Study of lateral scaling impact on the frequency performance of SiGe Heterojunction Bipolar Phototransistor

The influence of the lateral scaling such as emitter width and length on the frequency behavior of SiGe bipolar transistor is experimentally studied. Electrical transistors of different emitter sizes are designed and fabricated by using a commercial bipolar transistor technology. The effect of peripheral current and collector current spreading on electrical bipolar transistor performances are analyzed in regards to the state of the art. Furthermore, the lateral scaling effect on SiGe phototransistor electrical and opto-microwave frequency behavior is studied. The impact of the lateral flow of photo-generated carriers toward the optical opening in phototransistor structure is investigated. Moreover, the 2-D carrier flow effect on the opto-microwave frequency behavior of the phototransistor is characterized through optomicrowave scanning near-field optical microscopy measurements, in the course of which the intrinsic parameters, such as transit time and junction capacitances are extracted over the surface of the phototransistor. An intrinsic optical transition frequency of 6.5 GHz is measured for 10 × 10 μm2

Improving the opto-microwave performance of SiGe/Si phototransistor through edge-illuminated structure

This paper demonstrates the experimental study of edge and top illuminated SiGe phototransistors (HPT) implemented using the existing industrial SiGe2RF Telefunken GmbH BiCMOS technology for opto-microwave (OM) applications using 850nm Multi-Mode Fibers (MMF). Its technology and structure are described. Two different optical window size HPTs with top illumination (5x5µm 2 , 10x10µm 2) and an edge illuminated HPTs having 5µm x5µm size are presented and compared. A two-step post fabrication process was used to create an optical access on the edge of the HPT for lateral illumination with a lensed MMF through simple polishing and dicing techniques. We perform Opto-microwave Scanning Near-field Optical Microscopy (OM-SNOM) analysis on edge and top illuminated HPTs in order to observe the fastest and the highest sensitive regions of the HPTs. This analysis also allows understanding the parasitic effect from the substrate, and thus draws a conclusion on the design aspect of SiGe/Si HPT. A low frequency OM responsivity of 0.45A/W and a cutoff frequency, f-3dB , of 890MHz were measured for edge illuminated HPT. Compared to the top illuminated HPT of the same size, the edge illuminated HPT improves the f-3dB by a factor of more than two and also improves the low frequency responsivity by a factor of more than four. These results demonstrate that a simple etched HPT is still enough to achieve performance improvements compared to the top illuminated HPT without requiring a complex coupling structure. Indeed, it also proves the potential of edge coupled SiGe HPT in the ultra-low-cost silicon based optoelectronics circuits with a new approach of the optical packaging and system integration to 850nm MMF

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

An 850 nm SiGe/Si HPT with a 4.12 GHz maximum optical transition frequency and 0.805A/W responsivity

A 10 × 10 μm2 SiGe heterojunction bipolar photo-transistor (HPT) is fabricated using a commercial technological process of 80 GHz SiGe bipolar transistors (HBT). Its technology and structure are first briefly described. Its optimal opto-microwave dynamic performance is then analyzed versus voltage biasing conditions for opto-microwave continuous wave measurements. The optimal biasing points are then chosen in order to maximize the optical transition frequency (fTopt) and the opto-microwave responsivity of the HPT. An opto-microwave scanning near-field optical microscopy (OM-SNOM) is performed using these optimum bias conditions to localize the region of the SiGe HPT with highest frequency response. The OM-SNOM results are key to extract the optical coupling of the probe to the HPT (of 32.3%) and thus the absolute responsivity of the HPT. The effect of the substrate is also observed as it limits the extraction of the intrinsic HPT performance. A maximum optical transition frequency of 4.12 GHz and an absolute low frequency opto-microwave responsivity of 0.805A/W are extracted at 850 nm

Improved Nonlinear Model Implementation for VCSEL Behavioral Modeling in Radio-Over-Fiber links

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Flexible New Opto-Microwave Design Approach for Radio-Over-Fiber Applications: A Case Study of Low-Cost 60-GHz VCSEL-Based IF-RoF Link

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Use of SiGe Photo-Transistor in RoF links based on VCSEL and standard single mode fiber for low cost LTE applications

This paper presents for the first time the transmission of LTE signal based-on Radio-over-Fiber (RoF) scheme by using SiGe Hetero-junction Photo-Transistor as photo-receiver. The photo-transistor is fabricated based-on the commercial bipolar transistor technology. An 850 nm single mode Vertical Cavity Surface Emitting Lasers (VCSELs) and Standard Single Mode Fiber (SSMF) are used to complete the link so that makes the total cost of the system very low. The transmission of 20 MHz bandwidth LTE signal with high modulation format is achieved. The performance of the system is evaluated through the measurement of Error Vector Magnitude (EVM) under different operation conditions of the photo-transistor (in photodiode mode, photo-transistor mode and two terminals mode). We reach an EVM below 8% for the input power of -25 dBm to -2 dBm when the photo-transistor operates under two terminals condition. This EVM value fulfills the requirements of the LTE standard for 64-QAM modulation. The stability of the system with regard temperature variation and modal noise is also presented

Single Channel Microwave Photonics Digital Beamforming Radar Imaging System

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SiGe Photo-Transistor for Low-Cost SSMF-Based Radio-Over-Fiber Applications at 850nm

This paper investigates the possibility of implementing SiGe Hetero-junction bipolar Photo-Transistor (HPT) within low cost and low power consumption Radio-over-Fiber (RoF) systems based on standard single mode fiber (SSMF) operating in the first optical window for wireless communication networks scenarios. The SiGe HPT under study has potential lower cost compared to standard PIN photo-detectors as it is fabricated through the consolidated SiGe bipolar transistor process technologies, allowing the possible integration with Si-based circuits. Various operation modes of the photo-transistor (such as three-terminals and two-terminals) are investigated, analyzing their impact on the SSMF-based RoF link in terms of gain and noise (including modal noise fluctuations) for a frequency range up to 2.5 GHz. A 20 MHz LTE transmission is finally demonstrated as example of possible applications of the studied RoF link