QE Improvement of CMOS Image Sensors: Virtual Barrier Model Established for the PN-like Junctions of a BSI-PD With Gradient Doping

Quantum efficiency (QE) is crucial for enhancing the signal-to-noise ratio and imaging quality of CMOS image sensors (CISs). Experimental tests show that gradient-doped photodiode (PD) can achieve a high QE in a backside-illuminated CIS. In this paper, a modeling study is conducted on the above experimental phenomenon, and a PN-like junction virtual barrier model is proposed. After theoretical study and TCAD simulation, both the formation mechanism of the virtual barrier and its dynamic characteristics are revealed. Moreover, the virtual barrier caused by PN-like junction can make the partial QE (QE_P) of CIS blue and red shift, which is used to clarify the influence of the virtual barrier variation on total QE (QE_T) during the exposure process.