## 3 Results and Discussion
Despite the relatively good agreement with the literature showing the overall increase of the quantum efficiency with the increase of the wavelength, which is expected for silicon photodetectors, the numerical model of the p+/n-well photodiode fails to show, through the simulations, the expected decay in the 500-600 nm range, as predicted in Figure 2 and in the literature [1]. Figure 8: Current (nA) of n+/p-substrate, n-well/p-substrate and p+/n-well photodiodes in (a) 0.18 μm, (b) 0.35 μm and (c) 0.7 μm technologies in COMSOL. Figure 9: Responsivity (A/W) of n+/p-substrate, n-well/p-substrate and p+/n-well photodiodes in (**a**) 0.18 μm, (**b**) 0.35 μm and (**c**) 0.7 μm technologies in COMSOL. Figure 10: Quantum efficiency (%) of n+/p-substrate, n-well/p-substrate and p+/n-well photodiodes in (**a**) 0.18 μm, (**b**) 0.35 μm and (**c**) 0.7 μm technologies in COMSOL. Overall, the simulations showed that the 0.18 \(\upmu\)m technology presents a good performance characteristic within the visible optical range. Additionally, as previously stated, this technology is also the most suitable for integrating multisensing detection in the same chip (particularly high frequency acoustic sensors). In addition to the optical characterization plots, the dark current (at 300 K) was also obtained for each photodiode structure, as shown in Table 5. According to the results, the dark current of a photodetector is the result of balanced contributions from the channel length (related to the junction depth) and from the dopants' concentration. In particular, the dark current of a photodiode results from the contributions of the current due to carrier diffusion from the quasi-neutral regions, and the current due to generation in the space charge region. The major contributor for the dark current is the current due to generation in the space charge region, whose current density (\(j_{dc}\)) can be given by the approximation (14) [24; 25]. \[j_{dc}=\frac{q\cdot n_{i}}{2}\Bigg{(}\frac{x_{n}}{\tau_{0}^{n}}+\frac{x_{p}}{ \tau_{0}^{p}}\Bigg{)} \tag{14}\]  
where \(q\) is the electron charge, \(n_{i}\) is the intrinsic carrier concentration, \(\tau_{0}^{n}\) and \(\tau_{0}^{p}\) are the excess carriers' lifetime, and \(x_{n}\) and \(x_{p}\) are the n-side and p-side depletion widths, respectively. The width of the depletion region (\(x_{n}+x_{p}\)) is inversely proportional to the space between the space charge region and the ohmic contact, and a wider junction depth leads to a wider space between this space charge region and the ohmic contact [25]. Thus, the larger the junction depth, the larger the space between the space charge region and ohmic contact, and the smaller the width of the depletion region, theoretically leading to a smaller current produced by the p-n junction in the absence of illumination, i.e., the dark current (accordingly to (13)). However, according to the results presented in Table 5 (and as previously mentioned), it can be observed that the dark current values depend not only on the channel length but also on the dopants' concentration (acceptor and donor concentration (\(N_{0}\))). Their concentration is inversely proportional to the excess carrier lifetime (\(\tau_{0}^{n}\) and \(\tau_{0}^{p}\)) and, consequently, for higher concentration of dopants, the lifetime of excess carriers is shorter, leading to higher dark currents [24; 25]. The dopant concentrations (acceptors and donors) vary significantly between the technologies. Thus, the results show that the dopant concentration also has a significant effect in the dark current photodiode response, alongside the junction depth. Additionally, the n+/p-substrate and p+/n-well photodiodes are the photodetectors with the highest dark current for all technologies. Nevertheless, all the simulated photodiodes show low dark currents (femtoamperes) in the numerical results, so it is expected that this interference can be neglected in experimental tests.