Design And Research On Active Pixels Of TDI CMOS Image Sensor

Time-Delay Integration (TDI) image sensor is a type of sensor capturing imagesby scanning. Compared with the conventional line-scan image sensors, TDI imagesensors have the advantages of high sensitivity and high signal-to-noise ratio (SNR),which makes it more suitable for the application in dark or image capturing for fastmoving objects. Consequently, TDI image sensors have been widely applied in spaceremote sensing, aerial photography, industrial detection and security monitoring.Compared with CCD TDI image sensors, those sensors based on CMOSmanufacturing process exhibit their superiority in manufacturing cost, powerconsumption and integrability. As the photosensitive element of CMOS image sensor,the active pixels play an important role in the captured image quality. Based onCMOS process platform, the paper will focus on the research of the active pixeldesign for TDI CMOS image sensor.The operation mode of TDI CMOS image sensor is analyzed, and it is concludedthat the TDI mode is closely related to the charge transfer velocity as well as the darkcurrent level, which is the also the direction and key point of pixel design. Based onthe CMOS process platform, several pixel structures have been verified. On the basisof the analysis and evaluation of the experimental data, the optimized design of thelarge pixel has been achieved. Adopting the designed pixels, a TDI CMOS imagesensor with128stages is designed and verified by0.18um CMOS process，and thetest results indicates that the sensitivity and SNR of the sensor have been greatlyimproved. With respect to the special requirements on TDI CMOS image sensors,such as low power, high dynamic range and low cost, the corresponding optionalsolutions are proposed based on the optimization of pixel structures, which areconstructive suggestions on promoting the performance of TDI CMOS image sensor.The creative work of the paper includes:1. For improving the charge transfer in large pixels, a scheme adoptingnonuniform doping in the photodiode was proposed, and therefore a horizontalelectric field is formed, which is helpful to speeding up the charge transfer and satisfythe fast readout requirement of TDI image sensor consequently. 2. For lowering the power consumption of TDI image sensors, an in-pixel chargeaddition scheme was proposed, which could make the frequency of additionoperations performed in the accumulators decrease. As less constraint is imposed onthe readout rate, the readout circuits can be designed with lower consumption.3. For extending the dynamic range of TDI image sensors, two types of pixelswith higher and lower conversion gain are adopted in the pixel array, which aresuitable to image capture in low and high illumination respectively. By fusing the twokinds of pixels’ output signals in the process of TDI accumulation, a high dynamicrange as well as SNR can be achieved.