Low-Voltage and High-Responsivity Germanium Bipolar Phototransistor for Optical Detections in the Near-Infrared Regime

Germanium (Ge) bipolar phototransistors with high-responsivity performance are demonstrated using a low-temperature selective Ge epitaxy process. Large photocurrent and optical response enhancement are achieved over a conventional p-i-n Ge photodetector, which is predominantly attributed to the current gain induced by transistor actions. When illuminated with a photon wavelength of 1.55 mum , a Ge phototransistor shows a large responsivity of ~ 2.0 A/W for a low operating bias VA of 1.0 V. Optical measurement results further show that good photoresponse could be achieved for a spectral range of 1.31-1.62 mum, making such a device a very promising option for optical detections in the near-infrared (including L -band) wavelength regime.