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

The intrinsic frequency response of silicon- germanium heterojunction bipolar phototransistors (HPTs) at 850 nm is studied to be implemented in multimode fiber systems. The experimental analysis of an HPT with an optical window size of 10 × 10 μm
²
is presented. An opto-microwave (OM) scanning near-field optical microscopy 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
⁺⁺
subcollector 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
): fTopt value of 4.1 GHz given by raw measurement results increases up to 6 GHz after removing the substrate response. The influence of the 2-D carrier flows on the HPT intrinsic OM behavior is also studied. Design aspects of SiGe/Si HPT structures are finally discussed as a conclusion.