Monolithic integration of GaN DC-DC converters : technology and characterization

High-temperature (HT) power converters are increasingly important in extreme environments, such as electric vehicles, aviation, etc. Due to the limited temperature operation beyond 150 °C in Si-based devices, GaN-based power transistors are expected to be excellent candidates for power converters at high temperatures over 200 °C in electric vehicle applications. HT power converters with self-contained functionality (power, driver, microcontroller, sensors, etc.) without external heatsink or cooling systems are increasingly essential owing to reduced size and cost. The lateral AlGaN/GaN-based high electron mobility transistors (HEMTs) have been regarded as promising candidates in high frequency, high power density, and HT applications. GaN smart power integrated circuit (IC) provides an effective solution to achieve a systemon-chip scheme for HT power converters. This thesis uses normally-off GaN transistors with a recessed metal-insulated-semiconductor (MIS) gate, and it focuses on the development of GaN DC-DC converters with integrated gate drivers for HT power converters in extreme environments. To evaluate the recessed MIS gate for high-temperature GaN power converters, the impact of etch depth on the performance of mobility and RON is systematically studied at high temperatures. The mechanisms of carrier scattering are discussed at different etch depths, and full recess with dielectric engineering is proposed to improve the stability of GaN IC. On the lateral GaN smart power IC technology platform, this thesis focuses on three parts for HT power converters including 1) an integrated gate driver for a GaN boost converter; 2) integrated gate drivers with a half-bridge stage for a synchronous GaN buck converter; 3) an integrated technique of deadtime management for a synchronous GaN buck converter. Firstly, the GaN boost converter with the optimized gate driver exhibits a voltage conversion from 5 to 11 V at 100 kHz, and only 11% reduction of output voltage is observed at high temperatures up to 250 °C. Then, the synchronous GaN buck converter with an integrated half-bridge stage achieves a voltage downconversion with an input voltage of 25 V, and it shows good thermal stability and almost no reduction of output voltage at temperatures up to 250 °C, with a large gate swing of 10 V. Lastly, an integrated GaN buck converter with a no deadtime technique (NDT) exhibits a maximum efficiency of 80 % at high temperatures up to 250 °C , with an input voltage of 30 V at 100 kHz. At high temperatures, the optimized GaN NDT converter shows better performance than a synchronous GaN buck converter with a fixed deadtime technique (FDT) at high load currents, in terms of smaller voltage overshoots and oscillations of gate drivers and better converter efficiency as well. The proposed GaN NDT converter uses one control signal and provides a simple and effective method of deadtime management for high-temperature power converters.