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.