應用於毫米波之矽基板氮化鎵 高電子遷移率電晶體設計與製作

Design and Fabrication of AlGaN/GaN HEMTs on Silicon Substrates for Millimeter-Wave Applications

鄭智軒

指導教授 ： 徐碩鴻

國立清華大學/電機資訊學院/電子工程研究所/碩士(2012年)

摘要

近年來，氮化鎵高電子遷移率電晶體(GaN-based HEMTs)不管在功率電路與高頻電路的應用上上都具有相當驚人的發展潛力。早期HEMTs在基板的選擇上通常是採用Sapphire或是SiC材料，但最近Si基板也漸漸成為HEMT基板的選擇，由於考量到價格較低廉以及未來與CMOS的結合上，讓Si基板嚴然成為投入研究的議題。本論文著重在矽基板(Si substrates)之AlGaN/GaN HEMTs之元件設計及進一步的模型分析，並藉由等校模型分析所得到的結果提出幾種可以有效提升在矽基板上之GaN HEMT 元件之RF特性。首先我們利用縮小元件尺寸以及製作T型閘極結構來提高元件的高頻特性，並從中觀察元件佈局對其高頻特性的影響，進而得到元件佈局之最佳化設計。另外本論文藉由製作不同之閘極寬度觀察對元件高頻特性以及短通道效應對元件高頻特性的影響。本研究中所製作出之元件 fT達到35GHz以上，f_MAX 可達到100GHz以上( g_m=225"mS" ⁄"mm" ，I_d=822"mA" ⁄"mm" )。
另外本論文藉由分析不同元件佈局條件下之小訊號等校模型進一步得知更改不同佈局參數的條件時，元件等效電路內部之電容電阻參數對其高頻特性的影響。本論文最後則是藉由建立等效基板模型來模擬矽基板對元件高頻特性的影響，由於在氮化鎵緩衝層與矽基板接面處會產生一層寄生的反轉電子層，因而在高頻下使汲極與源極間形成額外的損耗路徑。
我們發現寄生的R_sub 〖與C〗_sub會嚴重影響元件的高頻功率輸出特性，因此藉由此等效電路所萃取的參數值可以評估不同矽基板之氮化鎵磊晶試片是否適合使用在高頻元件上，我們未來也可藉由此一分析方法做為依據來設計元件架構或本身基板磊晶結構，以改善基底功率散逸對元件特性的影響。

關鍵字

氮化鎵 ； 高電子遷移率電晶體 ； 矽基板 ； 毫米波

並列摘要

In recent years, GaN-based HEMTs have great potentials for high power and RF applications. The candidate of substrate for HEMTs was usually Sapphire or SiC material in previous day, however, Si has gradually been the choice of substrate for HEMTs recently. The GaN-based HEMT on silicon substrate, due to low cost and the integration to CMOS in the future work, has become a popular research topic.
In this thesis, we focus on the design of AlGaN/GaN HEMTs on silicon substrate and a small signal equivalent circuit modeling for AlGaN/GaN HEMTs. And by the results of the equivalent circuit model analysis, we propose some methods which can extremely improve the RF characteristics of AlGaN/GaN HEMTs.
First part of this thesis, for the purpose of improving the RF device characteristics, we scale down the device size and fabricate T-shaped gate structure. Then we investigate how the device layout affects the RF performance of AlGaN/GaN HEMTs, resulting in the design of optimized device layout. Second part of this thesis, we investigate how the short channel effect and the gate length (Lg) affects the RF performance. In this study, we have fabricated 2×(0.2×25) m AlGaN/GaN HEMTs grown on (111) high resistivity Si substrate with f_T more than 35 GHz and f_MAX more than 100 GHz (g_m=225"mS" ⁄"mm" ，I_d=822"mA" ⁄"mm" ).
Furthermore, by the analysis of the equivalent circuit model for different device layout, we figure out how the intrinsic parameters like capacitors and resistors affect the RF characteristics of devices. Final part of this thesis, we use the small signal equivalent circuit model to simulate the Si substrate parasitic effect, which largely affects the RF performance of the devices. Due to the GaN/Si structure, there will have high impact ionization and high electron density layer at the Si interface indicating a parasitic leakage path which decreases power at the high frequencies.
We figure out that the parasitic parameters such as Rsub and Csub provided by Si substrate largely affect devices’ output power for high frequency operations. By the parameters extracted by this small signal extraction modeling method, we can evaluate what kind of the substrate structure or the material suits for RF devices. In the future, we will use this extraction method as a basis for deign layer structure or device layout, in order to improve the RF performance of the devices.

參考文獻

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