Study Of The Charge Transfer Mechanism And Noise In CMOS Active Pixels

CMOS image sensors are widely used nowadays in the fields of consumer,industry,medical and aerospace applications due to its fast development in recent years.It has already exceeded the performance of traditional CCD image sensors.Although most of CMOS image sensors are still based on the classical structure of active pixels,devices in a pixel need to be well optimized according to the target application,especially in some high-end fields.The academia never stops the study on the mechanism of the classical active pixels.A lot of theories for the active pixels have been reported in recent years,but most of them can only explain one corresponding phenomenon,and even some of them contradict each other.So the deeper and more detailed study on the mechanism of active pixels is very necessary.In this work,a CMOS image sensor was designed and fabricated for the pixel measurement and analysis.Based on the measurement results and analysis of current theories,a charge transfer model and noise models for the active pixels are established.The models successfull explain the trade-off and some nonideal characteristics in pixel design.Major innovations in this thesis includes:1.A charge transfer model for CMOS active pixels is established.This model describes the charge transfer process between the pinned photodiode and the floating diffusion node,and it can explain how the design variables affect the charge transfer,such as transfer time,doping concentration of the photodiode,junction depth,area of the photodiode,barrier height under the transfer gate,width of the transfer gate,voltage and capacitance of the floating diffusion node.Futhermode,the extended model can explain some phenomenons in the active pixels,such as the method of pinning voltage measurement,feedforward effect,noise under long time exposure and the full well capacity variation.2.A method to judge the complete charger transfer is proposed,which is based on the analysis of the mean-variance curve.It can explain the effect of incomplete charge transfer on the output noise.This method can especially distinguish the less obvious incomplete charge transfer.So it can be used for optimizing the transfer time design in the active pixels.3.A model based on Monte Carlo method is established to study the random telegraph signal noise in the active pixels.This model can explain the statisticcharacteristics of the random telegraph signal noise in CMOS image sensors.Finally,a method based on the signal accumulation in spatial domain is proposed for the purpose of noise improvement.