氮化鎵異質結構場效電晶體在高功率以及微波射頻方面廣泛的被使用。氮化鎵的崩潰電場與電子飄移速度高於矽，所以它在高功率與高速元件的應用上是一個非常重要的材料。在許多文獻裡，AlGaN/GaN HEMTs製作在碳化矽(SiC)或藍寶石(Sapphire)基板上有非常好的高頻特性，包括它有較高的功率輸出與低的雜訊。然而，藍寶石基板有熱傳導能力(Thermal conductivity)較差的缺點，所以在高功率操作上，使用它做為元件的基板較不合適。在製作價格方面，電晶體使用碳化矽或藍寶石基板比使用矽基板昂貴，相較於藍寶石或碳化矽基板，AlGaN/GaN HEMTs製作在矽基板上有價格便宜以及它可以製作較大面積元件的優勢。近年來研究指出氮化鎵高速電晶體提供了良好的直流(DC)，不過它在高頻特性上與碳化矽或者藍寶石基板相對較差。我們推測主要原因可能是矽基板產生的寄生效應導致元件在高頻操作上的功率損耗，因此我們製作一個模型來分析氮化鎵電晶體的基板效應。
首先我們製作出閘極長度為0.5 贡m、閘極寬度為2×50 贡m的AlGaN/GaN異質結構場效電晶體，它的截止頻率(fT)達到21 GHz與最大震盪頻率(fmax)為37 GHz。為了分析基板效應，本論文製作了一個新的小訊號模型，它包含了基板的寄生效應，並且此模型的參數萃取是使用方程式求解的方法，參數值不需要任何假設即可求出。另外，我們製作兩種不同緩衝層厚度與不同尺寸的元件來驗證此模型，並且利用此模型模擬其基板寄生效應影響元件在高頻操作上的程度。
最後，我們根據模擬與量測結果作比較，驗證此模型的準確程度，再進一步的模擬出當元件去除基板效應之後，元件的fmax增加10%至20%，這個結果是如我們預期的。

GaN-based transistors have been developed successfully for high power and microwave RF applications. Compared with Si, GaN has a higher breakdown field and electron drift velocity, which makes GaN-based devices an excellent candidate for such applications. It has been reported that AlGaN/GaN high electron mobility transistors (HEMTs) on SiC or sapphire substrates demonstrated excellent RF performance such as high output power and low high-frequency noise. However, the cost of both SiC and sapphire substrates are relatively higher compared to the silicon substrate. In addition, the poor thermal conductivity of the sapphire substrate also degrades the device performance under high power operation. Compared with the SiC and sapphire substrates, GaN-on-silicon is much more attractive with low cost, large size substrates, and good thermal conductivity. Recent publications have shown that GaN-based HEMTs grown on silicon substrate could reach good DC characteristics, but their RF performance is not as expected mainly due to the parasitics from the silicon substrate.
In this work, a novel extraction methodology is proposed, allowing extraction of the small-signal model directly including the parasitic capacitance and resistance of substrate. The parameters of substrate parasitics can be extracted by solving the analytical equations without any assumption. The sub-micron AlGaN/GaN HEMTs are fabricated using the in-house developed technology. With a 0.5-贡m gate length and a width of 100 贡m, the device demonstrated fT= 21 GHz and fmax= 37 GHz on high resistivity silicon substrates. The proposed modeling approach are applied to several types of devices with different thicknesses of the GaN buffer layer. Based on the extracted small-signal equivalent circuit models, the fmax could be improved by 10 to 20 percent if the substrate parasitics are removed.

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