Temperature Effects on Feedforward Voltage in Standard CMOS Pinned Photodiodes

The dynamic range and the signal-to-noise ratio of a CMOS image sensor depend on the saturation level of the photodiodes. A very high charge handling capacity or the full well of the photodiode is desired. The thermionic emission of the electrons from the photodiode to the collection node of the pixel reduces the full well capacity of the photodiode. The effect of operating temperature on this thermionic effect on full well capacity is studied in this paper. As the temperature increases, electrons gain thermal energy and are able to overcome the potential barrier between the photodiode and the collection node. This is observed as an increased feedforward voltage. The feedforward voltage shows a logarithmic relationship with temperature changes. The rate of change of the feedforward voltage is not uniform and depends on the position of the quasi-Fermi level. For lower temperature ranges, the change in the feedforward voltage observed is linear, while, for higher temperature ranges, the change is logarithmic in nature.