# Advances in High-Speed, High-Power Photodiodes: From Fundamentals to Applications
## 4 Photodiode Photonic-Integrated Applications
### Photodiodes Integrated with Antennas
Figure 27a shows the THz UTC-PD chip integrated with a twin-diploe antenna [209] using an impedance matching circuit [90] to enlarge the output power and the resonant behaviors are found exhibiting the peak output powers of 5.9, 3.8, and 1.1 \(\upmu\)W at 0.78, 1.04 and 1.53 THz (Figure 27b), while generating a record output power of 10.9 \(\upmu\)W at 1 THz with a photocurrent of 14 mA [209]. Moreover, the THz wave and THz beam steering provide great opportunities for free-space applications, including imaging, sensing, radar, and wireless communications in the 6G era and beyond [15; 135; 210]. Figure 27c shows a 4 \(\times\) 1 UTC-PD array integrated with a 4\(\times\)4 unidirectional slot antenna to generate THz signals based on coupled fiber beats techniques [211]. The adaptive genetic algorithm is used to control the produced THz beam steering and the experiment achieves a precise pointing towards the intended direction of the 300 GHz beam, which greatly improves the THz power efficiency by beam steering and paves the way for THz wireless communication using large-scale UTC-PD array [211; 212; 213]. With the development of communication techniques, the frequency range (MMW to THz: 30 GHz\(\sim\)3 THz) between microwave and far-infrared is set to meet the growing demand. Exploiting and making the most of the MMW and THz wireless signals [7; 214; 215] with high-radiated power have long been studied and generated by the electronic system [26; 27; 28] or photonic system [6; 8; 9; 15; 16; 17; 18; 19; 20; 21; 22; 23] or hybrid electronic-photonic systems [216; 217], which have shown a very diverse range of application in information and communications, security screening, food and agriculture, biometrics, medical and medicine, drug inspection, semiconductor wafer inspection, and air pollution [218; 219; 220]. Figure 25: (**a**) UTC–PD integrated with slot antennas used as an MMW signal emitter for free–space applications [123; 124]. (**b**) The fabricated quasi–optical UTC–PD with Si–lens module for practical use [204]. (**c,d**) UTC–PD integrated with log–periodic antenna (micrograph as shown in the inset) attains 300 \(\upmu\)W output power and 20 mA photocurrent at a 1.5 V bias voltage [205]. Given the photonic-generated THz signal, ultrafast broadband, and high-power PDs, such as UTC-PDs [44; 54] and traveling-wave PD (TW-UTC-PD) [221], are necessary. By combining the TW-PD structure for high responsivity and the UTC-PD structure for high output power, a new TW-UTC-PD is proposed for integration with antennas for MMW and THz signal radiation [21; 222]. The TW-UTC-PD separately integrates with the resonant, bowtie, and log-periodic antennas for generating THz signals, covering frequencies up to 1 THz. The radiated power at different photocurrent levels is shown in Figure 28, where the 4 \(\times\) 15 \(\upmu\)m\({}^{2}\) PD integrated with a resonant antenna achieves an RF output power of 148 \(\upmu\)W at 457 GHz and 24 \(\upmu\)W at 914 GHz with a 10 mA photocurrent (Figure 28a). Figure 28b shows that the RF output power of 105 \(\upmu\)W at 255 GHz and 10 \(\upmu\)W at 612 GHz with a 13 mA photocurrent are attained by a 2.5 \(\times\) 50\(\upmu\)m\({}^{2}\) PD integrated with a bowtie antenna while exhibiting 70 \(\upmu\)W at 150 GHz with a 9 mA photocurrent for integrating with a log-periodic antenna. Another type of PD for THz applications is related to the design, usage, and optimization of the hybrid absorber UTC-PD (i.e., modified UTC-PD, MUTC-PD) [72], as shown in Figure 29. The band diagram for this type of MUTC-PD is shown in Figure 29a, where the absorber consists of a non-depleted absorber with p-doping and a depleted absorber without doping. The maximum 3-dB bandwidth can be calculated while just comprehensively analyzing the numerical relation of thickness among depletion (W\({}_{\text{dep}}\)), non-depleted (W\({}_{\text{an}}\)), and depleted (W\({}_{\text{ad}}\)) absorbers.