传输栅掺杂对CMOS有源像素满阱容量及暗电流的影响

Effect of Transfer Gate Doping on Full Well Capacity and Dark Current in CMOS Active Pixels

研究了传输栅掺杂,即N+TG和P+TG,对满阱容量以及暗电流的影响。沟道电势分布受传输栅与衬底功函数差的影响,随着钳位光电二极管和浮动扩散节点之间的势垒高度的增加,feedforward效应被抑制,满阱容量增加。另一方面,处于电荷积累状态的沟道可以降低暗电流。基于四管有源像素工作过程进行仿真,结果表明,在曝光期间不加负栅压的情况下,基于P+TG的像素的满阱容量相对N+TG的提高了26.9%,其暗电流为N+TG的0.377倍。当电荷转移效率大于99.999%时,N+TG的开启电压需高于2.3 V,而P+TG的开启电压需高于3.0 V。

In recent years,Pinned Photodiode(PPD) CMOS Image Sensors(CISs) are widely used in consumer electronics,medical,and other fields due to their advantages of high integration,low power consumption and low cost.CMOS active pixels play an important role in CISs.The design of the Transfer Gate(TG) affects image quality,which is related to Full Well Capacity(FWC) and dark current.TG affects the feedforward effect by channel potential.The feedforward effect directly influences FWC as the charges in PPD can flow into Floating Diffusion (FD) by thermal emission.In addition,due to the existence of interface states,dark current generates at the interface of the TG channel,which flows into PPD during the integration period.Several papers have analyzed the influence of TG on FWC and dark current,and have proposed different improvement techniques and designs.When a negative bias is added to TG,the channel is in a state of accumulation,isolating the interface state of the channel from the depletion region of PPD so that dark current is greatly reduced.Furthermore,adopting a negative bias to TG increases the channel barrier,inhibiting the feedforward effect and increasing in FWC.A positive voltage adopted to TG is also beneficial to reduce dark current,but will make FWC decrease.Adjusting the doping length of p-type impurities can change the position of the potential barrier,so that dark charges flow to FD.In this paper,the influence of two types of doped transfer gates,named N+TG and P+TG,on full wel capacity and dark current are investigated.Channel potential is affected by the work function difference between TG and substrate.As the barrier height between pinned-photodiode and floating diffusion increases,the feedforward effect is inhibited and the full well capacity increases.On the other hand,the channel in charge accumulation can reduce dark current.To analyze the influence of TG doping on FWC and dark current,a typical 4T-PPD pixel structure is used in this paper.The device consists of a PPD,a “special” TG transistor whose drain is a FD node,and three conventional transistors named Reset Transistor(RST) ,Source Follower(SF) ,and Row Select(RS) transistor.The two kinds of TG have the same structure except for different doping types.P-type doping is shared with p+doping used in PMOS transistors,so no additional steps need to be introduced.Device level simulation using Technology Computer Aided Design(TCAD) is performed based on 4T pixels working process,trap model is added to the simulation.The concentration of traps is set to 1×10traps·cmand the capture cross-section to 1×10cm~2.PPD of two doping types of TG integrates for 10 ms in dark conditions.In addition,the light intensity is set to 2×10W/cm~2 when testing the FWC of PPD.This paper compares P+TG and N+TG under the same channel and substrate doping conditions.FWC and dark current characteristics are simulated when the turn-off voltage(V) is 0 V.Simulation results demonstrate that the full well capacity of photodiode based on P+TG is 26.5% higher than that of N+TG.The dark current is 0.377 times that of N+TG without negative voltage during the exposure.In practical engineering,a negative voltage is usually applied to N+TG during exposure to obtain good full well capacity and dark current characteristics.The opening characteristics of TG affect image lag,which plays an important role in imaging quality and is usually determined by Charge Transfer Efficiency (CTE) .CTE of N+TG is greater than 99.999% at 2.3 V,while P+TG requires 3.0 V.P+TG requires a higher voltage to ensure excellent charge transfer.When the FWC of PPD is high,CTE will be negatively affected,resulting in image lag.At this point,the positive charge pump needs to be introduced to ensure transfer characteristics.Under the simulation conditions in this paper,two doping types of TG have good transfer characteristics at 3.3 V.