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An SiGe/Si Heterojunction Phototransistor for Opto-Microwave Applications: Modeling and first Experimental Results

An SiGe/Si Heterojunction Phototransistor for Opto-Microwave Applications: Modeling and first Experimental Results,J. L. Polleux,F. Moutier,A. L. Bill

An SiGe/Si Heterojunction Phototransistor for Opto-Microwave Applications: Modeling and first Experimental Results   (Citations: 1)
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A first SiGe bipolar heterojunction phototransistor developed in a commercial available SiGe/Si technology is presented. Emphasis on the development of a complete numerical model for the simulation of strained- SiGe based devices is given. The SiGe HPT exhibits a dc opto-microwave power gain of 3.46dB, i.e. a responsivity with 50Ohms loads of 1.49A/W, and a -3dB bandwidth of 0.4GHz at 940nm. Power budgets are drawn with the use of the opto-microwave power gain's monogram chart. I. INTRODUCTION Optical communications give the opportunity to carry high data rates on long distances, but efforts still have to be put on the last miles to distribute data to the home (FTTH) or also simply between two neighbor systems. In these short distances applications, like in 10Gbits/s optical back-planes in spatial and avionics applications, optical wavelength of 1.55µm is no longer an imperative, because low costs benefits can be found in working at lower wavelengths in the range of 0.8-1µm. This paper deals with a new bipolar heterojunction phototransistor developed in a commercially compatible SiGe/Si process, within which a single strained SiGe layer is used instead of MQW structures as in (1), (2). A special insight in the numerical simulations of the device will be given. The use of high base doping enables to enlarge the structure for convenient vertical optical coupling. Germanium is shown besides to warranty a sufficient detection in the all 0.8-1µm optical wavelength range, as well as improving the dynamic behavior up to operating frequencies of around 20GHz. The potential of such a structure is first theoretically demonstrated before prototypes were produced. A strained-SiGe layer model is presented. The absorption coefficient of strained-SiGe is worthily developed, taking into account its dependence on the Germanium content, the incident optical wavelength and on temperature. Main ways of applications of such layers are then depicted from the absorption curve. Then the structure of the SiGe HPT is proposed and prototypes are characterized at a wavelength of 940nm. Results are presented and compared to numerical simulations. Discussion is provided. then, the constraint effect on the electrical properties of the material has been well investigated. Strain is responsible for the enhancement of the material bandgap reduction, but also for energy shifts in the k-space that lead to modification in carrier's mobility, effective densities of states and intrinsic concentration. However, most of the proposed numerical models often only consider the bandgap reduction and neglect the others parameters changes, which nevertheless strongly affect the electrical behavior, especially at high Germanium rates. Also, not any absorption model was presented on single strained SiGe/Si layers before. In order to fulfill this need, physical laws are gathered from the literature and compared in (3). A synthesis is proposed here that is divided in three parts: the bandgap reduction, the valence and conduction bands effective density of states, and last the optical absorption coefficient. Bandgap reduction
Published in 2003.
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    • ...Experimental results were presented in [2], [3] and more completely in [4] within a single strained layer SiGe/Si HPT developed in a commercially compatible SiGe/Si process...
    • ...More recently a team from IBM [8] entered the field of SiGe HPT with similar structures than in [1], [4]...
    • ...For our 12µm-width HPT, a fT of 20GHz and a fMAX of 25GHz are experimentally demonstrated and presented in [3] and [4]...
    • ...For opto-microwave characterization of the HPT ([3] and [4]), a responsivity with 50Ω loads of 1.49A/W was measured and a −3dB-bandwidth of 0.4GHz is exhibited at 940nm in phototransistor mode: Vb e =0 .82V and Vc e =1 .5V ...
    • ...A complete physical model, presented precisely in [4] and [1], was built on a 2D drift-diffusion simulator: modules Atlas from Silvaco...
    • ...Physical simulation was compared to experimental datas in [4] obtained by a two laser beating technique presented in [11]...
    • ...To obtain Responsivity with 50Ω loads results, we need numerical computations to convert H parameters to S-Parameters, [1] and [4]...

    F. Moutieret al. Frequency response enhancement of a single strained layer SiGe phototr...

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