With the rapid advancement of the Internet of Things, Industry 4.0, and artificial intelligence, the global demand for communication capacity has grown exponentially. Looking toward the future, the industry is focused on researching and exploring the next-generation mobile communication technology (6G). 6G aims to leverage low, medium, and high-frequency spectrum resources for seamless global coverage, achieving peak data rates of Tbit/s. Photonic terahertz communication, offering advantages such as large bandwidth, low loss, and seamless integration with fiber optic networks, stands as a pivotal technology for future terahertz communications. Terahertz photodetectors play a critical role in these systems and have attracted considerable attention. In this paper, we summarize recent advancements in III-V terahertz photodetectors, germanium-silicon terahertz photodetectors, and heterogeneous integrated terahertz photodetectors within the communication band. This paper provides detailed insights into structural optimizations, advancements in fabrication technologies, and breakthroughs in achieving high bandwidth and power output for these photodetectors. By reviewing these developments, we aim to provide valuable guidance for the future development of high-speed, high-power photodetectors.

Photodetectors can be divided into two types based on the input light mode surface-incident and waveguide-incident photodetectors. From a bandwidth perspective, traditional surface-incident PIN photodetectors face a trade-off between bandwidth and responsivity. Increasing the thickness of the intrinsic absorption layer improves responsivity but also increases the transit time of photogenerated carriers, resulting in reduced bandwidth. Thus, bandwidth and responsivity are mutually limiting and difficult to balance. To achieve terahertz bandwidth, the thickness of the absorption layer is typically reduced, which decreases the transit time and reduces the device geometry to create terahertz photodetectors. However, the inherent bandwidth-responsivity trade-off limits further improvement in the device’s bandwidth-efficiency product. In contrast, waveguide-incident photodetectors overcome this inherent trade-off by decoupling the optical absorption process from the carrier transport process. The surface-incident III-V PIN photodetector achieves a bandwidth of 110 GHz and a bandwidth-efficiency product of 35 GHz. The latest waveguide germanium-silicon PIN photodetector achieves a bandwidth of 240 GHz and a bandwidth-efficiency product of 86.5 GHz, with a maximum bandwidth of 265 GHz (Fig. 15). From a power perspective, under high optical power conditions, photogenerated electron-hole pairs accumulate in the depletion region or at the heterogeneous interface, forming space charges. This reduces the electric field strength in the depletion region, causing the electric field to collapse, reducing the carrier drift velocity, and ultimately leading to a significant reduction in device bandwidth, responsivity, and output power. Compared to surface-incident photodetectors, waveguide-incident photodetectors experience severe local space charge effects that significantly limit their saturated output power. Traditional PIN photodetectors are limited by the slow drift velocity of the holes and strong space charge effects, which limit the output power of the device. To mitigate the space charge effect, a uni-traveling-carrier photodetector has been proposed. In this structure, the optical absorption process is decoupled from the carrier drift process by P-type doping of the absorption layer. Photogenerated holes in the P-type doped absorption layer are collected by the P-electrode through relaxation oscillation, while photogenerated electrons move to the collection layer by diffusion and are then collected by the N-electrode. In this process, only photogenerated electrons drift as effective carriers. This structure decouples the optical absorption process from the photogenerated electron transport process. Since the electron drift speed is much higher than that of holes, this structure effectively reduces the device transit time. At the same time, space charges in the depletion region can be quickly transported to the electrodes, effectively suppressing the space charge effect. On this basis, III-V uni-traveling-carrier photodetectors have achieved excellent performance by optimizing the epitaxial layer structure. The surface-incident uni-traveling-carrier photodetector achieves a bandwidth of 330 GHz, and provides an output power of -3.2 dBm@320 GHz (Fig. 12). The waveguide uni-traveling-carrier photodetector achieves a bandwidth of 220 GHz, with an output power of -1.69 dBm@215 GHz (Fig. 10). As system functions become richer and performance indicators improve, the demand for device integration increases. Therefore, integrated terahertz photodetectors and multifunctional integrated chips based on terahertz photodetectors are important research directions in photonic terahertz communication. By integrating high-performance photodetection, photonic terahertz signal processing, and high-performance terahertz signal generation functions into a single chip, system cost and power consumption would be greatly reduced, and system performance is improved. Integrated photodetectors with bandwidths of 70, 155, and 110 GHz have been implemented on SOI, silicon nitride, and thin-film lithium niobate platforms, respectively (Table 1).

The terahertz photodetector is a core device in terahertz optical communication systems. We summarize the recent progress in III-V terahertz photodetectors, germanium-silicon terahertz photodetectors, and heterogeneous integrated terahertz photodetectors. III-V materials with high electron mobility are widely used in the fabrication of terahertz photodetectors. At present, terahertz photodetector devices are relatively small in size and are usually fabricated by electron beam lithography. This method is highly precise and capable of producing ultra-small devices, but it is difficult to scale up for mass production. Therefore, it is necessary to develop stepper exposure methods based on ultraviolet/deep ultraviolet lithography to fabricate large-scale, high-yield terahertz photodetector chips. For germanium-silicon photodetectors, a terahertz bandwidth germanium-silicon photodetector has been realized using custom technology. The bandwidth of germanium-silicon photodetectors based on general processes has reached 103 GHz (Table 1). The device bandwidth is expected to be further improved by optimizing the device structure and electrode configuration. Heterogeneous integrated photodetectors based on wafer bonding or epitaxial growth have demonstrated 100 GHz bandwidth. The next step is to achieve ultra-large bandwidth and multi-function integration of heterogeneous integrated chips. This will meet the bandwidth, output power, responsivity, and integration requirements of photonic terahertz communication systems.