# Advances in High-Speed, High-Power Photodiodes: From Fundamentals to Applications
## 3 Solutions of Bandwidth-Responsivity Trade-Off
### Microhole Design for Photodiodes  
Moreover, the application of micro-holes [180; 181; 182; 183; 184; 185] etched from the top until through the active regions is also an effective way to assist the light-matter interactions. Figure 21 shows PDs with different kinds of micro- or/and nano-holes in active layers for enhancing light-matter interactions to balance the frequency response and responsivity [180; 184]. Figure 21(a1,a2) reports a novel design that uses the etched periodic or photon-trapping micro- and nanoholes (600-1700 nm diameters) with square (Figure 21(a3)) or hexagonal (Figure 21(a4)) shapes that penetrate from the top mesa to the bottom contact layer in the Si photodiode [180]. Such a structure reveals that the lateral propagating slow and  
Figure 20: UTC–PDs integrated with different kinds of mirrors. (**a**) PIN–PD integrated with 4 DBRs forming a four–mirror and three–cavity (M4C3) structure [157]; (**b**) UTC–PD integrated with high–reflectivity (HR) DBR mirrors, using HR–UTC–PD for short [34]; (**c**) UTC–PD integrated with non–periodic concentric chiral high–contrast grating (NP–CC–HCG, i.e., focusing reflection (FR) mirror), referred to it as FR–UTC–PD [35]; and (**d**) symmetric–connected UTC–PD array integrated with 2D–SWG beam–splitter [140].  
stationary optical modes can be generated only using normal incident light, which promotes a well-rounded interaction of light with Si that is an order of magnitude greater compared with the same thick Si-film absorber while ensuring an ultrafast carrier transport. This design enables the Si-based PDs to monolithically integrate with CMOS and extends its applications for optical data links in data communications and computer networks [180].  
Given the 1550 nm wavelength application, microstructure PIN-PD (MPIN-PD) is proposed and analyzed numerically by employing an InP substrate, as shown in Figure 21b [184]. The well-designed central obconical shape surrounded by a V-groove trench not only greatly facilitates the interaction of light and InGaAs absorber, but also increases the device responsivity and bandwidth simultaneously. With the optimized dimensions of the V-groove and center cone, together with the optimized circular electrodes, the efficiency and bandwidth of MPIN-PD are separately increased by 61% and 21%. In addition, the back-to-back type of PD [186], waveguide-coupled PD [187], dual-absorber PD [58; 161; 169] and near-ballistic PD [79] also can be used to achieve high responsivity.  
With the evolution of the RCE scheme and tapered-holes structures, a novel model for vertical-mesa PD, as shown in Figure 19d, which is realized based on tapered holes and bottom mirrors, could be used to solve the trade-off between bandwidth and responsivity. For waveguide PDs, the responsivity could be improved by exploiting an SWG high-focused mirror which is made by periodic (or non-periodic) strip or concentric circular SWG bonded at the back of the device (Figure 19f), while the usage of DBR mirrors (Figure 19e) have already been simulated [188] and achieved experimentally [189] showing similar performance for the trade-off.