The effect of photodiode shape on dark current for MOS imagers

The effect of photodiode (PD) shape was studied in the attempt to reduce the dark current in MOS imagers. In such imaging systems, each pixel ideally produces a voltage directly proportional to the intensity of light incident on the PD. Because of non-idealities, the PD performance is compromised by the presence of dark current, which becomes the most significant source of noise degrading overall image quality, particularly for low light environments. Unfortunately, due to the statistical variability of dark current, it is not possible to simply correct the readout voltage error via subtraction. To minimize the effect, recent research suggests that PD shape and features have an influence on dark current levels. We test that assertion by considering PDs with different corners while maintaining high fill-factor rates, along with rectangular and triangular shapes to exploit charge transfer characteristics. In all, five PD geometries were built to test the influence of PD shape on the dark current signal including a traditional square shape, two square shapes with increasingly rounded corners (135 and 150 degrees), a triangular design with sharp corners and finally, a triangular design with 120 degree corners. Results indicate the PDs with a square shape and 90 degree corners exhibit the lowest dark current and highest readout voltage. Furthermore, the triangular shape suggests improved charge transfer characteristics; however, this improvement appears to be negated by an increase in dark current response. Therefore, our findings indicate that the traditional PD square shape is the preferred design.