# Advances in High-Speed, High-Power Photodiodes: From Fundamentals to Applications
## 1 Introduction  
High data rate and high-quality transmission, large-capacity data storage, wireless short-range links, sensing, and imaging bring new challenges for current communication systems. Parts of the of microwave (MW), millimeter wave (MMW), and terahertz (THz) bands, which cover the frequency from 20 GHz to 3 THz, as shown in Figure 1, provide an appropriate choice for the above-mentioned challenges [1; 2; 3; 4]. The exciting MMW and THz bands (20 GHz\(\sim\)1 THz), which are located between the radio waves and light waves, combine both their qualities and advantages, including large available bandwidth, high information capacity, transparent for some kinds of materials, high-spatial resolution, smaller beam scattering than infrared and visible light, and greater safety for the human body than x-rays and gamma rays [5; 6; 7]. Based on the above-mentioned advantages of the frequency band, it would bring about many potential applications [7], such as high-capacity wireless links, short-range wireless communication systems, and inspection system, high-resolution imaging transmission, spectroscopy, remote gas sensing and detection, security system and astronomical radio telescope system. More-over, this frequency band would support 6G technology for a high data rate (100-100 Gbit/s), helping towards a fully connected world in the future. However, the large atmospheric attenuation (Figure 1) [8; 9] in wireless communication systems needs to be considered (i.e., the attenuation caused by the rain and fog, free-space propagation loss (FSPL), gas loss, and signal loss from fiber to chip [8; 9; 10; 11]) with the exception of focusing on the two key figures, i.e., the data rates or information capacityand transmission distance. To compensate for the FSPL limitation, high-gain and high-output power antennas or phased array antennas for beam forming/beam steering [12; 13] should be considered for both transmitter and receiver systems. For the transmitter in communication systems, using large-scale photonic integrated circuit (PIC) techniques has an unparalleled impact on the continually reducing footprint, coupling loss, low power consumption and cost, while gradually increasing stability, reliability, and functionality [14]. Photonics-based technology is not only used for the development of MMW to THz communication techniques in systems with lower loss in optical fibers, large bandwidth, and miniaturized device/system [6; 7; 8; 9; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25] but it can also reduce some drawbacks which are brought forth by electronics-based technology, including high propagation loss in coaxial cables, narrow bandwidth, incompatibility with system operations at high-frequency bands and bulky feeding methods [26; 27; 28; 29]. In this scheme, photonic-based techniques are derived from optical to RF signal conversions utilizing the photomixing method [30]. Since there is an output power limitation using a single photomixer, the InP-based semiconductor devices have been employed and led by a uni-traveling-carrier photodetector (UTC-PD) [31; 32], operating at a long fiber-optics communication band (1.31-1.55 \(\upmu\)m), which can be used to further enhance the output power up to a mW level around 300 GHz [9]. However, the new requirements of high data rate transmission in future 6G poses another challenge for THz systems. In addition, due to the lack of appropriate power amplifiers in the THz system transmitting terminally along with the difficulty of miniaturizing the monolithically integrated system employing Si-lens, the combination of high-speed, high-power UTC-PD (or UTC-PD array) and high-gain, high-output power antenna and/or arrays would be a better choice to tackle the power limitation problem. Furthermore, the optical-to-electrical (O/E) conversion efficiency and heat dissipation at a high bias voltage also needs to be considered.