Active pixel sensor using CMOS technology with reverse biased photodiodes
First Claim
1. An active pixel sensor using CMOS technology, comprising:
- a plurality of photocells, each configured to sense illumination falling on a respective region of the active pixel sensor and to generate an output signal corresponding to the sensed illumination, wherein each of the photocells includes;
a reverse-biased photodiode that generates a photocurrent that is proportional to the illumination falling on the photocell; and
a storage device coupled to and distinct from the photodiode with a stored charge that is discharged during an integration period as a function of the photocurrent, the amount of the stored charge discharged during the integration period determining the output signal.
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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.
187 Citations
38 Claims
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1. An active pixel sensor using CMOS technology, comprising:
a plurality of photocells, each configured to sense illumination falling on a respective region of the active pixel sensor and to generate an output signal corresponding to the sensed illumination, wherein each of the photocells includes; a reverse-biased photodiode that generates a photocurrent that is proportional to the illumination falling on the photocell; and a storage device coupled to and distinct from the photodiode with a stored charge that is discharged during an integration period as a function of the photocurrent, the amount of the stored charge discharged during the integration period determining the output signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27)
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28. An active pixel sensor method for use in each of a plurality of photocells, each configured to sense illumination falling on a respective region of an active pixel sensor and to generate an output signal corresponding to the sensed illumination, the method comprising the steps of:
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storing a charge on a storage device within the photocell; generating a photocurrent that is proportional to the illumination falling on the photocell using a photodiode within the photocell, the photodiode being distinct from the storage device; and discharging the storage node during an integration period as a function of the photocurrent, the amount of the stored charge discharged during the integration period determining the output signal. - View Dependent Claims (29, 30, 31, 32, 33, 34, 35, 36, 37, 38)
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Specification