Experimental and theoretical investigation of geometric and compositional factors in improving the off-state breakdown voltage, reliability, and enhancement-mode operation among GaN channel hetero-structure field effect transistors

经过二十多年对极性III-氮化物异质结场效应晶体管(HFET)的研究，高频功率放大器/射频开关和高温/高压电力电子开关领域的市场迅速增长。随着市场份额超过十亿美元大关，符合这些极性异质结构特点的创新解决方案正在获得更突出的地位，以改进其功率处理/热管理，并可靠地实现增强和耗尽操作模式(即分别具有正阈值电压和负阈值电压)。随着异质外延技术的进步和可靠源的出现，利用与GaN沟道的晶格匹配和研究较少探索的成分和几何结构来提高长期可靠性，增强实现稳定增强模式工作的可能性，以及延长击穿电压，都是非常及时的。将现已证实的将阈值电压转换为增强模式操作和减少自加热的可能性(两者都通过修改隔离特征的台面结构来实现)与由于费米能级钉扎侧壁小面的接近而提供的定制漏极感应电场的机会相结合，有望不仅提供实现可靠的增强模式III-氮化物HFET的方法，而且还将提高其关态击穿电压。虽然传统上关态击穿电压是通过采用所谓的场板(即栅极的延伸部分，因此是电容)来提高的，但是通过在沟道附近存在钉扎的侧壁小面来定制电场可以部分地减轻对较高击穿电压的频率响应的负担。在之前发现号拨款的帮助下，Pi的团队自豪地开发出了一种电子束光刻工艺，为成功完成当前提案中确定的实验任务做好了准备。除了PI对了解上述现象的贡献外，其他一些团体目前也参与了这一领域的活跃的技术努力。在目前Discovery补助金申请的支持下，Pi打算继续他的开创性工作，探索上述替代方案，以改善增强模式GaN沟道HFET的关态击穿电压、长期可靠性和频率响应。这项提案中提出的想法有可能产生可申请专利的概念，并从长远来看，与汽车和电信行业建立未来的合资企业。后者目前准备在5G网络中利用III-氮化物HFET，并开发物联网(IOT)应用。

More than two decades of research on polar III-Nitride hetero-structure field effect transistors (HFETs) has resulted in rapidly growing markets in the areas of high-frequency power-amplifiers/RF-switches and high-temperature/high-voltage power electronic switches. As the market-share is passing the billion-dollar mark, innovative solutions in-tune with the particulars of these polar hetero-structures for improving their power-handling/heat-management, and for reliable realization of both enhancement- and depletion-modes of operation (i.e. having positive and negative threshold-voltage, respectively) are gaining a more prominent status. With the improvement in techniques of hetero-epitaxy and availability of reliable sources providing the less explored AlInGaN/GaN hetero-structures, taking advantage of lattice-matching to the GaN channel and examining the less-explored compositional and geometric structures towards improving the long-term reliability, enhancing the possibility of achieving stable enhancement-mode operation, and extending the breakdown voltage are all very timely. Combing the now-proven possibilities of shifting the threshold-voltage towards enhancement-mode operation and reducing the self-heating (both realized through modifying the mesa structure of the isolation-feature), with the offered opportunity to tailor the drain-induced electric-field due to the proximity of Fermi-level pinned sidewall facets is expected to not only offer ways to achieve reliable enhancement-mode III-Nitride HFETs, but also to boost their off-state breakdown voltage. While traditionally off-state breakdown voltage is boosted by employing the so-called field-plates (i.e. extensions to the gate electrode and hence capacitance), tailoring the electric-field via the presence of pinned side-wall facets in the close proximity of the channel can partially lift the burden on the frequency response for a higher breakdown voltage. With the help of the previous Discovery grants, PI's team has proudly managed to develop an e-beam lithography process poised for successfully performing the experimental tasks identified in the current proposal. Along with the contributions of the PI to understanding of the aforementioned phenomena, a few other groups are also currently involved in lively technological endeavors in this area. Under the support of the present Discovery grant application, PI intends to continue his groundbreaking work on exploring the aforementioned alternatives for improving the off-state breakdown voltage, long-term reliability, and frequency response among enhancement mode GaN-channel HFETs. The ideas presented in this proposal have the possibility of generating patentable concepts and in the long-term establishing future joint ventures with automobile and telecommunication industries. The latter of which is presently poised to take advantage of III-Nitride HFETs in 5G networks and developing internet of things (IOT) applications.