Germanium photodetector hits 110 GHz bandwidth for 400-Gbps AI optical links

Researchers at Fudan University and Zhangjiang National Laboratory have built a germanium photodetector on a 300-mm CMOS silicon photonics platform that pairs a 110 GHz 3-dB bandwidth with 95.3% peak quantum efficiency, yielding a bandwidth-efficiency product of 104.8 GHz. The device, reported in Science Bulletin, also achieved 1.01 A/W responsivity at 1310 nm while keeping dark currents below 10 nA, a combination of speed, sensitivity and low leakage that the team says addresses the competing demands of next-generation optical interconnects for artificial intelligence and high-performance computing.

The rapid growth of large language models is driving data traffic among processors, memory and networking hardware, and silicon photonics moves that information using light. Germanium photodetectors on this platform must balance bandwidth, optical-to-electrical conversion efficiency, low dark current and compatibility with large-scale CMOS manufacturing, and improving one metric has typically come at the expense of another. The new device was designed to relax that trade-off through several structural choices rather than a single innovation.

At the core of the design is a semi-embedded germanium absorbing region, which strengthens the electric field, shortens the carrier transit distance and reduces 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, the researchers state, these features raise operating speed without sacrificing responsivity.

Wafer-level measurements across the 300-mm platform showed consistent performance. Most of 67 sampled reticles exhibited dark currents of 5 to 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 team obtained clearly open eye diagrams at 320 Gbit/s with PAM-4 modulation and 413 Gbit/s with PAM-6. Real-time filtering and feed-forward equalization were used only to compensate losses in the measurement link, not to enhance the device’s intrinsic performance.

The detector also maintained about 104 GHz bandwidth at -2 V and 80 GHz at -1 V, indicating usable speed at reduced bias voltages. Its responsivity remained close to 1.0 A/W from 25 to 85 degrees Celsius, and the device produced open 224 Gbit/s PAM-4 eye diagrams up to 85 degrees Celsius, a temperature range relevant to dense AI cluster hardware where thermal management is constrained. By combining these results with direct germanium epitaxy and a streamlined process flow, the authors position the photodetector as a scalable candidate for future single-lane 400-Gbps optical interconnects in AI and high-performance computing systems.

Subject of Research: Silicon photonics; germanium photodetectors; optical interconnects for AI computing

Article Title: Germanium photodetector targets 400-Gbps-per-lane optical links for AI clusters

Article References: Germanium photodetector targets 400-Gbps-per-lane optical links for AI clusters. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: germanium photodetector, silicon photonics, CMOS, optical interconnects, AI clusters, 400 Gbps per lane, PAM-4, PAM-6, bandwidth, quantum efficiency, Fudan University, Science Bulletin