Edge illuminated SiGe heterojunction phototransistor for RoF applications

The first edge illuminated SiGe phototransistor based on the available commercial SiGe/Si BiCMOS technology for a low-cost detector or mixer in radio-over-fibre applications. Its technology and structure are described. Edge mapping of the phototransistor performances, so as to observe the fastest and the highest sensitive areas of the structure, is performed. The phototransistor exhibits a cutoff frequency of 890 MHz and low frequency responsivity of 0.45 A/W at 850 nm. This new SiGe/Si phototransistor structure provides a cutoff frequency double that of a top illuminated SiGe/Si phototransistor obtained using the same BiCMOS technology.