## 1 Introduction  
Photodetectors are optical sensors that are increasing in importance, mainly due to their high ability for being miniaturized and on-chip integrated, in several devices for different application areas [1, 2]. In particular, photodiodes are optical sensors that convert light intensity into a photocurrent. Their operation principle is based on the photoelectric effect, where photons are converted into electron-hole pairs [3]. Photodiodes are typically characterized based on their generated current, dark current, responsivity and quantum efficiency features. The responsivity \(R\) (A/W) indicates the photodiode's ability to absorb incident light, relating the produced current (\(I_{ph}\)) to the incident optical power (\(P_{in}\)), as given by (1). The quantum efficiency (\(\varphi\)) describes the fraction of the incident optical power that contributes to the generated photocurrent, and it is given by (2), where \(h\) corresponds to the Plank constant (\(h=6.6261\times 10^{-34}\) J s), \(f\) is the incident frequency (Hz) and \(q\) the electron charge (\(q=1.6\times 10^{-19}\) C). \[R=\frac{I_{ph}}{P_{in}} \tag{1}\]  
\[\varphi=\frac{R\times h\times f}{q} \tag{2}\]  
Photodiodes can be microfabricated using Complementary Metal Oxide Semiconductor (CMOS) technology, which has enabled a major technological advance in the manufacturing of integrated sensors (namely optical sensitive surfaces), circuits and microchips in the last decades [3,4]. The reduction of the power consumption, the increase in the integration capacity and the low cost are strengths of CMOS technology, playing a key role in the most diverse areas of technology [5]. Over the years, CMOS has been the main competitor of CCD (Charge Couple Device) technology in the manufacturing of photodetectors. Although CCD is usually associated with higher quality images (due to their superior fill factor, when compared to the first CMOS devices), it is characterized by a high-power consumption and a large area per cell [6,7], when compared to CMOS technology. However, despite CMOS technology being more susceptible to noise, their low-power consumption is an advantage for portable and miniaturized devices. Additionally, back-illuminated CMOS photodiode sensors present improvements in the signal-to-noise ratio and in the fill factor, being able to compete with the CCD photodetectors in these features [8]. In addition to these features, CMOS active pixel sensors have also improved the speed and the signal-to-noise ratio of the CMOS sensors, despite having an even lower fill factor than the CCD detectors [9]. Moreover, CMOS photodetectors can be produced in the same substrate and same chip as their readout electronics. However, when compared to bipolar and other technologies used in microelectronics, CMOS technology has also some limitations. Unlike bipolar technology [5,7], CMOS manufacturing is more susceptible to contamination, due to the shallow diffusion on the silicon substrate and, therefore, even a small dust particle could compromise the entire process [5]. More recently, another technology has emerged, BiCMOS (or bipolar CMOS), to address the disadvantages presented by the CMOS and the bipolar technology. However, it presents a substantially higher cost by comparison, which makes it unfavorable for low cost and affordable processes and devices [7]. In this work, different CMOS vertical p-n junction photodiodes are studied, designed, simulated and compared for optical features. Besides the different designs of the photodiodes' p-n junctions, which will be further detailed, it is expected that, for the same p-n junction structure, their optical response also varies according to the used CMOS technology. Therefore, herein three CMOS technologies: the 0.18 \(\upmu\)m, the 0.35 \(\upmu\)m and the 0.7 \(\upmu\)m, are used for those studies. These values correspond to the minimum gate width of the metal-oxide-semiconductor field-effect transistors (MOSFETs) feasible by each technology.