The Effect of Pinned Photodiode Shape on Time-of-Flight Demodulation Contrast

An empirical investigation on improving the pinned photodiode (PPD) demodulation contrast by tailoring the geometry of the device is presented. Results of this TCAD simulation-based study are used to develop a structure especially suited for time-of-flight applications. In order to obtain a fair comparison between various PPD shapes, a square structure is adopted as a benchmark and all subsequent PPD geometries use the same process parameters. Five different PPD shapes are compared: 1) nominal square-shaped PPD; 2) triangular PPD; 3) constant-field PPD; 4) L-shaped constant-field PPD; and 5) proposed PPD. Device physics simulations are undertaken and the speed of each structure is evaluated on the basis of its demodulation contrast. It is shown that triangular and constant-field PPDs can provide significant improvement compared with a conventional square-shaped PPD, however they still lack effective lateral charge transfer in the final electron sorting stage. The final PPD proposed in this paper achieves this with a tailored PPD shape and doping gradient. In addition, the transfer gates are placed close to one another to make use of gate-induced fringe fields and thus improve the speed of electron sorting. Using these techniques, a PPD demodulation contrast of 61% is obtained at a frequency of 100 MHz, which is comparable to the contrast achieved in state-of-the-art photogate-based designs.