Raphaël Sommet

A pulsed-measurement based electrothermal model of HBT with thermal stability prediction capabilities

In this paper, a new electrothermal non linear model of HBT suitable for CAD purposes is presented. This model is fully determined by pulsed measurement techniques and for the first time, it is shown that the prediction of thermal instabilities (collapse of current gain) is obtained from the CAD model. The model has been validated both by DC and RF load-pull measurements.

Characterization and modeling of nonlinear trapping effects in power SiC MESFETs

Trapping effects in power SiC MESFETs are investigated using a pulsed I-V pulsed S-parameters measurement system. It is shown that the main effect comes from substrate (buffer) traps sensitive to the drain-source voltage. Moreover a nonlinear model of the trapping phenomenon, taking into account the electron capture and emission with different time constants allows one to predict experimentally observed I-V and RF power performances of the devices.

Characterization and modeling of bias dependent breakdown and self-heating in GaInP/GaAs power HBT to improve high power amplifier design

It is usual to say that power GaInP/GaAs heterojunction bipolar transistors (HBTs) have many advantages for power amplification at microwave frequencies, because of their high gain and high power density. Furthermore, the possibility of controling the base biasing conditions (voltage, current, self-bias control) compared to a field-effect transistor offers additive degrees of freedom to make a tradeoff between linearity and power-added efficiency. Nevertheless existing devices are limited because of the relatively low breakdown voltage whereas high collector voltage swings are required to achieve high power. This drawback makes them not appropriate for use in the next generation of mobile communication base station or radar systems. Silicon technologies such as LDMOS and III-V devices (MESFET and HFET) present competitive performances in term of high power level but for medium power added efficiency. Important improvements have been made in recent years which make possible large breakdown voltages for GaInP/GaAs HBTs. Breakdown value close to 67 V has been achieved. The aim of this work is to significantly improve the modeling of the breakdown voltage on this type of transistor. Furthermore, the in depth characterization and modeling of self-heating effects have been greatly improved in order to improve thermal management solutions which enable us to enhanced design solutions of HBT high power amplifiers.

Experimental Characterization and Modeling of the Thermal Behavior of SiGe HBTs

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 h12 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.

Low frequency parasitic effects in RF transistors and their impact on power amplifier performances

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.

Nonlinear RF Characterization and Modeling of Heterojunction Bipolar Transistors Under Pulsed Conditions

I(V) and S-parameters pulsed measurments have been performed on a heterojunction bipolar transistor for currents up to 105 Amps/cm2. S-parameters data have been acquired in the whole device output domain. So, a nonlinear DC to RF consistent model has been obtained with I(V) and RF pulsed measurements on a GaInP/GaAs HBT transistor, processed by the central research laboratory of Thomson CSF.

Coupled localized and distributed elements analysis applying an electromagnetic software in the frequency domain

A coupled localized and distributed elements analysis applying the F.E.M. in the frequency domain is described. First, two studies concerning a two port network and a gunn diode amplifier, are performed to prove with success the efficiency of our method. Then the main objective of this paper is to present an electromagnetic analysis of the passive area of a transistor (FET) taking into account all its physical and geometrical characteristics. Theoretical and experimental results are compared and they show encouraging agreement.

Thermal modeling and measurements of AlGaN/GaN HEMTs including thermal boundary resistance

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 S parameters. A special care about trapping effects has been followed where it is shown Error! Reference source not found. 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.

On the determination of the thermal impedance of microwave bipolar transistors

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.

An Electrothermal Model of High Power HBTs for High Efficiency L/S Band Amplifiers

This paper deals with an electrothermal model of high power heterojunction bipolar transistor (HBT) intended for CAD. The first section describes the model topology and sets the implemented equations that allow to take into account of the physical phenomena. The model also integrates scaling rules function of emitter length (W) and number of fingers (N). For the thermal aspect, low frequency impedance measurement approach has been led. Model simulations have been compared to quasi-isothermal I-V measurements, pulsed S parameters measurements, and load-pull multi-harmonics measurements.