The Key Techniques Research On 1T APS Image Sensors

The 1T pixel unit in CMOS image sensor includes a semi-floating gate transistor(SFGT)and a photodiode.The 1T pixel can achieve the photoelectric conversion by one SFGT compared with the 3T or 4T pixel in traditional CMOS image sensor.Its special structure makes it have the advantages of higher fill factor,higher pixel density,lower area,and lower cost.As the process of SFGT develops,the 1T pixel has a promising prospect in imaging.This thesis introduces the architecture of image sensor based on 1T pixel.An equivalent circuit model is proposed based on the analysis of 1T pixel structure and working principle.According to the theoretical calculation of the electrical characteristic parameters,this thesis analyzes the influences of the applied voltage and the parasitic capacitance of the device on the performance of the pixel,which plays a guiding role in the optimization design.The device is simulated by GSMC 0.18?m CMOS process.The simulation results show a good agreement with theoretical calculation,which verifies the accuracy of equivalent circuit model.A higher drain voltage improves the output swing,the maximum non-saturated light power,the conversion gain and the FWC.A lower floating gate capacitance increases the output level and the conversion gain at the expense of the light range and the FWC.This thesis designs the circuit to readout the output current signal.The structure of the transimpedance amplifier with feedback capacitance and switched capacitance amplifier is used to realize the conversion from current signal to the voltage signal and the double sampling.The readout circuit is simulated by GSMC 0.18?m CMOS process.It achieves the conversion from current signal to voltage signal.The effects of amplifier gain and the reference voltage on the circuit performances are discussed.This thesis analyzes the photosensitive process and the readout circuit design based on theoretical analysis,model establishment,device level simulation and circuit level simulation.The research proposes optimization for better pixel design and provides reference for the study of CMOS image sensor based on the 1T pixel.