Vertical gallium nitride power devices on bulk native substrates

Abstract

Lateral power devices based on AlGaN/GaN hetero-structures have achieved excellent performance in the medium power range applications. However for higher voltage higher current switches, a vertical structure is preferred since its die area does not depend on the breakdown voltage. This thesis studies vertical GaN power diodes and transistors grown on bulk GaN substrates. The first part of the thesis studies the PiN diode. Low p-GaN ohmic contact resistance is obtained through annealing in oxygen ambient. The breakdown voltage reaches 1200 V with optimized field plate design. The resistance components of the PiN diodes are also analyzed in this part of the thesis. The second half of the thesis presents a novel vertical power FinFET design with only n-GaN epi-layers. One of the key fabrication processes required for this device structure is to achieve a smooth vertical fin sidewall by combining dry/wet etch. The normally-off power FinFET demonstrates excellent performances without the need of p-GaN layer or material regrowth. With the optimization of edge termination structures, 800 V blocking voltage was achieved. A further reduction of on resistance is achieved by increasing the cap layer doping. Switching characteristics are investigated by capacitance measurements. The thesis concludes with the demonstration of spalling off the bulk GaN substrate after device fabrication. Thanks to the substrate spalling technology, the on resistance of the device can be further reduced and the bulk GaN substrate could possibly be reused to save cost.