The Characterisation of Gallium Nitride Devices for Power Electronic Applications

: Leakage Mechanisms and Device Reliability

Abstract

Highly efficient power conversion beyond the capabilities of silicon electronics is required to meet the growing global demand for power and to enable emerging technologies. The high breakdown field of wide bandgap semiconductors make these materials capable of meeting this demand in a power electronics revolution. Gallium nitride (GaN) is especially suited to this role due to its high electron mobility and its ability to form a high density 2D electron gas, resulting in enhanced efficiency. Compared to silicon, GaN systems can provide more efficient power conversion at higher voltages, all at a fraction of the system size.

Despite its promising material properties, a number of research topics remain before GaN technology can be fully exploited. In lateral devices, the substrate is usually grounded so increasing the vertical breakdown voltage of GaN-on-Si epitaxies is required to enable higher voltage devices while maintaining the low production cost associated with the use of silicon substrates. This has been approached from two directions in this thesis. Firstly from a material property perspective, by furthering the understanding of how carbon doping increases the resistivity of GaN. Through a combination of electrical measurements and device simulations, it is shown that carbon in GaN incorporates as donors as well as acceptors and that this self-compensation ratio of donors to acceptors is above 0.4. As the self-compensation ratio determines the material resistivity, it is an essential parameter in device design and future simulation works. Secondly, optimisation of epitaxial resistivity was approached at the device level. It is shown through electrical measurements that the resistivity of the epitaxy is reduced after processing Ti/Al based Ohmic contacts. These sub-contact leakage paths are further studied through a novel use of the quasi-static capacitance-voltage technique to reveal these paths extend up to 1.6 μm, all the way to the superlattice strain relief layers. The existence of these leakage paths is widely unknown and being aware of their impact is an important step forward for buffer design and accurate device simulation.

Vertical GaN-on-GaN devices are desired over lateral devices for their improved thermal performance, superior breakdown characteristics and reduced peak surface fields. The primary research efforts are focused on optimising vertical leakage and edge termination. The reverse leakage in vertical pn diodes is studied in the time domain and reveals the first evidence of impurity band conduction. The model required to explain the results also demonstrates the ability of charged point defects to control the conductivity of dislocations. This new understanding of the leakage dynamics could influence the way leakage is managed in future device designs. The reliability of these devices is also a topic of interest as qualification of this emerging vertical GaN technology is required before commercialisation. The mean time to failure of these vertical diodes was evaluated by adapting existing analysis techniques to facilitate the application of Weibull statistics to step stress measurements. These results represent the first lifetime estimations of this technology and this new technique will enable more rapid reliability testing of vertical GaN devices in the future.

Date of Award	1 Oct 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Martin H H Kuball (Supervisor)