Dependence of the Spill Back Effect on the Energy Level Distribution of Interface States Under the Transfer Gate in CMOS Image Sensor

The spill back effect easily occurs in CMOS image sensors (CISs), especially under high exposure conditions, which are common under visible light. This undesirable phenomenon can lead to image lag, which in turn severely degrades image quality. The presence of Si/SiO2 interface states under the transfer gate (TG) increases the probability of Coulomb scattering between the electrons in TG channel and the occupied interface states. As a result, the effect of Coulomb scattering on electron mobility cannot be ignored. Therefore, this article proposes a model of spill back effect related to the energy-level distribution of Si/SiO2 interface states under the TG for the first time. By deducing an expression for the mobility under the effect of Coulomb scattering related to the energy-level distributions of interface states, the number of electrons injected to pinned photodiode (PPD) from TG is modeled. Simulations were performed with Gaussian, exponential, and measured U-shaped interface state distributions, and the consistency between TCAD simulation results and theoretical model shows that the number of electrons injected to PPD from TG is not a single fixed value but a set of dynamic values changing with the energy-level distributions of interface states; moreover, it is at least twice as much as that of no interface states, and the higher the density, the greater the number of electrons.