Dynamic Capacitance Variations Due to Barrier Modulation in Pinned Photodiode of CMOS Image Sensor

Abstract

The pinned photodiode (PPD) capacitance is an important parameter in CMOS image sensor design since it provides useful information on the charge handling capacity of the photodiode. The model of the dynamic capacitance of the PPD includes a fixed depletion capacitance and a variable diffusion capacitance. The variable component of the photodiode capacitance is estimated by measuring the feedforward voltage at the floating diffusion node. However, the estimated feedforward voltage is highly nonlinear at its full-well condition. In this article, the transfer gate (TG) channel potential is shown to be dependent on the number of charges integrated in the PPD. The nonlinear behavior of TG channel potential influences the effective potential barrier. The nonlinearity observed in the feedforward voltage is also explained using the nonlinear behavior of TG channel potential. In the literature, the influence of the effective potential barrier on the photodiode capacitance models is not taken into account. In this article, the influence of the effective potential barrier on the full-well capacity, feedforward effect, and PPD capacitance is studied. The improved photodiode capacitance model using the effective potential barrier is in good agreement with the measurement results.