Large Signal Modeling and Optimization of HBTs
Graduate Student:
Apostolos Samelis
Professor
D. Pavlidis
U.S.Army Research Office DAAL03-92-G-0109, ARPA/COST MDA972-94-1-0004,
Bell Northern Research
Power Series based modeling is an attractive tool for the analysis of the
nonlinear behavior of HBTs under weak RF excitations.
Its advantage over other large signal modeling approaches is that it allows
one to isolate the behavior of each nonlinear component
of the HBT large signal model, in terms of its contribution to nonlinear output
products like, for example, the third order
intermodulation product (IMD3). Furthermore, this approach allows to
illustrate the interaction between the HBT nonlinear elements.
For example, it was shown that the interaction between the base-emitter
conductance and current source (alpha) generated nonlinear
currents results in mutual cancellation and therefore improved IMD3
performance.
The goal of this project is to determine the device design conditions which
allow optimization of the intrinsic nonlinear current
cancellation mechanisms. Furthermore, the harmonic loading conditions
leading in improved IMD3 behavior of HBT-based amplifiers are
investigated. Device power characterization is accomplished through load pull
measurements using automated mechanical tuners.
Large signal microwave characteristics of GaAs-based HBTs are modeled by
extending the conventional Gummel-Poon based BJT model to include self
heating effects. The model is incorporated as a user defined model in
LIBRA-HP/EEsof, a commercially available circuit simulator. The
experimental microwave
characteristics of HBTs are analyzed using the new model and Harmonic
Balance techniques and good agreement is obtained between measurements
and simulations. The newly developed approach permits extension of
commercially available CAD to incorporate temperature dependent HBT
characteristics as necessary in power applications.
Analytical modeling techniques are employed for the extraction of physically
significent HBT characteristics. Large-signal model for InP-HBTs are
developedby including both parameters calculated from S-parameter measurements
and DC characterization. Breakdown effects have been incorporated to account
for the breakdown characteristics of InP-HBTs. Transimpedance OEIC's are
studied under large-signal excitation conditions using these models.
Measured and simulated data for fundamental and second harmonic otuput power
and IMD3 of an AlGaAs/GaAs HBT. A Volterra series based nonlinear simulation was used
for the calculation and was based on analytically extracted models for the
devices. The model results fit the measured characteristics and the approach
allows physical insight into HBT nonlinearities.
Degradation of the transimpedance of an InP-based OEIC with increased
input power level. The results display as much as 3dB transimpedance variation
at an input power level as low as -5 dBm at 8 GHz. A large-signal model including
breakdown effects was used for InP HBT. The studies show the necessity for
large-signal modeling OEIC design.
[GaN]
[InP]
[GaAs]
[MOCVD]
[Mixer]
[Gunn (NDR)]
[PIN]
[HBTs]
[HEMTs]
[MMICs]
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Solid State Electronics Laboratory,
Department of Electrical Engineering and Computer Science,
University of Michigan
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