Physics and numerical simulation of single photon avalanche diodes

We present results of the numerical simulation of the transient behavior of shallow junction single photon avalanche diodes (SPAD´s). We developed a bidimensional model for above breakdown simulations and show that the initially photogenerated charge density builds up locally by an avalanche multiplication process and then spreads over the entire detector area by a diffusion-assisted process. To model real geometries, we developed a simplified model based on the obtained results. The importance of the photon-assisted spreading mechanism is evaluated and compared with the diffusive one. The contribution of the photon-assisted mechanism is minor in these geometries. The model is compared with the experimental data on the avalanche leading edge and the timing resolution; the agreement is good. We conclude that the model can be considered to be a useful tool for the design of improved structures