Impact-ionization-engineered waveguide Ge/Si avalanche photodiode with a record-high gain-bandwidth product of 5580 GHz

Abstract Avalanche photodiodes (APDs) featuring internal multiplication hold great promise for high-sensitivity detection in optical systems. The gain-bandwidth product (GBP) comprehensively characterizes APDs’ overall performance, intrinsically linked to the effective impact ionization ratio, namely the k factor. An ultralow k factor suggests an ultrahigh GBP in theory, underscoring that one of the keys for the APD design should focus on minimizing the k factor. Here, we propose and experimentally demonstrate a novel waveguide Ge/Si APD with a record-high GBP of 5580 GHz (more than 500% higher than those reported previously), corresponding to a high gain of 209 and a bandwidth of 26.7 GHz when operating under −30 dBm input optical power. Introducing a PN junction as the multiplication region leads to a highly non-uniform electric field distribution for impact ionization and enables impact-ionization engineering without requiring complex multi-layer epitaxy processes. The effective k factor is measured to be as low as 0.011, which is even much lower than the intrinsic value of bulk silicon. With the present APDs, very high-speed PAM4 signals with a bit rate of up to 160 Gbps are received successfully with a clear eye-diagram. The present APD is further packaged with a transimpedance amplifier chip for 50G-PON systems, successfully achieving a high sensitivity of −26.5 dBm for 50-Gbps NRZ signals. Such high-performance Ge/Si waveguide APDs developed with simple fabrication processes show great potential for photodetection across diverse application scenarios.