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

Abstract

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.

Study of Lateral Scaling Impact on the Frequency Performance of SiGe Heterojunction Bipolar Phototransistor

“…• Fragmentation of the HPT into smaller individual HPTs, as the electrical extension region may reach to the fragmented device size limit. • Considering laterally illuminated structures [22], [23] to improve the HPT opto-microwave speed.…”

Lateral GeSn waveguide-based homojunction phototransistor for next-generation 2000 nm communication and sensing applications

“…Moreover, avalanche PDs have also been reported for 2000 nm bands [25] but they require high reverse-biased voltage (more than −1 V) and also possess high noise which hampers the sensitivity of the device [26]. On the other hand, the common-emitter (C-E)-based three-terminals (3-Ts) phototransistors (PTs) can provide extremely high responsivity (due to its internal current gain) and 3 dB bandwidth while being biased with low voltages [27][28][29]. To this extent, in recent years a few research groups have focused on the design and fabrication of vertical bipolar junction transistor (BJT)based GeSn heterojunction phototransistors (HPTs) [30][31][32][33][34][35].…”

Effects of buried oxide layer on working speed of SiGe heterojunction photo-transistor*

“…However, due to the existence of junction capacitance and the slow-photon-generated carriers in the Si substrate, the working speed of the device is greatly limited. [7][8][9][10][11] To improve the working speed of the Si-based SiGe HPT, a buried oxide (BOX) layer is introduced into its collector in this paper. Due to the existence of series capacitance of BOX layer, the collector-substrate capacitance, the base-collector junction capacitance as well as the base-emitter junction capacitance are reduced to a certain degree.…”