The effect of size on photodiode pinch-off voltage for small pixel CMOS image sensors

The electron potential of a photodiode in a CMOS image sensor should be designed precisely since the charge capacity of the photodiode decreases as the pixel area shrinks. The pinch-off voltage of a photodiode, which also affects the electron capacity, is dependant on the doping profile of the pn junction as well as the size of the photodiode. The pinch-off voltage is lower in a smaller photodiode. A simple method that uses the lateral depletion of a photodiode for an estimate of the pinch-off voltage in small photodiodes is proposed, and is compared to the measured experimental data. Two constants are used to account for the doping profile and photodiode size. The measurement data shows the error of the estimation of the pinch-off voltage to be <0.05 V.