## 3 Results and Discussion  
Figure 7 represents the spatial distribution of the differences between the concentrations of n-type and p-type dopants (represented by the _semi.Na_ \(-\)_semi.Nd_ (1/cm\({}^{3}\)) variable in COMSOL Multiphysics) in a zoomed region on the top of the cross-sections of the n+/p-substrate (a), n-well/p-substrate (b) and p+/n-well (c) photodiodes. In these figures, the orange or red zones have a positive value of the _semi.Na_ \(-\)_semi.Nd_ difference, which means that they contain a higher concentration of p-type dopants (acceptors). The blue zones correspond to a higher concentration of n-dopants (donors), and because of that, the _semi.Na_ \(-\)_semi.Nd_ has a negative value. Such dopant distributions are in agreement with the expected behavior of each p-n junction. For all technologies, identical distributions to those shown in Figure 7 were obtained, varying only in the depth junctions and concentration of dopants. From these simulations, the results that characterize each photodiode for each of the technologies under study were obtained. Figures 8-10 presents the numerical results of the current, responsivity and quantum efficiency plots, in the 400 nm-850 nm spectral range (obtained by a parametric sweep of the light wavelength), for the n+/p-substrate, n-well/p-substrate and p+/n-well photodiodes using the 0.18, 0.35 and 0.7 \(\upmu\)m, respectively, in which the junctions' depths and the concentrations of dopants were set according to Tables 1 and 2, respectively. The spectral range 400 nm-850 nm was simulated, as it has been considered as the optical range needed for the optical detection system (as was the case in the development of a malaria diagnosis) [22]. Regarding the obtained results, the photodiode with the n+/p-substrate junction has a greater quantum efficiency across the spectrum for all simulated technologies. The n+/p-substrate photodiodes of the 0.18 \(\upmu\)m and 0.35 \(\upmu\)m technologies have a similar behavior and, thus, the choice also depends on the spectral range required for the target application. As the optical spectrum approaches the infrared region, the performances of the 0.35 \(\upmu\)m and 0.18 \(\upmu\)m photodiodes are very close, just slightly better for the 0.35 \(\upmu\)m technology photodiode. However, according to the simulations for the visible spectral range, the 0.18 \(\upmu\)m technology is the best option when a high quantum efficiency is required. Nonetheless, the 0.7 \(\upmu\)m technology also presents a curve with high quantum efficiency over the visible range, showing even higher quantum efficiency between 500-650 nm. The 0.7 \(\upmu\)m technology has the lowest peak quantum responsiveness and efficiency within the visible range for all structures. Comparing all the curves for each technology, the results obtained are in agreement with the literature [7; 23], since the n+/p-substrate photodiode seems to be the one with the highest quantum efficiency in the entire visible range of the optical spectrum. The n-well/p-substrate and p+/n-well photodiodes present, through the simulation, lower quantum efficiency values in the visible range of the spectrum. The n-well/p-substrate photodiode is the one with the lowest quantum efficiency over the  
Figure 7: Spatial distribution of the differences between the concentrations of n-type and p-type dopants (represented by the color range of the _semi.Na_ \(-\)_semi.Nd_ (1/cm\({}^{3}\)) variable, in COMSOL Multiphysics), at the 800 nm wavelength, in a zoomed region (around 10 \(\upmu\)m depth) on the top of the cross-sections of the n+/p-substrate (**a**), \(n\)-well/p-substrate (**b**) and p+/n-well (**c**) photodiodes in 0.18 \(\upmu\)m technology. entire spectrum.