CMOS image sensors are replacing conventional CCD sensors for applications requiring image pickup such as digital cameras, cellular phones, PDA (personal digital assistant), personal computers, and the like. Advantageously, CMOS image sensors are fabricated by applying present CMOS fabricating process for semiconductor devices such as photodiodes or the like, at low costs. Furthermore, CMOS image sensors can be operated by a single power supply so that the power consumption for that can be restrained lower than that of CCD sensors, and further, CMOS logic circuits and like logic processing devices are easily integrated in the sensor chip and therefore the CMOS image sensors can be miniaturized.
Most image sensors, such as pixel sensor cells, have a linear signal response to incoming incident photons until the light conversion/sensing element of the pixels becomes saturated and the image sensor signal reaches a maximum level. For example, FIG. 1 shows a pixel sensor cell response curve including a noise floor, a linear region and a saturated region. The linear region determines the dynamic range over which a pixel sensor cell can operate in a predictable manner. It is desirable for the pixel sensor cell to have a very large dynamic range, such that it is suitable in low light conditions as well as bright (e.g., sunny) conditions.
Typical image sensors handle varying light conditions by increasing or decreasing image sensor integration or shutter time to control the amount of the light signal collected by the sensor. This integration time and/or shutter time adjustment is done globally for the whole image sensor. When such image sensors are used to capture an image that has very large variations in light signal across the imager (such as, for example, outdoor images), portions of the image may be over-exposed or under-exposed impacting image resolution (e.g., very bright regions are washed out in white or very dark regions are washed out dark, thereby losing detail).
More specifically, blooming is a problem that may occur when integration time and/or shutter time adjustment is performed globally for the whole image sensor including thousands (or more) of individual sensors. Blooming occurs when the charge in a pixel exceeds a saturation level (e.g., enters the saturated region shown in FIG. 1) and the charge begins to migrate into adjacent pixels. For example, a pixel that reaches a saturation level does not stop generating electrons from incident photons; instead, the excess electrons that are generated post-saturation can make their way (e.g., bloom) into adjacent pixels. This blooming distorts that image data in the adjacent pixels since the adjacent pixels include charge that is not the result of photons impinging on the adjacent pixels.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.