The ever-increasing energy demand and accelerating electrification of modern society require unprecedented high efficiency and compact size of power conversion systems with power semiconductor devices as the key component. Wide-bandgap semiconductor GaN, with fundamentally superior properties than conventional Si, is taking up the task of performance advancement in power devices and recently entered the commercialization phase with deployment in consumer electronics and industrial power supply systems. For the widespread adoption of this new technology, GaN-specific evaluation methods and approaches must be developed to assure reliable and stable operation that meets lifetime requirements. Underlying mechanisms of device failure need to be revealed for continuous technology adva...[ Read more ]

The ever-increasing energy demand and accelerating electrification of modern society require unprecedented high efficiency and compact size of power conversion systems with power semiconductor devices as the key component. Wide-bandgap semiconductor GaN, with fundamentally superior properties than conventional Si, is taking up the task of performance advancement in power devices and recently entered the commercialization phase with deployment in consumer electronics and industrial power supply systems. For the widespread adoption of this new technology, GaN-specific evaluation methods and approaches must be developed to assure reliable and stable operation that meets lifetime requirements. Underlying mechanisms of device failure need to be revealed for continuous technology advancement and reliability enhancement. Focusing on the most vulnerable gate module in GaN power devices, a comprehensive study is carried out on gate stability and reliability in this thesis. There are two major parts:

The first part is devoted to the commercial p-GaN gate power HEMTs. These devices are emerging as the most widely used commercial GaN power devices in recent years. They do exhibit substantially different gate behaviors from the mainstream Si-based power MOSFETs, and the underlying physical mechanisms are not clearly and adequately revealed. In this part, several application-critical stability and reliability topics are comprehensively studied:

1. The threshold voltage (V_TH) instability of p-GaN gate devices is a concern, which may lead to insufficient drive voltage and subsequently degraded system efficiency. Threshold voltage instability under forward gate stress exhibits distinctive behavior under static and dynamic stress, showing bidirectional shifts in the former and monotonic positive shifts in the latter. A physical model based on charge dynamics in the p-GaN layer is developed to understand the V_TH instability. Besides, device stability strongly depends on the gate module concept. Different gate contact schemes, i.e., Schottky-type and ohmic-type gate, features distinct threshold voltage instability behaviors.

2. In terms of device lifetime in practical applications, the reliability of the vulnerable p-GaN gate structure has been characterized and studied under multi-scenario stress conditions. The Schottky-type p-GaN gate HEMTs, with a reverse-biased metal/p-GaN Schottky junction under forward gate stress, confronts with long-term reliability concerns. Apart from the static (dc) time-dependent gate breakdown (TDGB) evaluation that is straightforward to carry out, TDGB tests under dynamic (ac) gate stress with switched gate and drain bias are presented. The p-GaN gate shows distinctive degradation behavior that is different from the dielectric in MOS-gate devices.

3. Considering the positive temperature coefficient of the gate breakdown voltage, the p-GaN gate may suffer risks in low-temperature applications. The temperature-dependent gate lifetime of the Schottky-type p-GaN gate devices is discussed. The measurements are carried out in a wide range of temperatures from −100 °C to 150 °C. Time-efficient temperature-accelerated and voltage-accelerated stress tests are carried out to evaluate the device's lifetime. It is found that the time-to-failure at “use condition” predicted by acceleration tests at high gate bias could be overestimated under low temperatures. The root cause for such a discrepancy is that the gate leakage is dominated by different mechanisms at high/low gate bias. A physical-statistical approach to estimate the lifetime is further proposed.

The second part of this thesis is devoted to the MIS-gate (metal-insulator-semiconductor gate) GaN power devices that are currently under development. Compared to the p-GaN gate, the MIS-gate structure promises highly desired benefits of larger gate swing and stronger noise immunity, regarding the gate drive circuit. The most urgent need for qualifying this technology for practical applications mostly lies in the qualification of the gate dielectric. In this thesis, two types of normally-OFF MIS-gate GaN power devices, namely fully recessed MIS-FET and partially recessed MIS-HEMT, are systematically studied by comparing the static and dynamic performance, thermal stability, as well as bias instability. In conjunction with the high-reliability PECVD-SiN_x/LPCVD-SiN_x as gate dielectric, the superior manufacturability and gate stability suggest the fully recessed GaN MIS-FET be an attractive candidate for next-generation GaN lateral power devices.