# Review of CMOS Integrated Circuit Technologies for High-Speed Photo-Detection
## 3 Si-Based Photodetectors
### Si PD  
Silicon is a good waveguide material for telecommunication wavelengths due to its large bandgap energy equivalent to the energy of light with the wavelength of approximately 1100 nm. Therefore, ironically, silicon cannot be a good absorber for wavelengths longer than 1100 nm. Fortunately, some promising results have been obtained by enabling a sub-bandgap detection without introducing any other materials. It is known that silicon can also detect light of wavelength longer than 1100 nm if crystal defects are present, which cannot be easily explained theoretically, but can be proven experimentally [58; 59; 60; 61; 62]. By deliberately introducing defects into a p-n diode in a waveguide structure, the sub-bandgap detection in silicon can be realized. In [58], a silicon waveguide PD is implemented, wherein the quantum efficiency is enhanced by applying ion implantation. The implemented waveguide PD is illustrated in Figure 18. The diode is formed on a SOI substrate using a standard CMOS process. After the formation of the waveguide PD, the ion implantation is conducted to introduce the crystal defects. The lengths of the waveguides are 0.25 and 3 mm. The PD with a waveguide of length 0.25 mm shows a responsivity of sub-0.1 A/W at low reverse voltages; however, the responsivity can approach 1 A/W by increasing the reverse voltage at the cost of increased dark current, thereby degrading the minimum detectable power (MDP). The PD with a waveguide of length 3 mm can absorb virtually all the light, thus enhancing the responsivity at lower reverse voltages. However, the increased length directly results in higher junction capacitances, which severely degrades the frequency response. The estimated bandwidths of the PDs with the waveguides of lengths 0.25 and 3 mm are 10-20 GHz and 2 GHz, respectively. Further research advances the performance of the silicon waveguide PD through several process optimizations, providing a bandwidth of \(>\)35 GHz and an internal quantum efficiency of 0.5 to 10 A/W [59].  
As clearly observed in [58; 59], the silicon waveguide PD suffers from the tradeoff between the quantum efficiency and the bandwidth. In order to overcome this severe tradeoff, the work in [60] suggests that the adoption of a ring resonator can enhance the absorption with significantly reduced device dimension. As shown in Figure 19, the resonator-enhanced PD achieves a responsivity of 0.14 A/W with a length 10 times shorter than that of the straight waveguide for the same responsivity. The dark current level is also maintained below 0.2 nA, thus exhibiting an MDP of only 1.4 nW. Moreover, a resistive heater is employed for tuning the resonance wavelength.  
The silicon PDs discussed so far are all based on the SOI platform. However, the realization of a silicon PD on the bulk CMOS platform would be essential for a truly monolithic silicon photonic receiver. In [61], starting with a bulk silicon substrate, a micro-ring resonator PD using polysilicon is demonstrated, achieving a responsivity of 0.15 A/W, a dark current level of 40 nA, and a gigahertz frequency response. Recently, the work in [62] achieves further improvements of overall performance by applying mid-level implants to a typical p-i-n structure. The recent advances in silicon PDs are significant because they simplify the overall process with a minimal addition of steps, indicating a better compatibility with the standard CMOS processes than their Ge-based counterparts. Especially, the silicon PD incorporated in a waveguide has a distinct advantage that the integration with a waveguide is readily realized, which is challenging for the Ge-based detectors. Furthermore, the resonator-based structure directly enables the WDM function, which is a desirable feature for highly dense interconnection [50].