The explosive growth in data traffic has driven the relentless pursuit of higher data transmission speeds and bandwidths. Photonic devices with ultrawide bandwidths are crucial for next-generation networks. Moreover, these ultrafast optoelectronic devices hold significant promise for the oncoming sixth-generation (6G) systems and millimeter-wave (MMW) / terahertz (THz) wireless communication systems.

With electron-only transportation in the drift layer, uni-traveling carrier photodiodes (UTC-PDs) have significant potential for applications in optical communication, data interconnection, and microwave photonics owing to their exceptional characteristics such as wide bandwidth and high saturation. The bandwidth of photodiodes (PDs) directly limits the maximum data transfer rate. PDs with high responsivity not only alleviate tight fiber alignment tolerances but also enable extended link distances, reduce overall power consumption, and contribute to enhancements in the dynamic range and a higher signal-to-noise ratio (SNR). However, for surface-illuminated PDs, there is a tradeoff between bandwidth and responsivity. For waveguide PDs, the direction of light transmission is perpendicular to the direction of carrier transport, and high responsivity and wide bandwidth can be achieved simultaneously by decoupling the absorption efficiency and carrier transport.The bandwidths of the PDs are determined by the carrier transmission time and resistance capacitance (RC) time constant. Epitaxial structures should be optimized to take advantage of the electron overshoot effect, thereby reducing the electron transport time in the drift layer. To address the constant RC limitation, improvements can be made not only by optimizing the epitaxial structure but also by reducing the parasitic capacitance and lowering the contact resistivity. In addition, high-impedance lines can be used to compensate for the device capacitance, and parallel resistors can be added to decrease the overall resistance.

The development of chips and packaging technologies has been introduced. A carefully designed waveguide coupling structure can enhance quantum efficiency. Edge-illuminated detectors have the simplest waveguide structure, in which the absorber serves as the waveguide core. However, incident light is concentrated at the front end of the absorption region, which can easily lead to local saturation, making it challenging to improve the saturation output characteristics. Evanescently coupled waveguide photodiodes can achieve relatively uniform light distribution along the absorber, thus improving their high-power handling capability. To improve the coupling efficiency between the waveguide and the input fiber, PDs integrated with spot-size converters are typically adopted. Rouvalis et al. fabricated PDs integrating a horizontal taper. Demiguel et al. proposed evanescently coupled PDs integrating a double-stage taper with a bandwidth of 40 GHz and high responsivity of 0.6 A/W. Umbach et al. designed waveguide PDs with vertically tapered waveguides (Fig. 2). Campbell et al. proposed highly responsive evanescently coupled PDs integrating short planar multimode waveguides that are more compact, less complicated, and easily fabricated (Fig. 4). Our research group has proposed a thick multilayer waveguide with a gradually increasing refractive index as a coupling waveguide (Fig. 6).

The bandwidths of the PDs are determined by the carrier transmission time and RC time constant. Epitaxial structures should be optimized to take advantage of the electron overshoot effect, thereby reducing the electron transport time in the drift layer. To address the constant RC limitation, improvements can be made not only by optimizing the epitaxial structure but also by reducing the parasitic capacitance and lowering the contact resistivity. In addition, high-impedance lines can be used to compensate for the device capacitance, and parallel resistors can be added to decrease the overall resistance. The performance of the waveguide PDs is summarized in Table 1.

For practical use in these systems, UTC-PD chips must be packaged into modules. Three packaging forms of detectors—coaxial output, waveguide output, and antenna radiation—and their effects on the output electrical signal are introduced. The performance of the reported modules is summarized in Table 2.

The continuous growth in data traffic requires high-speed detectors to match the required bandwidth. High-speed waveguide UTC-PDs can be utilized as receivers in high-data-rate optical communications. Additionally, they can serve as transmitters in MMW/THz wireless communications to deliver high-power and high-frequency microwave signals. Using waveguide optical coupling, multiple photonic devices can be integrated on a chip to create a high-density photonic integrated circuit.