engleski

Color Detection Using a Capacitance of np Silicon Photodiode

This paper describes color detection using a capacitance of np silicon photodiode, which is part of a standard CMOS technological process. When a voltage step is apllied to a photodiode, the incremental charge distribution in the device is separated into positive and negative components, which are assigned to the respective contacts. This instantaneous change of charges is equalto the dark current and photocurrent. It corresponds both to the depletion capacitance charge of np junction and to the diffusion capacitance charge. Hence, since photogenerated charge within the photodiode structure is dependent on the wavelenght of absorbed light, the photodiode capacitance is also wavelenght dependent. Emphasizing the physical mechanism, the capacitance behavior observed in a one-junction Si photodiode is analized using one-dimensional numeric modeling. The interpretation of a color detection was based on the analysis of the transient current in response to a small voltage step at constant illumination. The analysis include quasi-neutral charge density and space-charge charge density components. Different transient current response (charge and discharge) times to a small voltage step can be ascribed to a light absorption. Using Fourier analysis method photodiode response on small voltage signal dependent on light wavelenght, can be translated from time domain to frequency domain. This enables use of np photodiode in color detection.

color detection; capacitance; photodiode

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