R. Sommet

University of Limoges, Limages, Limousin, France

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Publications (52)20.52 Total impact

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    ABSTRACT: For a number of decades, the modeling of microwave transistors relied on specific well-known characterization methods. Those methods include S-parameters measurement through vector network analyzers (VNAs) ranging from the lower end of the RF spectrum to the millimeter-wave (mm-wave) region and load pull measurements of transistors used for the design of power amplifiers (PAs). Later, the availability of more powerful computer-aided design (CAD) software enabled the need for models of active microwave devices, thus driving a huge amount of research activity on microwave transistor modeling. Simultaneously, new transistor technologies were invented, combining working concepts such as heterojunction bipolar transistors (HBTs), metal semiconductor field effect transistors (MESFETs) or high electron mobility transistors (HEMTs) and new materials such as gallium arsenide (GaAs), gallium nitride (GaN), indium phosphide (InP), and silicon germanium (SiGe), to cite only the main ones.
    IEEE Microwave Magazine 01/2014; 15(4):92-107. · 1.50 Impact Factor
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    ABSTRACT: This paper reports on the design of a new power cell dedicated to Ku-band power amplifier (PA) applications. This cell called “integrated cascode” has been designed in order to propose a strong decrease in terms of circuit size for PA. The technology used relies on 0.25-μm GaAs pseudomorphic high electron mobility transistors (PHEMT) of United Monolithic Semiconductors (UMS) foundry. A distributed approach is proposed in order to model this power cell. The challenge consists of obtaining, with a better shape factor (ratio between the vertical and horizontal sizes of the transistor), the same performances than a single transistor with the same gate width. In order to design a 2W amplifier, we have used two 12 × 100 μm transistors. Cascode vertical size is 413 μm whereas a transistor with the same gate width exhibits a vertical size of 790 μm. Therefore, the shape factor is nearly one as compared to a shape factor of 4 for a classical parallel architecture. This new device allows us to decrease the Monolithic microwave integrated circuit amplifier area of 40% compared to amplifier based on single transistors.
    International Journal of Microwave and Wireless Technologies 06/2013; 5(03). · 0.57 Impact Factor
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    ABSTRACT: The design of high speed integrated circuits heavily relies on circuit simulation and requires compact transistor models. This paper presents a non-linear electro-thermal model of SiGe heterojunction-bipolar transistor (HBT). The non-linear model presented in this paper uses a hybrid π topology and it is extracted using IV and S-parameter measurements. The thermal sub-circuit is extracted using low-frequency S-parameter measurements. The model extraction procedure is described in detail. It is applied here to the modeling of npn SiGe HBTs. The proposed non-linear electro-thermal model is expected to be used for the design of high-speed electronic functions such as broadband analog digital converters in which both electrical and thermal aspects are engaged. The main focus and contribution of this paper stands in the fact that the proposed non-linear model covers wideband-frequency range (up to 65 GHz).
    International Journal of Microwave and Wireless Technologies 12/2012; 4(06). · 0.57 Impact Factor
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    ABSTRACT: This reported work deals with an accurate characterisation method dedicated to the determination of the thermal impedance of gallium nitride based high electron mobility transistors (GaN HEMTs). The method is inspired by the `3ω method` initially proposed by Cahill (reference 8 of this Letter) in order to measure the thermal conductivity of bulk materials or layers and the present authors` previous works on thermal resistance measurements. It is demonstrated that the voltage oscillation at the third harmonic is a real image of the thermal impedance of the device in the frequency domain. Both the theoretical approach and the test bench are discussed.
    Electronics Letters 01/2012; 48(12):708-710. · 1.04 Impact Factor
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    ABSTRACT: This paper presents a behavioral model for RF power amplifiers including load-pull and self-heating effects for radar applications. The model topology combines nonlinear scattering functions with a thermal model. This work focuses on model identification from time domain load-pull measurements and thermal simulations of the power amplifier. Comparisons between model and pulsed measurements, demonstrate its ability to accurately reproduce the signals and temperature for arbitrary load impedances.
    Microwave Symposium Digest (MTT), 2012 IEEE MTT-S International; 01/2012
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    ABSTRACT: This paper reports on the design of a new power cell dedicated to Ku-band power amplifier (PA) applications. This cell called '“integrated cascode”' has been designed in order to propose a strong decrease in term of circuit size for Power Amplifier (PA). The technology used relies on 0.25-μm GaAs Pseudomorphic High Electron Mobility Transistors (PHEMT) of United Monolithic Semiconductors (UMS) foundry. A distributed approach is proposed in order to model this power cell. The challenge consists to obtain, with a better shape factor (ratio between the vertical and horizontal sizes of the transistor), the same performances than a single transistor with the same gate development. In order to design a 2W amplifier, we have used two 12×100 μm transistors. Cascode vertical size is 413 μm whereas a transistor with the same gate development exhibits a vertical size of 790 μm. Therefore the shape factor is nearly one as compared to a shape factor of 4 for a classical parallel architecture. This new device allows to decrease the MMIC amplifier area of 40 % compared with amplifier based on single transistors.
    Microwave Conference (EuMC), 2012 42nd European; 01/2012
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    ABSTRACT: This paper reports on the design of a new power cell dedicated to Ku-band power amplifier (PA) applications. This cell called “'integrated cascode'” has been designed in order to propose a strong decrease in term of circuit size for Power Amplifier (PA). The technology used relies on 0.25-μm GaAs Pseudomorphic High Electron Mobility Transistors (PHEMT) of United Monolithic Semiconductors (UMS) foundry. A distributed approach is proposed in order to model this power cell. The challenge consists to obtain, with a better shape factor (ratio between the vertical and horizontal sizes of the transistor), the same performances than a single transistor with the same gate development. In order to design a 2W amplifier, we have used two 12×100 μm transistors. Cascode vertical size is 413 μm whereas a transistor with the same gate development exhibits a vertical size of 790 μm. Therefore the shape factor is nearly one as compared to a shape factor of 4 for a classical parallel architecture. This new device allows to decrease the MMIC amplifier area of 40 % compared with amplifier based on single transistors.
    Microwave Integrated Circuits Conference (EuMIC), 2012 7th European; 01/2012
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    ABSTRACT: In this paper Low Frequency (LF) parasitic effects are assessed through three kinds of measurements. It is shown that LF S-parameters measurements allow to extract the thermal impedance of Heterojunction Bipolar Transistors (HBTs) and to put dispersive effects of AlGaN/GaN High Electron Mobility Transistors (HEMTs) into evidence. Large signal (RF pulsed and two tone intermodulation) confirm the impact of those parasitic effects on performances of Power Amplifiers.
    Wireless and Microwave Technology Conference (WAMICON); 01/2012
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    ABSTRACT: In this paper, a simple and accurate characterization method of the thermal impedance of silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs) is proposed. This method relies on low-frequency S-parameter measurements in the 100 Hz–3 GHz frequency range. It is shown that feedback hybrid parameter $h_{12}$ provides an image of the thermal impedance in the frequency domain, which is independent of the size of the transistor. Very short thermal time constants involved in SiGe HBTs are accurately determined by this method, as well as the temperature dependence of the thermal impedance in a truly simple way as electrical measurements can be performed in dc conditions either on a wafer or on an attached die. Finally, a nonlinear electrical equivalent circuit model is extracted, which can be readily implemented in computer-aided design software for nonlinear simulation of any SiGe HBT circuit.
    IEEE Transactions on Electron Devices 01/2012; 59(7):1921-1927. · 2.06 Impact Factor
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    ABSTRACT: This paper deals with a comparison between electro-thermal nonlinear simulation and measurements performed on a Silicon Germanium (SiGe) Heterojunction Bipolar Transistors (HBTs). The first part describes the simulation approach developed with ANSYS 3D Finite Element (FE) model. The second part describes the measurement process. This one is based on Low Frequency electrical impedance measurements generally achieved with S-parameters Vector Network Analyzer setup. Finally the FE model is compared to a Cauer circuit extracted to represent the measured data.
    Thermal Investigations of ICs and Systems (THERMINIC), 2012 18th International Workshop on; 01/2012
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    ABSTRACT: This paper reports on the design of new power cells based on GaAs PHEMT transistors with 0.25μm gate length in MMIC technology. The design starts from a fishbone power cell initially designed by United Monolithic Semiconductors (UMS). This one allows us to validate the elementary transistor model. This model is used for other power cells design as cascode cell.
    Integrated Nonlinear Microwave and Millimetre-Wave Circuits (INMMIC), 2011 Workshop on; 05/2011
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    ABSTRACT: This paper presents an electro-thermal behavioral advanced model of RF power amplifier including the load-pull effects for narrow band radar applications. An extension of nonlinear scattering functions is proposed for modeling large output impedance mismatches in the presence of high frequency memory and thermal effects. It combines a nonlinear scattering function block for the electrical response with a reduced order thermal model. The model implementation in an implicit system-level simulator will allow to achieve accurate and fast time-domain simulations to estimate temperature transients and signal distortions.
    01/2011;
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    ABSTRACT: Une conception d’une nouvelle cellule de puissance à base de transistor PHEMT AsGa de longueur de grille 0.25 $mu$m en technologie MMIC est présentée dans ce papier. Le point de départ est la cellule de puissance fishbone développée initialement par UMS. Cette dernière va nous permettre de valider un modèle de transistor de petite taille qui sera utilisé pour la conception d’autres cellules de puissances telles que la cellule cascode.
    Journées Nationales Micro-ondes (JNM); 01/2011
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    ABSTRACT: The microwave emission linewidth of spin transfer torque nano-oscillators is closely related to their phase and amplitude noise that can be extracted from the magnetoresistive voltage signal V(t) using single shot time domain techniques. Here we report on phase and amplitude noise studies for MgO based magnetic tunnel junction oscillators. The analysis of the power spectral densities allows one to separate the linear and nonlinear contributions to the phase noise, the nonlinear contribution being due to the coupling between phase and amplitude. The coupling strength as well as the amplitude relaxation rate can be directly extracted.
    Applied Physics Letters 11/2010; 97(18):182507-182507-3. · 3.52 Impact Factor
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    ABSTRACT: In this paper a systematic analysis of thermal and trapping behaviour of microwave power AlGaN/GaN HEMTs has been carried out through pulsed current-voltage (PIV) measurements and S parameters. It is shown that the thermal resistance of the device can be accurately determined provided that some assumptions on the trapping behaviour of the device are verified. The values obtained have been checked by three dimensional finite element (3D-FE) simulations with reasonable accuracy. Kink effects in the output characteristics have been analysed at different temperatures and it has been shown that they are more pronounced at ambient temperature. Finally the microwave behaviour of the device versus temperature has been assessed.
    Microwave Integrated Circuits Conference (EuMIC), 2010 European; 10/2010
  • A. El Rafei, R. Sommet, R. Quere
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    ABSTRACT: In this letter, we present a characterization method for the determination of the thermal impedance of heterostructure bipolar transistors. The thermal impedance was characterized using low-frequency S -parameter measurements. We will show that our method can be used to characterize the thermal impedance independent of the transistor size.
    IEEE Electron Device Letters 10/2010; · 2.79 Impact Factor
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    ABSTRACT: This paper has two main axis: firstly, we address the experimental characterization of the frequency-dependent thermal impedance of microwave bipolar transistors from continuous wave (CW)-only measurements (both DC and AC). From the experimental perspective, we will review some of the already available methods and propose a new method based on a recent observation. It will be shown that under proper measurement control, a reasonable precision of the computed value can be achieved. The method is applied to characterize the global (external) behavior of a multi-finger Heterojunction Bipolar Transistor (HBT), whose physical structure is known. A distributed thermal circuit, entirely derived from 3D thermal simulations, is incorporated into a complete distributed electrothermal model of the device, whose global behavior is validated by measurements. Then from a distributed electrothermal simulation perspective, we will address the power and temperature distribution between fingers as a function of the power dissipated by the device, and will show that the global behavior in measurements is close to the worst case in terms of highest temperature among individual fingers.
    Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2010 12th IEEE Intersociety Conference on; 07/2010
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    ABSTRACT: This paper presents a dual approach for a coherent determination and validation of heterostructure bipolar transistor (HBT) thermal impedance. This study relies both on an experimental characterization method and a 3D finite element simulation approach. One section reminds briefly the experimental approach. Another describes the 3D device modeling used for the physics-based thermal simulation. Thereafter, details on the reduction method used for the numerical computation of the thermal impedance are given. As complement to pure thermal simulation, an electrothermal distributed model is proposed and gives an interpretation of the distributed effects in multi-finger devices.
    Microelectronics Journal. 01/2010; 41:554-559.
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    ABSTRACT: In this paper an analysis of thermal behavior of microwave power AlGaN/GaN HEMTs has been carried out through pulsed current–voltage (PIV) measurements and small signal [S] parameters. A special care about trapping effects has been followed where it is shown that the thermal resistance of the device can be accurately determined provided that some assumptions on the trapping behavior of the device are verified. The values obtained have been checked by three dimensional finite element (3D-FE) simulations. Finally, the thermal boundary resistance (TBR) between GaN/SiC has been extracted and compared to literature. The results we have obtained are in line with what can be found. In addition to this first set of results, a second 3D-FE analysis of a larger transistor has been performed and checked thanks to liquid crystal measurements.
    Microelectronics Journal 01/2010; · 0.91 Impact Factor
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    ABSTRACT: This paper presents a behavioral electrothermal model implementation for high RF power amplifiers dedicated to the simulation of radar application in the Scilab/Scicos environment. This model, based on the direct coupling between a behavioral electrical model and a physics-based reduced thermal model, allows to predict nonlinear effects, high-frequency memory effects, and thermal effects due to the amplifier self-heating. System model implementation in Scilab/Scicos platform allows fast time domain simulation with very good convergence properties.
    International Journal of Microwave and Wireless Technologies 01/2009; 1(06). · 0.57 Impact Factor