High responsivity phototransistor with body-strapped base in standard SiGe BiCMOS technology

A new two-terminal phototransistor structure in standard BiCMOS technologies is proposed in this work. By connecting the floating base terminal to the local body terminal, the parasitic photo-generated carriers in the substrate are used to bias the base and to enhance the response of the phototransistor. Samples have been fabricated in a commercial 0.18 μm SiGe BiCMOS process. While the device without base-body connection shows a respectable 3.7 A/W peak responsivity at 630 nm wavelength, the phototransistor with base-body connection reaches an impressive 75 A/W peak responsivity at 750 nm wavelength and the dark current density is only 0.5 μA/cm². Characterization of the substrate current in the phototransistors verified that the response enhancement is due to the carrier generation in the substrate.