CMOS Image Sensor with Dual-Sensitivity Photodiodes and Switching Circuit for Wide Dynamic Range Operation

Abstract

Conventional CMOS image sensors (CISs) have a trade-off relationship between dynamic range and sensitivity. In addition, their sen-sitivity is determined by the photodiode capacitance. In this paper, CISs that consist of dual-sensitivity photodiodes in a unit pixel are proposed for achieving wide dynamic ranges. In the proposed CIS, signal charges are generated in the dual photodiodes during inte-gration, and these generated signal charges are accumulated in the floating-diffusion node. The signal charges generated in the high-sen-sitivity photodiodes are transferred to the input of the comparator through an additional source follower, and the signal voltages converted by the source follower are compared with a reference voltage in the comparator. The output voltage of the comparator deter-mines which photodiode is selected. Therefore, the proposed CIS composed of dual-sensitivity photodiodes extends the dynamic range according to the intensity of light. A 94 × 150 pixel array image sensor was designed using a conventional 0.18 μm CMOS process and its performance was simulated.