Wide dynamic range image sensor pixel cell
A pixel cell has a large and small photodiode, transfer transistors, a reset transistor, a dynamic range enhancement capacitor, a capacitor control transistor, a storage capacitor, a storage capacitor control transistor, an amplifier transistor in a source follower configuration and a rolling shutter row select transistor and a readout circuit block. The small and large photodiodes are exposed simultaneously, the large photodiode having a constant exposure while the small photodiode has a chopped exposure and charge transfer to a storage capacitor.
1. An image sensor pixel cell for use in a wide dynamic range image sensor, the image sensor pixel cell comprising:
a floating drain node which is useful to transform an imaging related charge into an imaging related voltage signal and is directly connected to five components comprising:
an input gate electrode of an amplifier transistor in a source follower configuration which is useful to couple out the imaging related voltage signal from the floating drain node through a row select transistor to a pixel cell array column line;
a drain electrode of a first transfer transistor which in turn has a source electrode connected to a first photodiode with low sensitivity to light incident from a scene;
a source electrode of a double conversion gain control transistor which in turn has a drain electrode connected to a reset transistor and also to a dynamic range enhancement capacitor;
a drain electrode of a second transfer transistor which in turn has a source electrode connected to a second photodiode with high sensitivity to light incident from a scene in comparison to the first photodiode; and
a drain electrode of a storage gate transfer transistor which in turn has a source electrode connected to the drain electrode of a third transfer transistor whose source electrode is connected to the first photodiode, wherein a storage capacitor is connected between the gate electrode and drain electrode of the third transfer transistor.
2. The image sensor pixel cell of claim 1 , wherein the electrode of the dynamic range enhancement capacitor not connected to the drain of the double conversion gain transistor is connected to ground.
3. The image sensor pixel cell of claim 1 , wherein the second photodiode has high sensitivity to light incident from a scene relative to the sensitivity of the first photodiode by virtue of it having a larger area relative to the first photodiode upon which to receive light incident from a scene.
4. The image sensor pixel cell of claim 3 , wherein the incident light reception area of the second photodiode is at least twice the incident light reception area of the first photodiode.
5. The image sensor pixel cell of claim 1 , wherein the first and second photodiodes share the same microlens and the same color filter element.
6. The image sensor pixel cell of claim 1 , wherein the dynamic enhancement capacitor and the storage capacitor are one of a metal-insulator-metal (MIM) capacitor, a metal-oxide-metal (MOM) capacitor, a metal-oxide-silicon (MOS) capacitor, a transistor gate to drain electrode parasitic capacitor, or a diode capacitor.
7. An image sensor pixel cell for use in a wide dynamic range image sensor, the image sensor pixel cell comprising:
a floating drain node which is useful to transform an imaging related charge into an imaging related voltage signal and is directly connected to four components comprising:
an input gate electrode of an amplifier transistor in a source follower configuration which is useful to couple out the imaging related voltage signal from the floating drain node through a row select transistor to a pixel cell array column line;
a drain electrode of a first transfer transistor which in turn has a source electrode connected to a first photodiode with high sensitivity to light incident from a scene;
a source electrode of a double conversion gain control transistor which in turn has a drain electrode connected to a reset transistor and also to a dynamic range enhancement capacitor, wherein the drain electrode of a second transfer transistor is connected to the drain electrode of the double conversion gain transistor and the source of the second transfer transistor is connected to a second photodiode with low sensitivity to light incident from a scene in comparison to the first photodiode; and
a drain electrode of a storage gate transfer transistor which in turn has a source electrode connected to the drain electrode of a third transfer transistor whose source electrode is connected to the second photodiode, wherein a storage capacitor is connected between the gate electrode and drain electrode of the third transfer transistor.
8. The image sensor pixel cell of claim 7 , wherein the electrode of the dynamic range enhancement capacitor not connected to the drain of the double conversion gain transistor is connected to ground.
9. A method to achieve wide dynamic range image sensing with the image sensing pixel cell as recited in claim 1 having one charge accumulation period and a sequence of four readout modes comprising the steps of:
resetting all of the components of the pixel cell; and then
simultaneously accumulating charge on the first and second photodiodes during an accumulation period, the charge representative of light incident on the photodiodes from a scene, wherein during the accumulation period the charge accumulating on the first photodiode is allowed to collect without disturbance by keeping the first transfer transistor disabled while the charge accumulating on the second photodiode is chopped by alternately first accumulating and transferring the charge to the storage capacitor during a time interval t 1 by enabling the third transfer transistor for a time interval t 1 and secondly draining the charge to the reset potential during a time interval t 2 by enabling the second transfer transistor for the time interval t 2 ; and next
reading out an imaging related voltage signal on the floating drain related to an imaging related charge transferred from the first photodiode in a High Conversion Gain mode, i.e. the dual conversion gain capacitor is not added to the floating drain; and next
reading out an imaging related voltage signal on the floating drain related to an imaging related charge transferred from the first photodiode in a Low Conversion Gain mode, i.e. the dual conversion gain capacitor is added to the floating drain; and next
reading out an imaging signal related voltage signal on the floating drain related to an imaging related charge transferred from the storage capacitor in a High Conversion Gain mode; and next
reading out an imaging signal voltage signal on the floating drain related to an imaging related charge transferred from the storage capacitor in a Low Conversion Gain mode; and next
combining the four imaging signals to produce a wide dynamic range composite signal representative of the incident light from a location within a scene.
10. The method of claim 9 wherein the number of cycles of the chopped exposure of the second photodiode, wherein during a single cycle charge is accumulated on the photodiode and transferred to the storage capacitor during an interval t 1 and then drained from the photodiode during an interval t 2 , exceeds three.