 2 microns and 8 microns, respectively. It is also shown that for the impurity concentrations typically employed in the fabrication of epitaxial silicon photodiodes, the effects of junction voltage are negligible. However, if a typical collector and substrate dopings are used, a significant variation of device quantum efficiency with voltage can be effected. Finally, a steady state lumped model using Linvill-lumped elements, but not requiring any binding assumptions regarding sample geometry, is derived for the p- and n-regions of the planar photodiodes. The utility of the model is demonstrated by showing the correlation between the photodiode quantum efficiency predicted by the model and that predicted by the general theory. (Author).

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Contents

INTRODUCTION
1
ABSORPTION AND QUANTUM EFFICIENCY
9
EFFECT OF JUNCTION VOLTAGE ON QUANTUM EFFICIENCY
66
MODELING OF THE PH0T0DI0DE
84
EXPERIMENTAL RESULTS
110
and S
117
CONCLUSIONS
132
APPENDIX B NUMERICAL ANALYSES
141

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