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- Thorsell, Mattias, 1982-
(författare)
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Characterization of Microwave Transistors for Robust Receivers and High Efficiency Transmitters
- 2009
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Licentiatavhandling (övrigt vetenskapligt)abstract
- 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.
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| 2. |
- Thorsell, Mattias, 1982-
(författare)
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Nonlinear Characterisation and Modelling of Microwave Semiconductor Devices
- 2011
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Doktorsavhandling (övrigt vetenskapligt)abstract
- There is an increasing need for more accurate models taking into account the nonlinearities and memory effects of microwave transistors. The memory effects are especially important for transistor technologies suffering from relatively large low frequency dispersion, such as GaN baed HEMTs. Nonlinear measurement systems are today available off-the-shelf, but the use of them is still limited. It is therefore important to demonstrate the possibilities these new systems brings to the device characterisation and modelling community. This thesis deals with electrothermal characterisation and modelling of GaN based HEMTs, and also development and utilisation of new nonlinear measurement systems. The electrothermal properties of the AlGaN/GaN heterostructure were characterised, and it was shown that a thermal response is present up to 100 MHz. Moreover, a new characterisation method, making use of nonlinear measurements, allowed for isothermal measurements of the current transport through the access resistances of a GaN based HEMT. A new current transport model was proposed to correctly reproduce the isothermal IV characteristics. Furthermore, the temperature dependence of the high frequency noise was characterised, showing that the major limiting factors for the low noise performance were the access resistances. The combination of high power and low noise makes the GaN based HEMT suitable for monolithically integrated GaN based transceiver front-ends. The first steps toward a transceiver were taken by designing and manufacturing a GaN based receiver front-end consisting of an SPDT switch and an LNA. A new fast multi harmonic active load-pull system was developed, with waveform acquisition capabilities. The speed of the load-pull system was increased by the use of an improved optimisation routine for presenting the wanted load impedances. The load-pull system was capable of presenting dynamically varying load impedances to a transistor, enabling faster device characterisation without the need to build complete amplifiers. The system was also used to characterise the nonlinear distortion in SiC varactors. It was shown that the nonlinear distortion increases the losses, and hence a new general Q-factor description was proposed. Furthermore, a new characterisation method was proposed which enabled the study of memory effects in transistors driven by modulated signals.
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