## 1 Introduction
Besides this width, these three technologies differ also in the junctions' depth and dopant concentration, as well as in the dimensions of contacts, metals and vias. The smaller the technology, the smaller and more compact the circuit [10]. The three different types of available CMOS vertical photodiodes' p-n junctions are based on n+/p-substrate; p+/n-well and n-well/p-substrate [7,11] as illustrated in Figure 1. Due to the different junctions' depths of these structures, according to the literature [7,12], it is expected that their photodiodes present different quantum efficiency spectra, as shown in the example of Figure 2 (data obtained from the results of 2 \(\upmu\)m CMOS technology photodiodes reported in literature). This occurs since the penetration depth of the light in silicon is wavelength dependent, due to the wavelength dependency of the silicon absorption coefficient in the visible spectrum [7,12]. Therefore, a shallow junction (as the n+/p-substrate or p+/n-well) collects the lower wavelengths more efficiently [7,12]. Additionally, the higher efficiency presented by the n+/p-substrate, above the 500 nm, is related to the different doping concentrations between the \(n\) and the \(p\) sides, which extends the \(p\) side depletion area more deeply. According to the literature report of Figure 2, with an example in 2 \(\upmu\)m technology, it is expected that the n+/p-substrate photodiode presents higher quantum efficiency than the p+/n-well and n-well/p-substrate photodiodes [7,11]. Therefore, to understand if such behavior also happens for the 0.18, 0.35 and 0.7 \(\upmu\)m CMOS technologies, this manuscript aims to model and numerically characterize the three types of photodiodes' junctions presented in Figure 1, for those three CMOS technologies. This will evaluate, for each structure, the differences in the optical performance, according to the manufacturing technology. The theory regarding optical detectors is well studied and there are many reports detailing the analytical solution of the problems. However, there is a lack of literature regarding numerical methods dealing with optical-semiconductor interfaces. Thus, this work intends to overcome this gap and explore the possibility of using a finite elements' numerical tool, in particular COMSOL Multiphysics, to understand the behavior of photodiodes in the entire optical spectrum. To the best of the authors' knowledge, there is no other work providing this numerical comparison in the literature in the junction photodiode's performance using different CMOS technologies for the three different types of photodiodes' p-n junctions using the 0.18 \(\upmu\)m, 0.35 \(\upmu\)m and 0.7 \(\upmu\)m CMOS technologies; this work thus fills a research void. Finally, the authors also detail the layout of the photodetectors, designed with CADENCE tools, using the 0.18 \(\upmu\)m technology and chosen due to the better performance, including the optical response, achieved even when high frequency electronic circuits are used, since the authors intend to further integrate such photodiodes, their readout electronics and also acoustic sensors in the same CMOS chip for multisensing detection [13].