Method and apparatus for biasing a CMOS active pixel sensor above the nominal voltage maximums for an IC process
First Claim
1. A method for operating an active pixel sensor having a reset transistor, a transfer transistor, and a readout transistor fabricated according to a process having a nominal operating voltage comprising the steps of:
- applying a voltage having a value greater than the nominal operating voltage to a gate of the reset transistor during a reset period to maximize the dynamic range of the active pixel sensor;
applying a voltage having a value greater than the nominal operating voltage to a gate of the transfer transistor during said reset period;
continuing to apply said voltage having a value greater than the nominal operating voltage to said gate of the transfer transistor during an integration period; and
applying a voltage to said gate of the reset transistor during said integration period that limits an electric field applied across a gate dielectric of the transfer transistor during said integration period.
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Abstract
An active pixel sensor is operated with voltages that exceed the nominal operating voltages for a particular integrated circuit process. Voltages that exceed the nominal operating voltages are employed during the reset, integration and readout periods in the operating cycle of the active pixel sensor. The lower limit of the voltage representing the capture of photocharge in the active pixel sensor is fixed by setting the voltage applied to the gate of a reset transistor in the active pixel sensor to a level during integration which prevents the voltage across the dielectric of a transfer transistor from exceeding a preselected value. Read disturb caused by impact ionization current from a readout transistor to a storage node is reduced by lowering the voltage applied to the drain of a readout transistor to reduce the Vds of the readout transistor at the start of a readout period.
128 Citations
12 Claims
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1. A method for operating an active pixel sensor having a reset transistor, a transfer transistor, and a readout transistor fabricated according to a process having a nominal operating voltage comprising the steps of:
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applying a voltage having a value greater than the nominal operating voltage to a gate of the reset transistor during a reset period to maximize the dynamic range of the active pixel sensor;
applying a voltage having a value greater than the nominal operating voltage to a gate of the transfer transistor during said reset period;
continuing to apply said voltage having a value greater than the nominal operating voltage to said gate of the transfer transistor during an integration period; and
applying a voltage to said gate of the reset transistor during said integration period that limits an electric field applied across a gate dielectric of the transfer transistor during said integration period. - View Dependent Claims (2, 3, 4, 5, 6)
applying a first voltage level to a drain of said readout transistor during said reset period and integration period; and
applying a second voltage level lower than said first voltage level to said drain of said readout transistor during a readout period to control the impact ionization current generated in said active pixel sensor during said readout period.
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3. A method for operating an active pixel sensor as in claim 1, further including the steps of:
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applying a first voltage level to a drain of said reset transistor; and
applying a second voltage level lower than said first voltage level to said drain of said readout transistor to control the impact ionization current generated in said active pixel sensor.
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4. A method for operating an active pixel sensor as in claim 1, further including the steps of:
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applying a first voltage level to a drain of said reset transistor and a drain of said readout transistor during said reset period and said integration period; and
applying a second voltage level lower than said first voltage level to said drain of said reset transistor and said drain of said readout transistor during a readout period to control the impact ionization current generated in said active pixel sensor during said readout period.
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5. A method for operating an active pixel sensor as in claim 1, further including the steps of:
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applying a first voltage level to a drain of said readout transistor during said reset period and integration period; and
applying a second voltage level higher than said first voltage level to said drain of said readout transistor during a readout period, said second voltage level set to control the impact ionization current generated in said active pixel sensor during said readout period.
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6. A method for operating an active pixel sensor as in claim 5, further including the steps of:
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applying a third voltage level to a drain of said reset transistor during said reset period and integration period; and
applying a fourth voltage level higher than said third voltage level to said drain of said reset transistor during a readout period, said fourth voltage level set to control photo generated charge during said readout period.
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7. An active pixel sensor fabricated according to a process having a nominal operating voltage comprising:
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a transfer transistor having a gate coupled to a voltage having a value greater than the nominal operating voltage during a reset period and an integration period in an operating cycle of said active pixel sensor; and
a reset transistor having a gate coupled to a voltage having a value greater than the nominal operating voltage during a reset period in the operating cycle of said active pixel sensor to maximize the dynamic range of said active pixel sensor, and to a voltage that limits the electric field applied across a gate dielectric of said transfer transistor during said integration period in said operating cycle of said active pixel sensor. - View Dependent Claims (8, 9, 10, 11, 12)
a reset transistor having a drain coupled to a first voltage level during said reset period and said integration period, and to a second voltage level lower than said first voltage level during a readout period; and
a readout transistor having a drain coupled to said first voltage level during said reset period and said integration period, and said second voltage level lower than said first voltage level during a readout period to control the impact ionization current generated in said active pixel sensor during said readout period.
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11. An active pixel sensor as in claim 7 wherein a drain of said readout transistor is coupled to a first voltage level during said reset period and said integration period of said operating cycle of said active pixel sensor, and coupled to a second voltage level higher than said first voltage level during a readout period in said operating cycle of said active pixel sensor, said second voltage level set to control the impact ionization current in said active pixel sensor during said readout period.
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12. An active pixel sensor as in claim 10 wherein a drain of said reset transistor is coupled to a third voltage level during said reset period and said integration period of said operating cycle of said active pixel sensor, and coupled to a fourth voltage level higher than said third voltage level during a readout period in said operating cycle of said active pixel sensor, said fourth voltage level set to control photo generated charge during said readout period.
Specification