The effect of photodiode shape on charge transfer in CMOS image sensors

The output signal of a CMOS image sensor is derived from the electrons generated by light incident on the pinned photodiode in each pixel. The output voltage depends on the transfer of the signal electrons from the pinned photodiode to the readout node. In large photodiodes it becomes difficult to fully extract the generated electrons because of the reduction in lateral electric field that pushes the electrons to the transfer transistor. To enhance the transfer, a triangular shaped photodiode was proposed which increases the lateral electric field applied to the photo generated electrons. Compared with a conventional rectangular shaped pixel, the electrons are extracted more easily in a triangular shape since the narrow tail end forms an increased potential gradient. A VGA type image sensor chip showed that the triangular photodiode had 50% higher output voltage than a conventional rectangular photodiode. It was verified that the output voltage is enhanced more by the electric force than the light receiving area. The noise due to the residual electrons was not seen in the image acquired from image sensor having triangular photodiode.