An Analytical Model for the Spectral Response and Terminal Characteristics of CMOS Integrated Silicon Phototransistors

M.G. Guvench (USA)


– CMOS, photodiodes, spectral response, integrated photosensors.


An analytical model for the circuit equivalent of CMOS integrated photodiodes (or phototransistors) is being presented. The model takes into account the second P-N junction created between the N Well and the Substrate, and the transistor action taking place in between the two P-N junctions. The resulting circuit equivalent is shown to behave like an Ebers-Moll model of a bipolar transistor with two additional photocurrent components. The model calculates Ebers Moll parameters and the added photocurrent components, and determines the spectral response of them. Therefore, it can be used as a SPICE simulation/verification tool in the design of CMOS image sensors, photodiode arrays and peripheral circuitry. The substrate photocurrent is shown to have spectral characteristics distinct from its photodiode and can be amplified with unexpected results in design due to strong transistor action built in the structure. The distinct spectral photo-responses of the two junctions , however, can be taken advantage of to discriminate color within a single pixel for high density image sensing.

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