Intrinsic Frequency Response of Silicon–Germanium Phototransistor Associated With 850-nm Multimode Fiber

Abstract : —Theintrinsic frequency response of Silicon-Germanium Heterojunction bipolar Photo-Transistors (HPT) at 850nm is studied to be implemented in multimode fiber systems. The experimental analysis of an HPT with an optical window size of 10x10µm 2 is presented. An Opto-Microwave Scanning Near-Field Optical Microscopy (OM-SNOM) is performed to observe the variation of the HPT dynamic behavior versus the illumination location of the phototransistor. The photocurrent generated by the photodiode at the interface between the n++ sub-collector and the p+ guard ring is analyzed and its impact on the performance of the HPT is investigated. Then, we propose a technique to remove the substrate photocurrent effect on the optical transition frequency (f Topt): the f Topt value of 4.1GHz given by raw measurement results increases up to 6GHz after removing the substrate response. The influence of the two-dimensional carrier flows on the HPT intrinsic Opto-Microwave (OM) behavior is also studied. Design aspects of SiGe/Si HPT structures are finally discussed as a conclusion.