Responsivity-Bandwidth Limit of Avalanche Photodiodes: Toward Future Ethernet Systems

Design of avalanche photodiodes (APDs) and their limiting factors on operating speed and responsivity are discussed with consideration of their application to optical receivers for optical-fiber communications systems. Each type of APD, with variable structures such as vertical illumination and waveguides, has inherent performance tradeoffs related to responsivity, carrier transit time, capacitance-resistance time constant, and avalanche build-up time. The key to designing APDs for application to optical-fiber communications systems is maximizing their responsivity and bandwidth according to the required bitrate and receiver sensitivity while taking into account their reliability. The design consideration and performance of our recent APDs targeting the 50-GBd operations are presented. Device structure, including geometry, epitaxial-layer configuration, and materials, is carefully chosen for the required bitrate and sensitivity. The proposed APD employs an inverted p-down configuration with a vertical-illumination structure. A thin InAlAs avalanche layer contributes to large gain-bandwidth product and a hybrid absorber combined with a triple-mesa inverted p-down structure enables maximum 3-dB bandwidth of 35 GHz and responsivity of 0.7 A/W. The proposed APD successfully demonstrated 50-GBd transmission over a 40-km single-mode fiber. In addition, some attempts to break the performance limits on traditionally used materials and structures for APDs are introduced.