Analytical Modeling of Charge Behavior in Pinned Photodiode for CMOS Image Sensors

The thermionic emission mechanism is usually employed to model the charge packet, which is not affected by the potential barrier; however, the electric fields induced by the charge itself are not included in the previous work. We introduce a proposed physical-based model to characterize the whole charge behavior characteristics of the pinned photodiode (PPD) when employing the thermal diffusion, self-induced drifting, and thermionic emission mechanisms together. In addition, the dynamic PPD parameters such as PPD capacitance ( ${C}_{\text {PPD}}{)}$ , PPD potential ( ${V}_{\text {PPD}}{)}$ , and charge transfer potential barrier ( ${V}_{\text {B}}{)}$ are included to characterize the integration and transfer process together based on iterative methods. The model is verified with technology computer-aided design (TCAD) simulations, and the test devices were fabricated in a 0.11- $\mu \text{m}$ CMOS image sensor (CIS) process. The proposed analytical behavior model presents a great convenience for circuit-level simulation of the PPD-based pixels.