# Advances in High-Speed, High-Power Photodiodes: From Fundamentals to Applications
## 3 Solutions of Bandwidth-Responsivity Trade-Off
### High-Reflected Mirrors for Group-IV Photodiodes  
However, the PDs based on III-V compound semiconductors are not easy for CMOS compatibility, so the group-IV PDs are gradually paid more attention by researchers, which more easily enables electronic photonic integrated circuits (EPICs) with low-cost, compact, and large-scale integrations [190]. It is to be noticed that the Si-based PD cannot be directly used for the C-band and L-band fiber-optic telecommunication regimes due with the bandgap of 1.12 eV only resulting in a \(\sim\)1107 nm cutoff wavelength.  
Therefore, germanium (Ge) will play an important role for the above-mentioned telecommunication bands owing to a relatively small bandgap (less than 0.7 eV at room temperature), though its optical response rolls off rapidly beyond 1500 nm [191] while this defect can be solved by depositing the Ge-layer on the Ge-on-insulator (GOI) platform [190]. By introducing 0.16% tensile strain to the Ge active layer, the operation wavelength is  
Figure 21: Photodiodes with different kinds of micro–holes for enhancing light–matter interactions. (**a**) Schematic of Si–based photodiode with micro–holes [180]: (**a1**) PD with GSG–pad; (**a2**) scanning electron micrograph (SEM) of PD active region; (**a3**) SEM of square holes; (**a4**) SEM of hexagonal holes. (**b**) Microstructure PIN photodiode (MPIN–PD) [184]: (**b1**) schematic diagram; (**b2**) design of V–groove and center cone.  
extended to cover the entire range of telecommunication C- and L-bands (1530-1620 nm). The high refractive index Ge absorber with the low refractive indices of the bottom insulator and the deposited SiO\({}_{2}\) top layer separately serve as two mirrors which finally constitute a RCE-Ge PD on the GOI platform [190] to increase the responsivity, similar to the model shown in Figure 19b. Such RCE-Ge PD on GOI (i.e., GOI-PD) indicates that the responsivity can be greatly increased up to 0.15 A/W at 1550 nm while covering the entire C- and L-bands due to the resonant cavity effect.  
Meanwhile, germanium-tin (GeSn) PD on Si is another typical group-IV candidate for SWIR applications which only needs to incorporate Sn into Ge shrinking the direct bandgap and causing redshift of the absorption edge from C-band to the mid-infrared range and beyond [192; 193]. GeSn-based (2.5% Sn) PIN-RCE-PD on SOI use the buried oxide (BOX) and the deposited SiO\({}_{2}\) as the bottom and top mirrors forming multiple reflections to enhance light-matter interaction, which shows \(\sim\)0.4 A/W responsivity at 0 V bias voltage in the C-band, which is much higher than that of the conventional GeSn PIN-PDs with the values of 0.1-0.3 A/W [194]. The introduction of 10% Sn extends the operation wavelength to a 2 \(\upmu\)m band, where the metal-semiconductor-metal (MSM) RCE-GeSn-PD can work at 1550 nm and 2 \(\upmu\)m wavelengths by optimizing the cavity length. Based on the RCE resonance effects, a record responsivity of 0.43 A/W is obtained with a \(-\)3 V bias voltage at 2 \(\upmu\)m wavelength at room temperature, revealing that it is promising for CMOS-compatible photonic-integrated circuit applications covering the entire telecommunication bands in a 2 \(\upmu\)m range [195].