Characterization of Microwave Transistors for Robust Receivers and High Efficiency Transmitters
Licentiate thesis, 2009
The next generation of integrated transceiver front-ends needs both robust
low noise amplifiers and high power amplifiers on a single-chip. The Aluminium
Gallium Nitride / Gallium Nitride (AlGaN/GaN) High Electron Mobility
Transistors (HEMT) is a suitable semiconductor technology for this purpose
due to its high breakdown voltage and high electron mobility. In this thesis
the AlGaN/GaN HEMT’s thermal properties, noise and survivability have
been characterized for the intended use in robust high power transceivers.
Furthermore, a new characterization setup for load modulated high efficiency
power amplifiers have been developed.
The thermal properties of AlGaN/GaN HEMTs have been carefully investigated
considering self-heating and its effect on small-signal parameters and
high frequency noise. Self-heating is a severe problem for a high power transistor
on any semiconductor material, including GaN. In addition to reliability
problems, the performance of the operating HEMT degrades with temperature.
The access resistances showed a large temperature dependence, which was also
verified with TLM measurements. Due to the large self-heating, the temperature
dependence of the access resistances has to be taken into account in the
modeling of the AlGaN/GaN HEMT. A temperature dependent small-signal
noise model was derived and verified through fabricated amplifiers. Design
strategies for robust low noise amplifiers are discussed and implemented using
the derived model.
The new characterization setup gives new possibilities to characterize the
performance of load modulated amplifiers. Recent results on load modulated
amplifiers show promising efficiency improvements in back-off operation.
Therefore a new measurement setup was developed that performs dynamic
load modulation at the transistor terminals. This method should be useful
to further improve the performance of load modulated amplifiers for high efficiency
operation. The measurement setup is based on an active load-pull
setup, where a modulated input signal is used to synthesize a time varying
output power. The load impedance is dynamically controlled with the envelop
of the input signal, following an optimum efficiency load trajectory. This gives
better insight into device operation and possible improvements.
Kollektorn, Kemivägen 9, Chalmers
Opponent: Dr. Matthias Rudolph, Ferdinand-Braun-Institut für Höchstfrequenztechnik, Berlin, Germany