Method and apparatus for achieving uniform low dark current with CMOS photodiodes
First Claim
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1. An apparatus for reducing dark current associated with a CMOS photodiode, the apparatus comprising;
- a reset transistor connected to the photodiode, the reset transistor having a threshold voltage set to a value that allows dark current to be removed during signal integration; and
wherein the reset transistor is a MOSFET, and the threshold voltage is set according to the following equation;
where Vx=nkT/e n is the ideality of a reset MOSFET, Jdark is a dark current density for the photodetector, Adet is a photodetector area., μ
is a MOSFET mobility, Wrst is a width of the reset MOSFET, Lrst is a length of the reset MOSFET, and Cox is a MOSFET capacitance density.
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Abstract
An apparatus and method for achieving uniform low dark currents with CMOS photodiodes. A threshold voltage of a reset FET is set to an appropriate value such that the dark current from a photodiode is actively removed through the reset FET during signal integration. This reduces the dark current by over 3 orders of magnitude as compared to conventional active pixel sensors, without requiring pinned photodiodes.
35 Citations
19 Claims
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1. An apparatus for reducing dark current associated with a CMOS photodiode, the apparatus comprising;
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a reset transistor connected to the photodiode, the reset transistor having a threshold voltage set to a value that allows dark current to be removed during signal integration; and
wherein the reset transistor is a MOSFET, and the threshold voltage is set according to the following equation;
where Vx=nkT/e n is the ideality of a reset MOSFET, Jdark is a dark current density for the photodetector, Adet is a photodetector area., μ
is a MOSFET mobility, Wrst is a width of the reset MOSFET, Lrst is a length of the reset MOSFET, and Cox is a MOSFET capacitance density.- View Dependent Claims (2, 3, 4, 5, 6)
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7. A method for reducing dark current associated with a CMOS photodiode in an active pixel sensor, the method comprising:
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setting, a threshold voltage of a reset transistor to a predetermined value; and
removing the dark current via subthreshold conduction during signal integration, wherein the predetermined value is set according to the following equation;
where Vx=nkT/e n is the ideality of a reset MOSFET, Jdark is a dark current density for the photodetector, Adet is a photodetector area, μ
is a MOSFET mobility, Wrst is a width of the reset MOSFET, Lrst is a length of the reset MOSFET, and Cox is a MOSFET capacitance density.- View Dependent Claims (8, 9, 10)
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11. A CMOS active pixel sensor comprising:
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a photodetector producing a dark current;
an amplifier FET connected to the photodetector;
a reset FET, having a threshold voltage VT, connected to a node formed by the photodiode and the amplifier FET; and
an access FET;
wherein the threshold voltage VT is set to a predetermined value to drain off the dark current through the reset FET during integration and the threshold voltage is set according to the following equation;
where Vx=nkT/e n is the ideality of a reset MOSFET, Jdark is a dark current density for the photodetector, Adet is a photodetector area, μ
is a MOSFET mobility, Wrst is a width of the reset MOSFET, Lrst is a length of the reset MOSFET, and Cox is a MOSFET capacitance density.- View Dependent Claims (12, 13, 14, 15, 16, 17)
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18. A CMOS imager comprising a plurality of active pixel sensors, each active pixel sensor comprising:
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a photodiode producing a dark current;
an amplifier FET connected to the photodiode;
a reset FET, having a threshold voltage VT, connected to a node formed by the photodiode and the amplifier FET; and
an access FET;
wherein the threshold voltage VT is set to a predetermined value such that a subthreshold conduction level of the reset FET is comparable to the photodetector dark current.
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19. A CMOS imager having a plurality of active pixel sensors, each active pixel sensor of the type having a photodiode that produces a dark current, and a reset transistor, the improvement characterized by a predetermined threshold voltage connected to the reset transistor, wherein the value of the predetermined threshold voltage allows the dark current to be removed through the reset transistor during photocurrent integration and where a subthreshold conduction level of the reset FET is comparable to the photodetector dark current.
Specification