Modelling of RF High Power Bipolar Transistors
Koen Mouthaan
PhD Thesis
Summary
In recent years wireless mobile communication has emerged as a
mass communication medium. Wireless mobile communication is in
essence a bidirectional link between a mobile telephone on the one hand
and a base station on the other. In base stations, power amplifiers are
found that amplify the transmitted signals to power levels of a few Watts
and possibly up to over 100 Watts. Power transistors are used to construct
power amplifiers. The power transistors are normally built using a package,
matching capacitors, dies consisting of multi-call active devices and
bondwires.
In the design of the dies, device simulation and analysis tools
are used extensively. In the design of power transistors, however, trial and
error methods are often applied due to a lack of electrical models for
the bondwires, package and matching capacitors. Furthermore, power
transistors dissipate substantial amounts of power and, as a consequence
power transistors heat up significantly affecting the electrical
behaviour due to the electro-thermal effect.
Therefore there is a clear need for an electrical, a thermal model and
an electro-thermal model for RF high power transistors.
In this work the segmentation approach is applied to modelling
power transistors: first, models are derived and tested using
measurement or rigorous simulation for the separate components.
These models are connected together to form the full model
for the power transistor. This approach is applied both to the electrical
modelling and the thermal modelling. All models are implemented in the
commercially available microwave simulator Microwave Design System (MDS) of
Agilent Technologies.
The models may also be used in the modelling of, for example, mutual
inductances between bondwires in plastic packages or in the
modelling of the thermal behaviour in integrated circuits.
By implementing the models in MDS, the full
electrothermal simulation of power transistors under DC, RF small signal and
RF large signal conditions is now possible.
As a demonstrator the modelling approach is
applied to the Philips BLV 910 power transistor. This transistor is designed
to deliver 10 Watts of RF power around 900 MHz. The results clearly
demonstrate that the model accurately represents the DC behaviour and the
small signal scattering parameters.
Most important of all, the model also predicts very accurately the RF large signal
parameters such as gain, efficiency and output power.
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Last updated: 15-DEC-2004.