Nonlinear Characterisation and Modelling of Microwave Semiconductor Devices

Doctoral thesis, 2011

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.