CMOS imaging array with improved noise characteristics
A pixel cell and imaging arrays using the same are disclosed. The pixel cell includes a photodiode that is connected to a floating diffusion node by a transfer gate that couples the photodiode to the floating diffusion node in response to a first gate signal. A shielding electrode shields the floating diffusion node from the first gate signal. An output stage generates a signal related to a charge on the floating diffusion node. In one aspect of the invention, the photodiode is connected to the floating diffusion node by a buried channel, and the shielding electrode includes an electrode overlying the channel and positioned between the transfer gate and the floating diffusion node. The shielding electrode is held at a potential that prevents charge from accumulating under the shielding electrode when the floating diffusion is at the second potential.
1. A pixel cell comprising:
a photodiode;
a floating diffusion node;
a transfer gate that couples said photodiode to said floating diffusion node in response to a first gate signal;
a shielding electrode that shields said floating diffusion node from said first gate signal; and
an output stage that generates a signal related to a charge on said floating diffusion node, said shielding electrode being held at a constant potential while said transfer gate couples said photodiode to said floating diffusion node.
2. The pixel cell of claim 1 wherein said photodiode is connected to said floating diffusion node by a buried channel and wherein said shielding electrode comprises an electrode overlying said channel and positioned between said transfer gate and said floating diffusion node.
3. The pixel cell of claim 2 wherein said floating diffusion node operates between first and second potentials, said first potential being greater than said second potential, said potential on said floating diffusion node being dependent on charge transferred from said photodiode, and wherein said shielding electrode is held at a potential that prevents charge from accumulating under said shielding electrode when said floating diffusion is at said second potential.
4. The pixel cell of claim 1 wherein said output stage comprises a capacitive transimpedance amplifier having a gain greater than one.
5. The pixel cell of claim 1 wherein said photodiode is a pinned photodiode.
6. An image sensor comprising:
a two-dimensional array of CMOS pixel cells organized as a plurality of rows and columns, said pixel sensors in each column being connected to a corresponding bit bus, and each of said pixel sensors in each row being connected to a corresponding row select line, each pixel cell comprising:
a photodiode;
a floating diffusion node;
a transfer gate that couples said photodiode to said floating diffusion node in response to a first gate signal;
a shielding electrode that shields said floating diffusion node from said first gate signal, said shielding electrode being held at a constant potential while said transfer gate couples sais photodiode to said floating diffusion node; and
an output stage that generates an output signal related to a charge on said floating diffusion node and couples that output signal to said corresponding bit bus in response to a row select signal on said row select line corresponding to that pixel cell; and
a row decoder that generates said row select signal on a selected one of said row select lines in response to a row address being coupled thereto.
7. The image sensor of claim 6 wherein said photodiode is connected to said floating diffusion node by a buried channel and wherein said shielding electrode comprises an electrode overlying said channel and positioned between said transfer gate and said floating diffusion node.
8. The image sensor of claim 7 wherein said floating diffusion node operates between first and second potentials, said first potential being greater than said second potential, said potential on said floating diffusion node being dependent on charge transferred from said photodiode, and wherein said shielding electrode is held at potential that prevents charge from accumulating under said shielding electrode when said floating diffusion is at said second potential.
9. The image sensor of claim 6 wherein said output stage comprises a capacitive transimpedance amplifier having a gain greater than one.
10. The image sensor of claim 6 wherein said photodiode is a pinned photodiode.
11. The image sensor of claim 9 further comprising:
a two-dimensional array of CMOS pixel cells organized as a plurality of rows and columns, said pixel sensors in each column being connected to a corresponding bit bus, and each of said pixel sensors in each row being connected to a corresponding row select line, each pixel cell comprising:
a photodiode;
a floating diffusion node;
a transfer gate that couples said photodiode to said floating diffusion node in response to a first gate signal;
a shielding electrode that shields said floating diffusion node from said first gate signal;
an output stage that generates an output signal related to a charge on said floating diffusion node and couples that output signal to said corresponding bit bus in response to a row select signal on said row select line corresponding to that pixel cell; and
a row decoder that generates said row select signal on a selected one of said row select lines in response to a row address being coupled thereto wherein said output stage comprises a capacitive transimpedance amplifier having a gain greater than one, said image sensor
further comprising a column amplifier connected to one of said bit buses, said column amplifier having a gain of substantially equal to said transimpedance amplifier.