Ultra-Compact Lateral Junction Germanium Photodiode with a Bandwidth of 136 GHz

As optical interconnects evolve toward high-bandwidth and high-density integration, germanium (Ge) photodiodes (PDs) are becoming increasingly significant for achieving high responsivity and bandwidth in compact configurations. Electrically, conventional vertical junction Ge PDs encounter challenges in reducing device area due to constraints in electrode deposition on the Ge, while optically, conventional evanescent-coupling scheme necessitates extended optical absorption lengths to ensure sufficient absorption efficiency. These structural compromises result in a large device area and increased junction capacitance, limiting further bandwidth enhancement. Here, we implement a lateral junction to reduce device area and junction capacitance, while adopting a butt-coupling structure to compensate for the responsivity degradation caused by reduced device area. The bandwidth is further enhanced by combining the gain peaking technique, which leverages the elevated resonant frequency enabled by the reduced junction capacitance and on-chip inductor. Experimental results demonstrate a 136 GHz bandwidth Ge PD with a responsivity of 0.66 A·W–1 at 1550 nm. Based on the soft-decision forward error correction threshold of 2 × 10–2, the measured sensitivities for 170 Gb·s–1 on–off keying, 270 Gb·s–1 four-level pulse amplitude modulation, and 375 Gb·s–1 eight-level pulse amplitude modulation signals are −4, −3, and 0 dBm, respectively. Due to its extremely small device area, the device achieves the highest responsivity density and bandwidth density, facilitating high-density integration. This work provides an ultracompact and large-bandwidth Ge PD, promoting low-cost and high-speed optical detection.