Pixel-grouping image sensor
The invention relates to matrix image sensors organized into pixel rows and columns, and more specifically to image sensors produced with active pixels in MOS technology. The matrix is organized into groups of at least two pixels with means for grouping the charges engendered in the two pixels into one pixel, with the aim of improving sensitivity. Provision is made for at least one gate for temporarily storing charges, of dissymmetric form, arranged between the photodiode of the first pixel and the photodiode of the second pixel, and means for applying to the temporary storage gate a succession of potentials that allow prohibition first of all of the passage of charges between the first and second photodiodes during the charge integration period, then collection, under the gate, of the charges accumulated in the photodiodes, then discharge of these charges only into the second photodiode, on account of the dissymmetry of the gate. The charges grouped in this way in a single photodiode are read in the pixel.
1. An image sensor having a matrix of pixel rows and columns, each pixel having a photodiode and a node for storing charges in order to collect in said node, at the end of a charge integration period, charges accumulated in the photodiode, the matrix being organized into groups of at least two pixels with means for grouping into one pixel charges generated in the two pixels, wherein a group of pixels has at least one temporary storage gate for temporarily storing charges, said at least one gate exhibiting dissymmetry between an upstream side and a downstream side and being located between the photodiode of the first pixel and the photodiode of the second pixel, and means for applying to the temporary storage gate a succession of potentials that are:
a low potential prohibiting a passage of charges between the first and second photodiodes during the charge integration period,
then a high potential temporary storage control signal permitting a pouring towards the temporary storage gate of charges accumulated in the two photodiodes,
then a low potential allowing a pouring towards the first photodiode only of charges stored under the temporary storage gate.
2. The image sensor as claimed in claim 1 , wherein the photodiodes are produced in an active semiconductor layer of a first conductivity type and have a diffusion of a second conductivity type covered by a superficial layer of the first type connected to a reference potential for the active layer, and in wherein the temporary storage gate is arranged above an active layer portion of the first type, and is:
adjacent to the photodiode of the first pixel by an end of at least one narrow finger extending towards the photodiode of the first pixel,
adjacent to the photodiode (PD 2 ) of the second pixel by a side having a width larger than a width of the narrow finger,
and, with the exception of the ends of the narrow fingers, separated from the photodiode of the first pixel by a doped insulation region of the first conductivity type, which is more doped than the superficial regions of the photodiodes, and which is brought to the potential of the active layer.
3. The image sensor as claimed in claim 2 , wherein the active layer portion situated under the gate has a plurality of narrow fingers extending toward the photodiode of the first pixel and having an end adjacent to this photodiode.
4. The image sensor as claimed in claim 1 , wherein the matrix is organized into groups of four pixels, a first pixel and a second pixel in a group of four being two adjacent pixels in a first row of pixels and a third and a fourth pixels in the group being two adjacent pixels in another row, adjacent to the first, and at least three dissymmetric temporary storage gates being provided in order to permit charges on the photodiodes of the four pixels to be grouped into the photodiode of the fourth pixel.
5. The image sensor as claimed in claim 4 , wherein a first temporary storage gate is provided between the first and second pixels, a second gate is provided between the second and third pixels, and a third gate is provided between the third and fourth pixels, and means are provided in order to control the gates in succession, a storage control signal for the second gate being applied to the latter only after the end of a storage control signal applied to the first gate, and a storage control signal for the third gate being applied to the third gate only after the end of the storage control signal applied to the second gate.
6. The image sensor as claimed in claim 4 , wherein a first temporary storage gate is provided between the first and fourth pixels, a second gate is provided between the second and third pixels, and a third gate is provided between the third and fourth pixels, and means are provided in order to control the gates in two successive steps, the storage control signal for the first gate being applied to the first gate at the same time as the storage control signal for the second gate, and the storage control signal for the third gate being applied to the third gate only after the end of the storage control signal applied to the first and second gates.
7. The image sensor as claimed in claim 1 , wherein the matrix is organized into groups of n×m pixels, where (n+m)>=2 and has (nxm−1) temporary storage gates between photodiodes of adjacent pixels in order to provide for the grouping of charges from a group of n×m pixels.
8. The image sensor as claimed in claim 7 , wherein means are provided for setting up n+m−2 control signals for controlling the temporary storage gates of the matrix.