Substrate diode effect on the performance of Silicon Germanium phototransistors

This paper provides a study on the substrate effect on the opto-microwave behavior of Silicon-Germanium Heterojunction bipolar Photo-Transistors (HPT). An Opto-Microwave Scanning Near Field Optical Microscopy (OM-SNOM) is performed to observe the distribution of photocurrent and dynamic behavior over the structure of the phototransistor. The photocurrent generated in the photodiode created by a n++ sub-collector and p+ substrate is extracted and analyzed. A maximum substrate diode current of 700μA is observed at 850nm with a related cutoff frequency of 0.42GHz. We have extracted low frequency responsivity (at 50MHz) bandwidth product of 109.2 MHzA/W. Finally, this study will provide a design guide line for Si base phototransistors.