Active pixel sensor using cmos technology with reverse biased photodiodes

Abstract

An active pixel sensor implemented with CMOS technology that employs a plurality of photocells, each including a photodiode to sense illumination and a separate storage node with a stored charge that is discharged during an integration period by the photocurrent generated by the photodiode. Each photocell includes a switching network that couples the photocurrent to the storage node only during the integration period while ensuring that a relatively constant voltage is maintained across the photodiode during integration and non-integration periods. The transistors in the switching network operate in a forward active subthreshold region, ensuring linear operation and the diode voltage is clamped to a small positive voltage so that the diode is always reverse-biased. A source-follower generates a output signal correlated to the charge on the storage node that is coupled to column output circuitry that samples the signal. An operational scheme is employed wherein the storage node is first set to a defined voltage, the photocurrent is allowed to discharge the storage node and then the remaining charge coupled as a first signal to the column output circuitry, which samples and stores the first signal. The storage node is then reset to the same defined voltage and the resulting charge on the storage node is coupled as a second signal to the column output circuitry. The column output circuitry computes the difference of the first and second signals, which provides a reliable measure of the photocurrent during the integration period.