Abstract
"There is a fundamental need for improved hardware efficiency to enable the next generation of mobile communications, radar, satellite and power electronics applications. The need for improved efficiencies is further highlighted by the continuous growth in the demand for capacity and coverage of cellular networks. Furthermore, the demand for slim and portable electronic appliances requires high power density power converters switching at high frequencies. To continue to improve the thermal design and efficiency of semiconductor devices, this dissertation examines new applications of transient thermoreflectance (TR) measurements and new die designs using multiphysics simulations. These techniques have significant advantages over the state-of-the-art and directly contribute to improve the transistor performance and reliability for both RF/microwave and power applications. The new techniques also provide novel insight into the distributed electrical and thermal characteristics of semiconductor devices.
More specifically, the research work covered in this dissertation investigates the complex interaction between the electrical and thermal behaviour that occurs within gallium nitride (GaN)-based transistors. High spatial and temporal TR measurements revealed non-uniform heating and allowed, for the first time, the measurement of the dynamic temperature operation of transistors. These measurements captured more accurate dynamic channel temperatures within the transistor finger and across the width of the die while operating in DC/DC converters at switching frequencies up to 5 MHz. The combination of the TR, pulsed I-V and load-pull measurement systems allowed the characterisation of microwave transistors under realistic microwave excitations and the investigation of the isothermal condition assumed during pulsed I-V measurements. Finally, a multiphysics circuit-based simulation methodology was developed and used to optimise GaN transistors operating at 26 GHz. By optimising the metallization, the device has a 2 percentage points improvement in the drain efficiency. The multiphysics simulation is then validated with TR measurements."