The present disclosure relates to a technique of increasing pixel conversion gain in a CMOS image sensor, specifically an active pixel sensor.
Electronic image sensors obtain an electrical image of a subject. The sensor converts the incoming light photons to an electronic signal. The sensitivity of conversion between the photons and electrons is often measured with a quantity called conversion gain.
Many image sensor devices convert the incoming photons to charge using a photo-gate. That charge is stored in a substrate. Other devices convert the incoming photons to electrons using a photodiode. In the devices using photodiodes, it was found to be advantageous to enlarge the size of the photodiode. The large size allows more light to be received by the photodiode.
The conversion gain of a photodiode pixel, expressed in volts per photon, is often inversely proportional to pixel size. Thus, although the number of incident photons on a pixel increases with pixel area, the corresponding reduction in conversion gain prevents a matching increase in pixel sensitivity. This is often addressed by using photo-gate pixels or other structures that isolate the sensor node of the pixel from the collection area. However, these solutions often involve an increase in the number of transistors within the pixel. The increase in the transistor count reduces fill factor. Further, the source follower output stage introduces a non-linearity in pixel response.
In one embodiment of the present invention, a pixel sensor system is disclosed. The system includes a photo-sensor, an output amplifier, and a feedback capacitor. The photo-sensor is configured to receive photons and to convert the photons into charge. The output amplifier has at least two transistors in a cascoded configuration. The amplifier converts the charge into electronic signal. The feedback capacitor is disposed between the photo-sensor and an input of the output amplifier.
In another embodiment of the present invention, a method for increasing a pixel conversion gain is disclosed. The method includes providing a feedback capacitor between a photo-sensor and a pixel circuit, and adjusting the feedback capacitor to increase pixel sensitivity.
In a further embodiment, an image sensor is disclosed. The sensor includes an array of pixel sensors. Each sensor includes a photo-sensor, an output amplifier, and a feedback capacitor. The photo-sensor is configured to receive photons and to convert them into charge. The output amplifier includes at least two transistors in a cascoded configuration. The amplifier converts the charge into electronic signal. The feedback capacitor is disposed between the photo-sensor and an input of the output amplifier. The image sensor further includes a controller configured to provides timing and control signals to enable read out of signals stored in each pixel sensor.