# Advances in High-Speed, High-Power Photodiodes: From Fundamentals to Applications
## 3 Solutions of Bandwidth-Responsivity Trade-Off  
### High-Reflected Mirrors for Photodiodes  
High-performance PD based on III-V compound semiconductors serves as a key component and has already been demonstrated in various applications in wireless communications and radio-over-fiber and antenna systems [5]. The key parameters of PDs, which are responsivity, bandwidth, and saturation power, have been well designed for increasing one of them. For tackling the trade-off between bandwidth and responsivity in PD, several kinds of structures have been reported. The first structure is the resonant cavity enhanced (RCE) structure [33]. By selecting distributed Bragg reflectors (DBRs) as a bottom mirror in the PD, the incident light is reflected to the active layer, to achieve secondary absorption, as shown in Figure 19a. The detailed requirements for DBR design, which is also amenable to the RCE scheme, i.e., many number of different semiconductor material combinations, have been already reported in a review article [153]. Based on the reflection theory of RCE, many kinds of reflected mirror models, as shown in Figure 19b-f, which are used in vertical-mesa and waveguide types of PDs to improve the responsivity without sacrificing the bandwidth, have been proposed in the last 30 years. Most of the fabrication of reflected mirrors are the material combinations of DBRs (Figure 19b) [154; 155; 156], while periodic (or non-periodic) strip or concentric circular subwavelength gratings (SWGs), two-dimensional square bulk SWGs  
Figure 18: Two mW RF output power UTC–PD on SiC substrate in 300 GHz band [134]. (**a**) UTC–PD chip and the packaged module; (**b**) schematic of UTC–PD and effective–medium (EM) waveguide, while showing the electrical field distribution; (**c**) UTC–PD chip on SiC, where a short-stub matching circuit and a metal–insulator–metal (MIM) capacitor is used to increase the output power; (**d**) UTC–PD chip RF output power and photocurrent at 300 GHz; and (**e**) RF output power and photocurrent at 273 GHz for the packaged module (UTC–PD chip integrated with tapered slot antenna). (Figure 19c) [161; 166; 167; 168; 169; 170; 171; 172; 173; 174], Au mirrors [149], and even the scattering structure formed by a periodic cylindrical particle array [175], also can be employed as reflected ones. Those types of mirrors are usually either on the PD bottom (i.e., bottom mirrors) realized by direct epitaxial growth by metal-organic chemical vapor Deposition (MOCVD) or quasi-monolithically integrated using micron-level-thick benzocyclobutene (BCB) bonding (or integrated [69]) techniques, or on the top of the PD (i.e., top mirrors) implemented by employing the metal contacts (or strip gratings) by magnetic sputtering (or electron beam evaporation) or the flip-chip method with chemical etching [158; 176; 177; 178]. Figure 20 shows PIN-PD and UTC-PDs integrated or quasi-integrated (BCB bonding) with two kinds of bottom mirrors [34; 35; 140; 157; 179] It can be seen from Figure 20a that the GaAs-based PIN-PD is integrated with 4 DBRs (three GaAs/AlGaAs DBRs and one Si/SiO\({}_{2}\) DBRs) forming a four-mirror and three-cavity(M4C3) structure operating at 1550 nm [157]. The fabricated M4C3 structure achieves 70% peak quantum efficiency, 36 GHz 3-dB bandwidth, and quite a narrow spectrum linewidth (full-width at half-maximum (FWHM)) of 0.75 nm which is well-suited to high-density wavelength division multiplexing (WDM) communication systems [155]. The HR-UTC-PD [34], as shown in Figure 20b, is quasi-integrated with high-reflectivity (HR) DBR mirrors, which makes the responsivity increase by 23.2% without sacrificing the 3-dB bandwidth. As depicted in Figure 20c, the FR-UTC-PD [35] is quasi-integrated with a focusing reflection (FR) mirror, which is designed with a non-periodic concentric circular high-contrast grating (NP-CC  
Figure 19: Photodiodes (PIN–PDs or UTC–PDs) are integrated or quasi–integrated with different kinds of bottom mirrors or reflectors.