# Advances in High-Speed, High-Power Photodiodes: From Fundamentals to Applications
## 4 Photodiode Photonic-Integrated Applications
### Photodiodes Integrated with Antennas  
Another photonic-integrated device is PD with antennas for increasing output power for practical use. The general schematic diagram of the photonics-based transmitter and receiver is shown in Figure 23. One of the unique performances of UTC-PDs [44; 54] is both a large bandwidth and high saturation output power, which can be used as an MMW signal emitter [123] for wireless applications. To obtain V-band-to-G-band (50 GHz-220 GHz) photonically driven emitters and extend applications to MMW and THz signal wireless links, high-power photonic-integrated emitters with three types of planar antennas have been proposed [22; 202], as shown in Figure 24. The high-speed and high-power broadband CC-MUTC-PD [78; 101] with 0.2 A/W responsivity and 12 dBm output power is used for the MMW and THz emitter design. The fabricated emitter with the sinuous antenna and 8 um PD shows a record output power of 20 dBm at 90 GHz with a photocurrent of 20 mA while achieving an output power of  
Figure 23: General schematic diagram of (**a**) PD photonic–integrated emitter and (**b**) receiver. Figure 22: Photonic MMW generator (PMG) equivalent circuits and fabricated micrographs [196]. (**a,d**) PMG–A works at W–band; (**b,e**) PMG–B works at W–band; (**c**) relative output power comparisons for PMG–A, PMG–B, and UTC–PD without PMG; and (**f**) PMG works at J–band [168]. 9.9 dBm at 70 GHz, 11.5 dBm at 100 GHz, all with 7 mA photocurrent, for log-periodic and spiral antennas, respectively [22]. Using flip-chip technology, UTC-PD can be mounted on a planar slot antenna on the Si substrate [123; 124; 203], as shown in Figure 25a, and then boned on a hemispherical Si-lens to collimate the MMW signal direction, which generates a larger than 0.5 mW output power from the PD chip while radiating a bigger than 0.3 mW power into the free space, at a 120 GHz bandwidth [123; 124]. Meanwhile, this design indicates the potential applications in future wireless communication, sensing, radar, measurement, and imaging systems based on photonic-assisted MMW generation and transmission techniques. Beyond the F-band, the electrical transmission line will bring loss and reflection, so the miniaturized PD-integrated antenna with a quasi-optical configuration (Figure 25b) is necessary for sub-MMW signal handling [204]. As shown in Figure 25c, a UTC-PD integrated with a log-periodic antenna achieves a saturation output power of 300 mW at a photocurrent of 20 mA (Figure 25d) [205]. Using a similar configuration, the UTC-PD integrated with a log-periodic antenna exhibits a high output power of 2.6 mW at 1.04 THz with a saturation current of 13 mA [204]. However, a single PD integrated with an antenna suffers some limitations, including the limited emitted power resulting from the saturation of PD and the limited antenna directivity due to the finite effective aperture of the planar antenna [206]. Here, four InP-based photodiodes operating at 300 GHz, which are integrated with a linear bowtie antenna array mounting on the Si-lens, are proposed, as shown in Figure 26, to increase the radiation power. Compared to the straight radiation of a single emitter, the array emitter realizes a 10.6 dB higher power (theoretical gain: 12 dB), while the 3-dB beamwidth is reduced by 8.5\({}^{\circ}\) and up to 22.5\({}^{\circ}\), which achieves the output power increase and higher directivity. These photonic-integrated antenna arrays provide new solutions to wireless communications beyond 5G and will further exploit combinations of other photonic devices for application to future complex MMW and THz transceivers. Figure 24: Schematic diagram of CC–UTC–PD photonic–integrated emitter with Si–lens, the three different planar antennas, and the top/bottom views of the fabricated emitter [22]. In addition, a resonant antenna integrated with UTC-PD is another excellent candidate to be employed for output power radiation in the THz bands [207; 208; 209].