image: The device combines a semi-embedded germanium absorbing region, optimized doping and octagonal contact pads. Wafer-level tests showed low dark current, high responsivity and 110 GHz bandwidth, while high-speed measurements produced open eye diagrams at 320 Gbit/s PAM-4 and 413 Gbit/s PAM-6.
Credit: © Wang Xu
Artificial intelligence and large language models are rapidly increasing data traffic among processors, memory and networking hardware. Silicon photonics can move this information using light, but its germanium photodetectors must balance bandwidth, optical-to-electrical conversion efficiency, low dark current and compatibility with large-scale CMOS manufacturing.
A team from Fudan University and Zhangjiang National Laboratory has developed a germanium photodetector on a 300-mm CMOS silicon photonics platform that addresses these competing requirements. The device achieved a bandwidth-efficiency product of 104.8 GHz, combining a 110 GHz 3-dB bandwidth with 95.3% peak quantum efficiency. It also reached 1.01 A/W responsivity at 1310 nm while maintaining dark currents below 10 nA.
The Ge PD uses a semi-embedded germanium structure to strengthen the electric field, shorten carrier transit distance and reduce junction capacitance. A triple-gradient doping profile lowers contact resistance while preserving the depletion region, octagonal contact pads suppress parasitic capacitance, and a lateral p-i-n junction limits metal absorption loss. Together, these features improve speed without sacrificing responsivity.
Wafer-level measurements showed consistent performance: most of 67 sampled reticles exhibited dark currents of 5-8 nA at -2 V, selected sites exceeded 0.97 A/W responsivity, and nine dies reached 110 GHz bandwidth at -3 V. In large-signal tests, the researchers obtained clearly open eye diagrams at 320 Gbit/s with PAM-4 and 413 Gbit/s with PAM-6. Real-time filtering and feed-forward equalization were used only to compensate measurement-link losses.
The device also maintained about 104 GHz bandwidth at -2 V and 80 GHz at -1 V. Its responsivity remained close to 1.0 A/W from 25 to 85 degrees Celsius, with open 224 Gbit/s PAM-4 eye diagrams up to 85 degrees Celsius. By combining these results with direct germanium epitaxy and a streamlined process, the work provides a scalable photodetector candidate for future single-lane 400-Gbps optical interconnects in AI and high-performance computing systems.