Abstract
A dynamic range expansion technique for CMOS image sensors with dual charge storage in a pixel and multiple sampling technique is presented. Each pixel contains a photodiode and a storage diode which is connected to the photodiode via a separation gate. The sensitivity of the signal charge in the storage diode can be controlled either by a separation gate which limits the charge to flow into the storage diode or by controlling the accumulation time in the storage diode. The operation of the sensitivity control with separation gate techniques is simulated and it is found that a blocking layer to the storage diode plays an important role for high controllability of sensitivity of the storage diode. A prototype chip for testing multiple short time accumulations is fabricated and measured.
Highlights
Wide dynamic range image sensors are required for many applications such as digital cameras, security systems, automobiles and industrial cameras
If the SP gate perfectly controls the sensitivity in the SD, wide dynamic range imaging with small motion blur is possible with only controlling the SP gate
The sensitivity control with the separation gate technique has been simulated and the simulation result shows that the sensitivity ratio of the storage diode signal to the photodiode signal can be controlled by the separation gate
Summary
Wide dynamic range image sensors are required for many applications such as digital cameras, security systems, automobiles and industrial cameras. Many methods to enhance the dynamic range of CMOS image sensors have been reported The nonlinear responses such as the logarithmic response [1] and well capacity adjustment [2] are useful for dynamic range expansion with a relatively simple structure and processing. These techniques are not compatible with a pinned photodiode technology, which is a key technology for low noise image sensors. Another group of dynamic expansion methods uses multiple exposure-time signals in one frame [3,4,5] and the pinned photodiodes can be used. The pixel structure, the dynamic range expansion methods, the enhancement of SP gate controllability and the implementation of a prototype sensor chip are described
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