Feedforward Effect in Standard CMOS Pinned Photodiodes

The charge handling capacity or the full well of the photodiodes used in CMOS image sensors is a very important characteristic because it affects the saturation level and the dynamic range of the image sensor. The scaling of the pixel size to increase the spatial resolution is also reducing the barrier separating the photon detection and the collection node in a standard pinned photodiode (PPD). The barrier reduction and the thermionic emission of the electrons allow some of the charges from the photodiode well to feed into the collection node, resulting in a feedforward voltage. In conventional readout of the pixels, this feedforward voltage is neglected and lost when the collection node is reset. The barrier height of the transfer gate (TG) determines the quantity of electrons held back in the photodiode well. Thus, the knowledge of this barrier height is very important in determining the true charge handling capacity of the photodiode potential well. Experiments with standard PPDs showed that a barrier height of around 0.5 V is needed to hold the electrons in the photodiode potential well. This is analogous to the barrier potential for charge-coupled devices reported in the literature. Furthermore, the barrier height dependence on the charge storing time in the photodiode well and the structural dimensions of the TG and photodiode length are also explored in this paper.