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

Abstract

The intrinsic frequency response of silicongermanium 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 optomicrowave (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 ): f Topt 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.Index Terms-Microwave photonics, opto-microwave scanning near-field optical microscopy (OM-SNOM), siliconbased photodetectors, SiGe phototransistor, substrate effect.

Substrate resistivity influence on silicon–germanium phototransistor performance

“…HPT mode OM responsivity (in A/W) topological map of 10 × 10 µm 2 phototransistor fabricated based‐on a HR silicon substrate [10] b LR silicon substrate (new result)…”

“…In [6,7], the phototransistor performances are investigated through optoelectronic compact circuit modelling. Regardless of the resistivity of the silicon substrate, the existence and impact of substrate photocurrent on the speed of SiGe HPTs were demonstrated in [8][9][10]. However, the effect of the silicon wafer resistivity on the performance of the HPTs, by comparing high-resistivity (HR) and low-resistivity (LR) silicon substrate technologies, has never been studied.…”

“…SiGe/Si HPT structure and technology: We use the commercial SiGe2RF Telefunken GmbH technology to fabricate SiGe/Si HPTs as it is presented in [10]. In our previous works, we used HR silicon substrate with a resistivity of 2000 Ω•cm to realise the HPTs [10,11]. However, the phototransistor under study is realised on the LR silicon wafer, i.e.…”

“…20 Ω•cm. The detailed schematic and specifications of the bipolar transistor technology used are the same as presented in [5,10,11]. The HPT fabrication does not modify the vertical stacks of layers of the standard bipolar transistor (HBT) technology.…”