Based on the thermionic emission theory, a charge transfer model has been developed which describes the charge transfer process between a pinned photodiode and floating diffusion (FD) node for CMOS image sensors. To simulate the model, an iterative method is used. The model shows that the charge transfer time, barrier height, and reset voltage of the FD node affect the charge transfer process. The corresponding measurement results obtained from two different test chips are presented in this paper. The model also predicts that other physical parameters, such as the capacitance of the FD node and the area of the photodiode, will affect the charge transfer. Furthermore, the model can be extended to explain the pinning voltage measurement method and the feedforward effect.@en