Low-Cost and Low-Voltage Si/SiGe Phototransistor With High Responsivity at 900nm for Microwave Photonics Applications

This letter presents performances of Si/SiGe heterojunction bipolar phototransistors (HPT) used for microwave photonics communication receiver link around 850nm. SiGe phototransistors are designed in an industrial 55-nm SiGe BiCMOS from STMicroelectronics without any change in the layers technology process. Different horizontal geometries of HPTs and biasing techniques were evaluated. The static responsivity of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20\times 20~ {\mu }$ </tex-math></inline-formula>m2 optical window HPT reached 20.7 A/W at 900nm. This is the highest static responsivities reported for a fully-integrated Si/SiGe HPT with vertical illumination. A 1.6-GHz bandwidth for a 10.5-A/W static responsivity was achieved for a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times 5 {\mu }$ </tex-math></inline-formula>m2 optical window HPT using current bias.