Method and apparatus for providing offset for light detector
First Claim
1. A light imaging system comprising:
- a plurality of row electrodes and column electrodes;
an array of rows and columns of photo detector elements, each said element including a photo diode having an anode and a cathode, and a transistor having first and second power electrodes and a control electrode for controlling current flow between said first and second power electrodes, said control electrode being connected to one of said row electrodes, said first power electrode being connected to one of said column electrodes, said second power electrode being connected to said cathode of said photo diode, and said anode of said photo diode being connected to a negative voltage -Vb ;
means for charging said photo diodes by connecting said column electrodes to a voltage Vc and connecting said row electrodes to a voltage Von which is positive relative to Vc ;
means for exposing said array to photons after said charging means charges said photo diodes;
means for impeding said photo diodes from being recharged, while said exposing means exposes said array to photons and after said exposing means exposes said array to photons, said impeding means including said transistors and including means for connecting said row electrodes, prior to said array being exposed to photons but after said photo diodes are charged, to a voltage which is negative relative to Vc and negative relative to -Vb ;
means for sequentially connecting the rows to the voltage Von, for a predetermined period of time, after said exposing means exposes said array to photons and after said impeding means impedes said photo diodes from being recharged;
means for respectively measuring, at the column electrode for each photo diode in the row connected to Von by said connecting means, the charge required to restore the photo diode to its voltage before it was exposed to photons and after it was charged by said charging means, for a period of time longer than the predetermined period of time so as to provide a settling period;
means for resetting the measuring means during a reset period after the settling period and before the next row is connected to Von ; and
means for providing, on a row other than the row being measured by said measuring means, a negative voltage transition, starting when or before the row being measured is connected to Von and ending during the reset period, so as to provide an offset against charge retention by the transistors in the row being measured.
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Accused Products
Abstract
The invention provides a solid state light imager or x-ray detector including an array of rows and columns of elements, each element including a photo diode, which converts photons to an electrical signal, and a transistor. Each photo diode has a capacitance associated with it. The cathode of the photo diode in each element is connected to the source of the transistor in the element. The amount of charge removed from each photo diode, after exposure to light, is used to create an image. The imager is capable of accurate measurement of charge removed from the photo diodes after the array has been exposed to light, using unipolar measuring circuitry, in spite of charge retention by the transistors and a problem caused by the combination of changes in row voltage and parasitic row to column capacitance.
155 Citations
20 Claims
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1. A light imaging system comprising:
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a plurality of row electrodes and column electrodes; an array of rows and columns of photo detector elements, each said element including a photo diode having an anode and a cathode, and a transistor having first and second power electrodes and a control electrode for controlling current flow between said first and second power electrodes, said control electrode being connected to one of said row electrodes, said first power electrode being connected to one of said column electrodes, said second power electrode being connected to said cathode of said photo diode, and said anode of said photo diode being connected to a negative voltage -Vb ; means for charging said photo diodes by connecting said column electrodes to a voltage Vc and connecting said row electrodes to a voltage Von which is positive relative to Vc ; means for exposing said array to photons after said charging means charges said photo diodes; means for impeding said photo diodes from being recharged, while said exposing means exposes said array to photons and after said exposing means exposes said array to photons, said impeding means including said transistors and including means for connecting said row electrodes, prior to said array being exposed to photons but after said photo diodes are charged, to a voltage which is negative relative to Vc and negative relative to -Vb ; means for sequentially connecting the rows to the voltage Von, for a predetermined period of time, after said exposing means exposes said array to photons and after said impeding means impedes said photo diodes from being recharged; means for respectively measuring, at the column electrode for each photo diode in the row connected to Von by said connecting means, the charge required to restore the photo diode to its voltage before it was exposed to photons and after it was charged by said charging means, for a period of time longer than the predetermined period of time so as to provide a settling period; means for resetting the measuring means during a reset period after the settling period and before the next row is connected to Von ; and means for providing, on a row other than the row being measured by said measuring means, a negative voltage transition, starting when or before the row being measured is connected to Von and ending during the reset period, so as to provide an offset against charge retention by the transistors in the row being measured. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. An x-ray imaging system comprising:
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a plurality of row electrodes and column electrodes; an array of rows and columns of photo detector elements, said array including amorphous silicon, each said element including a photo diode having an anode and a cathode, and transistor means having first and second power electrodes and a control electrode for controlling current flow between said first and second power electrodes, said control electrode being connected to one of said row electrodes, said first power electrode being connected to one of said column electrodes, and said second power electrode being connected to said cathode of said photo diode, and said anode of said photo diode being connected to a negative voltage; means for charging said photo diodes; means for exposing said array to photons after said charging means charges said photo diodes; means for impeding said photo diodes from being recharged, while said exposing means exposes said array to photons and after said exposing means exposes said array to photons, said impeding means including said transistor means; means, operable after said exposing means and said impeding means, for respectively measuring the charge required to restore each photo diode to its voltage before it was exposed to photons and after it was charged by said charging means; and means for providing an offset against charge retention by the transistor means that are connected to the photo diodes being measured while said measuring means measures the charge required to restore at least one of said photo diodes. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19)
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20. A method of reading image data from an array of rows and columns of photo detector elements that have been exposed to photons, each element including a photo diode having an anode and a cathode, and electronic valve means having first and second power electrodes and a control electrode for controlling current flow between the first and second power electrodes, the control electrode being connected to one of the row electrodes, the first power electrode being connected to one of the column electrodes, the second power electrode being connected to the cathode of the photo diode, and the anode of the photo diode being connected to a negative voltage, said method comprising:
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respectively measuring the charge required to restore each photo diode in one of the rows to its voltage before it was exposed to photons; and providing, on a row other than the row being measured, a voltage transition to at least partially offset against charge retention by the transistors in the row being measured.
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Specification