Study On Multi Gain High Dynamic Range CMOS Active Pixel

CMOS image sensors are widely used in consumer electronics,biomedical,security monitoring,aerospace and other fields.As a key performance parameter of CMOS image sensor,dynamic range determines all the details of the image taken.The pixel with multi-gain and dual-photodiode active pixel realized high sensitivity with weak light and high full well capacity(FWC)with strong light in a single exposure by using the high gain ratio within the pixel,thus expanding the dynamic range.However,the high dynamic range(HDR)is achieved at the expense of the signal-to-noise ratio(SNR)of dual photodiode signal switching points.In order to further improve the dynamic range and maintain good SNR with HDR.Based on the mechanism of multigain active pixel extending dynamic range,this article analyzes the parameters limiting dynamic range and SNR under different pixel sizes by building a mathematical model,and designs a HDR active pixel.Firstly,the mechanism of active pixel with multi-gain and dual photodiode is studied.The shot noise,readout noise,k TC noise and fixed pattern noise under the high and low conversion gain signals of large and small photodiodes are analyzed.Secondly,the mathematical model of switching and output process of pixel signals is established combining with performance parameters such as FWC,dynamic range and SNR.The relationship between the dynamic range and SNR with the sensitivity ratio of large to small photodiodes and lateral overflow integration capacitor(LOFIC)is determined.The output signals of the pixel and the overall SNR curve are given.Finally,a HDR active pixel is designed based on the model.Device simulation and layout design based on finger photodiode were carried out,and the source of parasitic capacitances under different floating diffusion nodes was studied.Capacitance distribution of each node was achieved combining layout and device simulation.For 10μm×10μm pixel,when LOFIC is 30 f F and the sensitivity ratio of two photodiodes increases from 13 to 18,the dynamic range is extended by 2.83 d B,but the SNR at the signal switching point decreases by 1.51 d B.When the sensitivity ratio is 13 and the LOFIC increases from 20 f F to 40 f F,the dynamic range is extended by 5.31 d B,while the SNR decreases by 0.61 d B.Considering the dynamic range and SNR,the sensitivity ratio and overflow capacitance were designed to be 14 and 28.49 f F,respectively.The FWC of 56Ke-and 12Ke-for large and small photodiodes are achieved under the condition of complete charge transfer.The capacitors of the three conversion nodes in the simulated pixel are 0.883 f F,8.886 f F and 40 f F respectively,and the corresponding conversion gains are 181.2μV/e-,18μV/e-and 4μV/e-,respectively.The dynamic range of pixel is up to 125.4d B,and the SNR at signal switching is above29 d B.To summary,the multi-gain active pixels designed with the proposed model can ensure the SNR at the signal switching on the basis of extending the dynamic range.