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 applied 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 equal to the dark current and photocurrent. It corresponds both to the depletion capacitance charge of np junction and to the diffusion capacitance charge. Hence, since the photogenerated charge within the photodiode structure is dependent on the wavelength of absorbed light, the photodiode capacitance is also wavelength dependent. Emphasizing the physical mechanism, the capacitance behavior observed in a one-junction Si photodiode is analyzed using one-dimensional numerical 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 included 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 light absorption. Using Fourier analysis dependent on light wavelength, can be translated from time domain to frequency domain. This enables use of np photodiode in colour detection.