高頻高功率氮化鎵微波元件技術研究

Abstract

氮化鎵磊晶成長並無晶格匹配的基材，僅能選用SiC 及sapphire 作為磊晶成長的 基板，其晶格不匹配分別高達4%及16%，如此大的晶格不匹配會造成大量的差排 (dislocation)， 而劣化磊晶品質。幸而SiC、sapphire 及GaN 均為六方晶結構 (hexagonal)，運用緩衝層(buffer layer)的成長，可使差排互相結合減少其密度，有效提 昇磊晶品質。以兩種基版而言，高品質的高頻高功率氮化鎵微波元件皆已被實現，目前 主要的研究方向轉而以改變磊晶結構提升元件特性。現在氮化鎵微波元件有三種改良結 構被提出，一為插入AlN 於AlGaN/GaN 之間，即AlGaN/AlN/GaN 高電子遷移率電晶 體(HEMT)，二為使用InGaN 當作通道之HEMT，三為使用InAlN Schottky layer 之 HEMT。三種結構都利用HEMT 結構的量子井而使電子侷限於量子井中，形成二維電子 氣(2 dimen- sional electron gas，2DEG)， mobility>1300cm2/V-s， 工作頻段可輕易 >10GHz。此外，此三種HEMT 結構的量子井結構由於更大之conduction band discontinuity 與更強烈的壓電效應，可獲得比一般AlGaN/GaN HEMT 結構更高之載子 濃度(sheet carrier concentration)，加上極高的電子飽和傳輸速度，故可以有極高的飽 和電流值。本計劃主要運用有機金屬氣相沈積(MOCVD)成長buffer GaN 以求得較佳之 buffer layer 平整度，接著使用分子束磊晶(MBE)成長AlGaN/AlN/GaN 之磊晶結構以取 得較佳之量子井介面平整度，運用兩種成長方式之優點成長高品質之磊晶。並以製程方 式增加元件的崩潰電壓及輸出功率，以製作出操作在3GHz，Power density 為6W/mm 的氮化鎵高功率放大器。

In the epitaxial growth of GaN, due to the lack of lattice matched substrate. The epilayer can only be grown on the SiC or sapphire, which has large lattice mismatch of 4% and 16% respectively. This value of mismatch will introduce dislocation in the GaN film and degraded the quality. However, using optimized buffer layer growth, dislocation density can be effectively reduced. For both substrates, high frequency, high power microwave devices has been realized. The research now turned to epilayer structure engineering to further improve the performance. Three approaches have been proposed, including insertion of AlN layer between AlGaN/GaN, InGaN channel HEMT and InAlN schottky layer HEMT. These structures use quantum wells to confine the electrons within 2DEG. Furthermore, higher sheet carrier concentration compared to the standard AlGaN/GaN HEMT is possible because of the larger conduction band discontinuity and stronger piezoelectric effect. Thus results in higher saturation current. This proposal uses MOCVD to grow buffer GaN on sapphire, and MBE for sequential AlGaN/AlN/GaN epitaxial layer growth. Breakdown voltage and output power density will be optimized by process parameter and structure design. A power HEMT with output power density of 6W/mm operating at 3GHz will be demonstrated.