高頻及高功率應用之氮化鋁鎵/氮化鎵高電子遷移率電晶體之研究

Abstract

本研究成功製作並量測氮化鋁鎵/氮化鎵高電子遷移率電晶體。為了進一步提升此電晶體之特性，本論文提出形成歐姆接觸與蕭特基接觸新的方法。除此之外，也討論了元件佈局設計對於其特性的影響。 在這篇論文裡，首先藉由插入3 nm矽於氮化鋁鎵/氮化鎵結構與傳統(鈦/鋁/鎳/金)歐姆金屬層之間，有效地降低接觸阻值到0.23 ohm-mm。本論文藉由穿透式電子顯微鏡之觀察，解釋了其形成機制。歐姆接觸表面粗糙度的改良，則使用了WSiN作為退火保護層，其後利用電漿蝕刻將部分移除，此種方法可以製作出具有平整表面之歐姆接觸且不會導致元件電性的退化。 至於蕭特基接觸，藉由有系統地研究氮化鎢內氮的比例對於此材料的特性變化，選擇了合適比例之氮化鎢(W0.52N0.48)並將之作為電晶體之閘極材料。此氮化鎵高電子遷移率電晶體具有耐電流時效，熱穩定度，且有著在2 GHz時，最高功率密度5 W/mm的功率輸出。 另外，此論文還包括了氮化鎵高電子遷移率電晶體佈局及尺寸與特性的關係。當調變閘極-汲極與源極-閘極距離時，觀察到閘極特性與其有著顯著相關性。此結果可以做為元件設計時之準則

AlGaN/GaN high electron mobility transistors (HEMTs) have been fabricated and characterized. To further improve the performance of the HEMTs, Ohmic and Schottky contacts have been extensively re-examined and new approaches are proposed. The effect of the device layout to the device characteristics is also discussed. In this dissertation, the contact resistivity of Ohmic contacts of AlGaN/GaN HEMTs is improved by the insertion of a thin Si layer (3nm) between the semiconductor layer and traditional Ti/Al/Ni/Au metal stacks. Using this approach, low contact resistivity of 0.23 ohm-mm is realized, and the mechanism for this improvement is investigated by means of transmission electron microscope (TEM). As for improvement of the Ohmic contact surface morphology, WSiN cap layer is placed on top of the ohmic contact before annealing, afterward it is partially removed by low damage induced coupled plasma (ICP) etcher. Smooth surface with no degradation of the electrical performance are observed. For Schottky contacts, we systematically investigated the characteristics of tungsten nitride with respect to the nitrogen composition, then diodes and GaN HEMTs are realized using the optimized W0.52N0.48 as Schottky contact metal. Reliability after electrical stress, thermal stability as well as excellent RF performance of 5W/mm at 2 GHz are measured with the WNx gate device. In addition, the geometry and layout effecting the GaN HEMTs are also investigated. Both gate-drain and source-gate distances are varied and the gate behaviors are strongly dependent on these parameters. The information could provide a guideline when designing the device.