Monolithic Germanium/Silicon Photodetectors With Decoupled Structures: Resonant APDs and UTC Photodiodes

The Ge/Si system is useful to realize avalanche photodetectors (APDs) operating at 1310~1550 nm because of the intrinsic advantages of complementary metal-oxide-semiconductor (CMOS) compatibility, high light-absorption of Ge, low ionization rate ratio of silicon, and high thermal conductivities of Si and Ge. With the Ge/Si system, it is convenient to realize photodetectors with decoupled structures including resonant Ge/Si APDs as well as uni-traveling carrier (UTC) photodiodes. The resonant Ge/Si APD with a separated absorption-charge-multiplication (SACM) structure, which decouples the light absorption and avalanche process, has high speed, high gain, and high gain-bandwidth product. The UTC photodiode, which decouples the light absorption and the carrier collection, is useful for high-power applications. This paper first reviews the structure and model of decoupled Ge/Si (A)PDs, particularly, the equivalent circuit models for explaining the peak enhancement of the frequency response in resonant SACM APDs. This model is also applied to UTC Ge/Si PDs developed recently for the high-power applications.