Simulation Study of Vertical p-n Junction Photodiodes' Optical Performance According to CMOS Technology

CMOS photodiodes have been widely reported in microsystem applications. This article presents the design and numerical simulation of p-n junction photodiodes, using COMSOL Multiphysics, for three CMOS technologies (0.18 mu m, 0.35 mu m and 0.7 mu m) and three different p-n junction structures: n+/p-substrate, p+/n-well and n-well/p-substrate. For these simulations, the depth junctions and dopant concentrations were set according to the different technologies. Then, each photodiode was spectrophotometrically characterized regarding the current, responsivity and quantum efficiency. The obtained numerical results show that the 0.18 and 0.35 mu m CMOS technologies are those with the highest peak of efficiency when visible spectral ranges are needed, comparative to the 0.7 mu m technology. Furthermore, the three most common p-n vertical junction photodiode structures were compared. The n+/p-substrate junction photodiode appears to be the one with the highest quantum efficiency in the visible range, which is in agreement with the literature. It can be concluded that the photodiodes' characteristic curves and dark current values are consistent with reports in the literature. Therefore, this numerical approach allows to predict the photodiodes' performance, helping to select the best structural design for each required application, before their microfabrication.