## 4 High-Performance Photodetector Based on Graphene  
Due to its transparency in the telecommunications C-band near 1550 nm, silicon is inefficient in converting light into an electrical signal. Up to now, the mainstream of integrated photodetectors in silicon photonics for the communication wavelengths includes employing germanium (Ge) or monolayer graphene as photodetection material [53,54]. Ge-Si hybrid photodetectors have been widely studied and reached significant maturity thanks to their ability to absorb light in the telecommunications band near 1550 nm. High responsivity larger than 1 A/W has been achieved and an impressive bandwidth of 265 GHz has been demonstrated recently [53,55]. Compared to Ge/Si photodetectors, graphene/Si photodetectors hold great potential in reaching ultra large operation bandwidth thanks to the ultrahigh carrier mobility of graphene as well as its absorption ability in a broader wavelength range. For graphene/silicon photodetectors, there are three different mechanisms for the photodetection, which are photovoltaic effect (PV), photo-thermoelectric effect (PTE) and photo-bolometric effect (PB). The graphene photodetectors based on these three mechanisms are discussed in detail below. The PV effect relies on the separation of photoexcited electrons and holes by an applied electric field to generate photocurrent, which can be utilized by graphene/silicon photodetector structures with normal light incidence [53,54]. As the representative work of early endeavors in graphene-based photodetectors, [PERSON] et al. demonstrated that the graphene/Si photodetectors can reach 40 GHz [55]. Moreover, the intrinsic response time of graphene photodetectors is experimentally demonstrated to be 2.1 ps, indicating a high bandwidth of 262 GHz (Figure 3a) [56]. However, the responsivities of graphene-based photodetectors with normal light incidence are normally low. To increase the light-graphene interaction and obtain better performances, waveguide-based photodetectors have been extensively researched [18,46,57,58,59]. A typical waveguide integrated structure is shown in Figure 3b [46]. In their design, the graphene lies on top of the silicon slot waveguide as the absorption layer. Here, the graphene monolayer absorbs the light within the waveguide mode's evanescent field, and a responsivity of 0.273 A/W is measured. Meanwhile, hybrid integration of the silicon waveguide and graphene could also be fabricated at the wafer-scale, thanks to its low fabrication complexity [60]. The proposed photodetector holds a bandwidth of 41 GHz and the maximum responsivity is 46 mA/W. To further optimize the performance of the photodetector, [PERSON] et al. proposed a waveguide-coupled integrated graphene plasmonic photodetector (Figure 3c) [57]. The plasmonic slot waveguide is formed by two different metallic slabs with a gap of 120 nm. This structure induces subwavelength light confinement within the surface of the graphene, dramatically increasing light absorption within the graphene. Meanwhile, different types of metal cause different doping levels in the graphene, enhancing the internal electrical field in the gap and more effectively separating the photogenerated carriers, leading to a high responsivity. Owing to these two points, the device features responsivity up to 0.36 A/W and an operation bandwidth larger than 110 GHz. When the plasmonic gap becomes even narrower (less than 50 nm), there exist a competition between the PV and PB effect within the photodetection process, as [PERSON] et al. thoroughly investigated [61]. An impressively high responsivity of 0.7 A/W at 1310 nm is achieved based on their work with PB effect. The PB effect refers to the modification of the channel resistance by either a change in the number of carriers or a change of the temperature-dependent carrier mobility. Thus, the PB effect leads to a negative photocurrent due to the increased channel resistance caused by the smaller mean free path induced by a temperature change due to photon absorption.