使用氮化鎢閘極之氮化鋁鎵/氮化鎵高電子遷移率電晶體之高溫應用

Abstract

近年來應用於高頻之高功率放大器元件材料選用上,氮化鎵高電子遷移率電晶體逐漸脫穎而出,此乃由於它為寬能隙材料,且其有高崩潰電壓,高飽和電子移動速度及高溫操作的材料特性.本研究針對氮化鎵高電子遷移率電晶體的高溫操作的行為及所需之熱、電穩定特性進行探討.研究方向著重於高溫操作下此元件之材料之電性變化,因此對於電極材料的選用上有非常高之要求.本研究困難點於選用之材料需兼顧有熱穩定性,及高溫下保有良好的電性,更為重要的是選用的電極材料與基材不能產生反應,故對於材料界面、高溫之相變化及金屬間的擴散行為所造成的電性變化亦是本研究的課題.除電極材料需求外,氮化鎵元件製作流程仍尚未成熟亦為本研究的困難點,亦由於氮化鎵的寬能隙及化學穩定性,在製程上未如砷化鎵成熟：未成熟的蝕刻技術,物理蝕刻所引起基材傷害,物理蝕刻對於基材所引起的電子極化影響所造成二維電子氣層的衰減,絕緣區的深度,保護層厚度,皆為目前重要之課題.本研究審慎選用較為穩定之製程方式,製程上遭遇的問題及解決之方法皆於本論文中詳述. 另外,本研究重要之課題為高溫操作下之電晶體直流特性的行為及其不穩定原因之探討.對於源極及汲極歐姆接觸的熱穩定性,低接觸電阻,閘極蕭特基接觸於高溫下的熱穩定性及擁有良好的理想因子,較高的能量障礙,此為形成電晶體高溫操作之必要條件. 本實驗驗證了氮化鎵高電子遷移率電晶體製程,並完成了用濺鍍方式的T型閘極氮化鎢結構,最後元件的直流特性於室溫、閘極偏壓0伏特下電流密度為30~32mA/mm,電導為46~50mS/mm,-1.4伏特下操作,磊晶層二維電子氣濃度及載子移動率於300K為1.6×1012cm-2、800cm2V-1s-1.

GaN HEMTs have enormous potential for high-power solid-state amplifier application at microwave frequencies, due to their characteristics such as high breakdown field, high electron saturation velocity and high operating temperature. These characteristics are attributed to the wide bandgap nature of the GaN material. The study focuses on the GaN HEMTs under high temperature operation. The selections of electrode materials are very critical for HEMTs to operate at high temperature. The candidates must have thermal stability and good electrical properties under high temperature operation. No reactions between electrode materials and GaN substrate should occur. Therefore, the changes of electrical properties caused by material interactions at high temperature were focused in this study. Besides electrode material study, the GaN HEMT fabrication processes were also completed in the study. Due to the wide band-gap nature, the GaN material was chemically very stable, however, its process technology is not as mature as other Ⅲ-Ⅴ compounds. Good etching technique, damages caused by physical etching, 2DEG degradation in epitaxial layer caused by changing in electron polarization caused by physical and chemical etching, isolation technique and the passivation technique are among the most important issues in GaN process now. The fabrications process development was described in details in this thesis. Another important issue in this study is the thermal stability of the transistor DC characteristics. Transistors must have thermally stable Ohmic contacts for Source & Drain, and stable Schottky for the Gate with good ideality factor (n) and high barrier height (ψb) under high temperature operation. We have developed a WNx T-gate process by sputtering and lift-off process. A AlGaN/GaN HEMT with the WNx T-gate were processed in this research. The DC characteristics of the AlGaN/GaN HEMT processed is Imax =30~32 mA/mm, gm =46~50 mS/mm under 0 V gate bias and at 300K, and pinch-off voltage is –1.4 V.