Thermal Modelling of Microwave Bipolar Transistor from 3D finite Element Simulation
Xiong, XA; Sommet, SR; A.L. de Souza, ALSA; Quéré, QR
XLIM

Abstract
This study is devoted to the thermal aspects of microwave Heterojunction Bipolar Transistors (HBT). A thermal impedance determination based on low frequency input impedance measurement has been developped. Jointly, a 3D thermal simulation based on finite element decomposition of the HBT device has been performed. In agreement with both methods, the compact thermal model is obtained.

Introduction
With the increase of the current density and the high power dissipation in microwave devices, thermal modelling has become necessary because thermal phenomena impact electrical behaviour. As a consequence, several models have been investigated and many characterization methods have been developped. Most of thermal studies rely on the determination of the thermal resistance of the device. However the full behavior is only determined by the knowledge of the thermal impedance. We describe in a first part the characterization method we have developped. The second part is devoted to thermal simulation based on 3D physics-based modelling. The good agreement between results validates both approaches.

Thermal Impedance Measurement
The experimental characterization of thermal impedance is based on the low frequency measurement of the microwave bipolar transistor input impedance. From a measurement protocol and assuming that the base-emitter voltage VBE is a nonlinear function of the base current IB and the temperature T, a frequency-dependent expression of the thermal impedance can be derived. The major advantages of this method versus other techniques rely on the fact that it is purely an electrical method and it does not require temperature control or pulsed measurements. Measurements can be realized on a wide frequency range (1Hz to 1MHz) in order to get a maximum of thermal time constants.

3D Thermal Simulation
For 3D thermal simulation, the finite element ANSYS simulator has been used. Like most of thermal simulation software, ANSYS simulation consists in modelling and meshing a structure. Appropriate and pertinent mesh must be applied regarding the complexity of a structure. In most case, symmetric considerations allow to model only a half (or a quarter) of a full device and hence also reduce number of elements. This physics-based analysis generally gives good results for power devices.

Results and Comparison
Characterization campaigns have been performed on several technologies, including SiGe and GaAs devices. Three differents structures on SiGe transistors have been studied, standard, low cost and SOI. GaAs transistors have been mounted onto two different carriers. Comparison between measurements and simulations gives very good agreements.