A Waveguide InAlAs / InGaAs Superlattice Avalanche Photodiode with a 120-GHz Gain-Bandwidth Product

An avalanche photodiode (APD) with both an ultra-wide bandwidth and a high quantum efficiency is attractive for use in compact, highly-sensitive photoreceivers in optical communication systems at a wavelength of 1.55 μm. APDs with superlattice (SL) avalanche multiplication structures [1, 2, 3] are advantageous for obtaining a large gain-bandwidth (GB) product because their SL multiplication layers have high ionization-rate ratios. However, it is difficult to obtain a wide bandwidth above 20 GHz, while keeping a high quantum efficiency in conventional top- or back-illuminated SL-APDs [11] because there is a tradeoff between the quantum efficiency and the 3dB bandwidth which is limited by the carrier-transit time. On the other hand, in a photodiode with a waveguide (WG) structure, the quantum efficiency is independent of the bandwidth because the incident light and photogenerated carriers travel in different directions [4].