## 4 Conclusions and Future Works  
This paper describes the design, numerical simulation, and characterization of vertical CMOS photodiodes in silicon p-n junctions. Three CMOS photodiode structures were considered: n+/p-substrate, n-well/p-substrate and p+/n-well. Initially, the CMOS photodiode structures were characterized for the 0.18 \(\upmu\)m, 0.35 \(\upmu\)m and 0.7 \(\upmu\)m CMOS technologies. The simulation results show that the photodiodes with 0.18 and 0.35 \(\upmu\)m technologies are those with the highest peak quantum efficiency in the visible range (400-800 nm). Similarly, the photodiode in the 0.7 \(\upmu\)m technology proved to be the one with the lowest peak quantum efficiency, showing that, from the simulations, the greater the depth of the junctions, the lower the quantum efficiency in the visible range. Additionally, when comparing the different photodiodes' junction structures, the n+/p-substrate photodiode presented the best quantum efficiency curve in the visible range, which is in agreement with the literature. Regarding the dark current values and comparing with the literature, the results are in similar magnitude orders (pA and fA), but the variation of dark currents between technologies and p-n junctions is not entirely clear, being dependent both on the junctions' depth and on the dopant concentrations. Additionally, regarding the effect of oxide interference in the performance of the photodetectors, it was found that they significantly influence the quantum efficiency curves (both the number of layers, thickness and refractive index of the materials). Furthermore, the foundries where the photodiodes are manufactured also lead to slight variations in the fabrication parameters and, consequently, different performance of the photodiodes. These results demonstrate that, even with technologies that allow high integration, it is possible to obtain photodiodes with good performance features. Furthermore, the results obtained demonstrate that COMSOL is a tool that enables the numerical characterization of silicon photodiodes in CMOS with relative confidence. However, COMSOL still has some relevant limitations in the integration of optics and semiconductor physics, in particular when describing the expected decay in the quantum efficiency curves of p+/n-well photodiodes, in the 500-600 nm range. Nevertheless, based on the achieved results, the authors designed the layout of n+/p-substrate, n-well/p-substrate and p+/n-well photodiodes, aiming the fabrication and integration of such photodetectors into a multiphysics sensing CMOS chip based on 0.18 \(\upmu\)m technology. Integration into a detection and readout system for malaria diagnosis will be the next step of the development work [22]. Conceptualization, G.M.F., G.M. and S.O.C. ; methodology, G.M.F., V.S., G.M. and S.O.C. ; validation and analysis, G.M.F., V.S. and S.O.C. ; investigation, G.M.F. and V.S. ; resources, G.M. and S.O.C. ; writing--original draft preparation, G.M.F. ; writing--review and editing, G.M. and S.O.C. ; supervision, G.M. and S.O.C. ; project administration, S.O.C. ; funding acquisition, S.O.C. All authors have read and agreed to the published version of the manuscript. This work was supported by Project NORTE-01-0145-FEDER-028178 funded by NORTE 2020 Portugal Regional Operational Program under PORTUGAL 2020 Partnership Agreement through the European Regional Development Fund and the Fundacao para a Ciencia e Tecnologia (FCT), IP. This work was also supported by national funds, through the Portuguese FCT, under the reference projects UIDB/04436/2020 and UIDP/04436/2020. [PERSON] thanks FCT for the grant SFRH/BD/137529/2018 funded by COMPETE 2020. [PERSON] thanks FCT for her contract funding provided through 2020.00215.CEECIND. Not applicable. **Data Availability Statement**: All data are available upon request to the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.