Small pixel for image sensors with JFET and vertically integrated reset diode
AssigneeIntellectual Ventures II LLC
InventorJaroslav Hynecek
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Abstract
A pixel and a pixel array of an image sensor device of the present invention have small pixel sizes by resetting sensed charge using a diode built vertically above a substrate. The pixel and the pixel array also have low noise performance by using a JFET as a source follower transistor for sensing charge. The pixel includes a floating diffusion node configured to sense photo-generated charge, a reset diode configured to reset the floating diffusion node in response to a reset signal, and a junction field effect transistor configured to output a signal having an output voltage level corresponding to a charge level of the floating diffusion node.
Claims19
A method for driving a pixel of an image sensor, wherein the pixel includes a source follower JFET configured to receive a signal from a floating diffusion node, the method comprising:
- addressing the pixel to readout the signal during a readout sequence, wherein the readout sequence includes a reset operation to charge the floating diffusion node to a reset voltage and a transfer operation to charge the floating diffusion node to a pixel voltage corresponding to a photo-generated charge;
- directing the pixel to a non-readout state at a time after completion of the readout sequence;
- and
- providing a predetermined set voltage on the floating diffusion node when the pixel is in the non-readout state;
- wherein the predetermined set voltage has a magnitude sufficient to forward bias a p-n junction between a gate and drain of the source follower JFET, but insufficient to drive the source follower JFET to an active readout state, and wherein the forward bias of the p-n junction at the predetermined set voltage is sufficient to allow a small current to flow from the floating diffusion node through the p-n junction thereby inhibiting voltage drift of the floating diffusion node from the predetermined set voltage.
A method for driving a pixel array, wherein the pixel array includes a plurality of pixels, each pixel having a JFET configured as a source follower for providing a respective pixel output signal and a reset diode operatively connected between a reset line and a floating diffusion node, the method comprising:
- switching a drain voltage to the JFET from a low-level drain voltage to a high-level drain voltage;
- switching a reset voltage on the reset line to a high-level reset voltage to reset the pixel;
- activating a transfer signal to transfer a photo-generated charge generated by the pixel to a gate of the JFET to thereby provide the respective pixel output signal;
- and
- switching the drain voltage from the high-level drain voltage to the low-level drain voltage and driving the reset voltage on the reset line to a mid-level reset voltage, wherein the mid-level reset voltage has a magnitude between the high-level reset voltage and a low-level reset voltage provided on the reset line to address the pixel, and wherein a difference in magnitude between the mid-level reset voltage and the low-level drain voltage forward biases a p-n junction between a gate and a drain of the JFET to a level that is insufficient to drive the JFET to an active state.
A method for driving a pixel array having a matrix of rows and columns, wherein each pixel in the pixel array includes a JFET configured as a source follower to receive a signal, corresponding to a photo-generated charge, from a floating diffusion node, and wherein each pixel in the pixel array further includes a reset diode operatively connected between a reset bus of the row to which the pixel belongs and the respective floating diffusion node, the method comprising:
- providing a high-level reset voltage on the reset bus of a row of pixels that are to be read;
- providing a low-level reset voltage on the reset bus of the row of pixels that are to be read after the pixels of the row have been reset;
- providing the photo-generated charges to the respective floating diffusion nodes during readout of the row of pixels;
- maintaining reset buses of non-addressed rows of the pixel array at a mid-level reset voltage while pixels of the addressed row are read out, wherein the mid-level reset voltage has a magnitude between the high-level reset voltage and the low-level reset voltage;
- and
- providing a set voltage on each of the floating diffusion nodes of the row of pixels if the pixels of the row are not addressed, wherein the set voltage has a magnitude that forward biases a p-n junction between a gate and drain of the respective JFET, and wherein the magnitude of the set voltage is insufficient to drive the respective JFET to an active state.
Dependencies are inferred from the claim text.