Image sensor with stacked pixels having high dynamic range and low noise

An image sensor comprising:

• a logic die, comprising column readout circuits and bitlines connected to the column readout circuits;
• a sensor die, overlaid on the logic die;
• and
• an array of detector elements, each detector element comprising:
• a sensing circuit, which is disposed on the sensor die and comprises:
• a photodiode having cathode and anode terminals;
• a floating diffusion node connected to one of the terminals of the photodiode;
• a reset transistor coupled between the floating diffusion node and a reset voltage;
• and
• a source follower transistor having an input connected to the floating diffusion node and an output;
• and
• a pixel circuit, which is disposed on the logic die and comprises:
• a select transistor, which has an input coupled to the output of the source follower and an output coupled to one of the bitlines;
• and
• a current memory circuit, which is coupled to the input of the select transistor and is configured to sense and output a signal indicative of a level of noise in the detector element,
• wherein during each image frame, the reset transistor is switched on in a first reset period to reset the floating diffusion node prior to an exposure period of the frame, and
• the select transistor is switched on in a first readout period, following the exposure period, so as to read out photocharge from the floating diffusion node, and
• the reset transistor is switched on in a second reset period, following the first select period, to reset the floating diffusion node, and
• the select transistor is switched in at a second readout period, following the second reset time, to read out noise accumulated by the detector element.

A method for image sensing, comprising:

• providing a logic die, comprising column readout circuits and bitlines connected to the column readout circuits;
• overlaying on the logic die a sensor die;
• and
• forming an array of detector elements on the sensor die and the logic die, each detector element comprising:
• a sensing circuit, which is formed on the sensor die and comprises:
• a photodiode having cathode and anode terminals;
• a floating diffusion node connected to one of the terminals of the photodiode;
• a reset transistor coupled between the floating diffusion node and a reset voltage;
• and
• a source follower transistor having an input connected to the floating diffusion node and an output;
• and
• a pixel circuit, which is formed on the logic die and comprises:
• a select transistor, which has an input coupled to the output of the source follower, and an output coupled to one of the bitlines;
• and
• a current memory circuit, which is coupled to the input of the select transistor and is configured to sense and output a signal indicative of a level of noise in the detector element;
• and
• coupling a lateral overflow integration capacitor (LOFIC) circuit between the floating diffusion node and the reset transistor in each sensing circuit, the LOFIC circuit comprising one or more charge storage capacitors and one or more LOFIC transistors coupled between the floating diffusion node and the charge storage capacitors, wherein the reset transistor is coupled between the LOFIC circuit and the reset voltage.