# High-Speed Photodetectors for Microwave Photonics
## 4 High-Power Photodiodes on Si
### Ge-on-Si Photodiode Arrays  
For monolithic integration, germanium-on-silicon has been widely investigated for near-infrared optoelectronics, and despite a lattice mismatch of 4%, Ge heterogeneous epitaxy on Si substrates has been successfully demonstrated with sufficiently low defect densities [41]. Compared to III-V growth on Si [42], Ge-on-Si is more compatible with the Si CMOS process. In addition, Ge can be directly grown on Si without buffer layers, which makes optical coupling between passive  
Figure 12: (a) Device structure of heterogeneous MUTC PD on silicon-on-insulator (SOI); (b) Measured frequency responses for PDs with different area. The inset shows a schematic view of the PD. structures and Ge photodetectors easier. As a result, Ge-on-Si has become a well-developed platform for photonic integrated circuits [43]. Today, many foundries offer complete suites of technologies based on this platform, including the American Institute for manufacturing Integrated Photonics (AIM Photonics), IMEC, GlobalFoundries, Institute of Microelectronics (IME), STMicroelectronics, Taiwan Semiconductor Manufacturing Company (TSMC), and TowerJazz [44]. Ge-on-Si PDs with a low dark current, large bandwidth and high responsivity are well-developed in monolithic Si photonic platforms for digital applications [45; 46; 47], and large scale digital systems have been realized [48]. However, only a few Ge-on-Si PDs for high-power analog applications have been demonstrated. In addition to high responsivity and large bandwidth, PDs for microwave photonics applications should possess high-power handling capability and high linearity in order to achieve large RF gain and high spur-free dynamic range. For example, in order to achieve an RF link gain of 0 dB with a modulator with V\({}_{\pi}\) of 3 V, the photocurrent should be as high as 27 mA for an intensity-modulated direct detection link and 14 mA for a phase-modulated link. These photocurrents are much larger than the ones that typically occur in digital applications. The fact that the PDs need to maintain a large bandwidth and high linearity at these levels of photocurrents, puts stringent requirements on PDs in Si photonics. Lack of material versatility limits the design freedom of epi-layers in monolithic Ge-on-Si PDs. To mitigate this drawback, PD arrays have been proposed. In [49], 8 Ge PDs were connected in a traveling-wave fashion and the optical signal was fed from both sides of the active region in order to enhance the power handling capability. In ref. [50], four Ge PDs were connected in parallel in a compact array to enhance the RF output power. The optical image and schematic of the PD array are shown in Figure 13a. The device layout was designed using the AIM Si photonics Process Design Kit (PDK) [51]. Three optical waveguide Y-junctions were used to equally split the input optical power to feed the four PDs. The outputs of the PDs were connected in parallel to the shared ground-signal-ground (GSG) pad on the top dielectric layer through vias. The device was fabricated on a multi project wafer run in AIM's Si photonics foundry. The I-V characteristics of the PD array are shown in Figure 13b. The dark current is as low as 0.3 uA at \(-\)2 V and 1 uA at \(-\)5 V bias. The PD array has a high external responsivity of 0.58 A/W at 1550 nm wavelength (no anti-reflection coating at the waveguide edge coupler) when measured with a lensed fiber with 3 um spot size. The fiber-chip coupling loss was estimated to be 1.7 dB. The frequency response of the PD array was measured at \(-\)5 V bias at various photocurrents from 1 mA to 20 mA. The results are shown in Figure 14a. The 3-dB bandwidth is 15 GHz up to a photocurrent of 15 mA and remains as high as 12 GHz at 20 mA photocurrent. The RF output power of the PD array was measured at the 3-dB bandwidth frequency under different reverse voltages.