在本論文中，分別在摻雜通道式場效電晶體與變形晶格式高電子移動率電晶體中採用組成漸變之砷化銦鎵通道。由於使用漸變式通道，因此使得元件操作特性獲得明顯改善。
　　首先，我們成長具有摻雜的漸變式通道之對稱型雙邊�-摻雜砷化鋁鎵/砷化銦鎵異質結構場效電晶體，其中銦的組成從通道的底部(x = 0.22)漸變到通道的上層(x = 0.12)。在室溫和77 K可獲得改善的二維電子雲密度及電子遷移率分別為3.6 (3.2)×1012 cm-2 及4850 (18000) cm2/V．s，同時可獲得大於2伏特之閘極工作電壓擺幅 (gate voltage swing)。此外，由於使用了超晶格砷化鋁鎵/砷化鎵作為緩衝層，因此可以有效改善元件特性隨著溫度的升高而呈現劣化之缺點，包括減少漏電流、輸出電導、臨限電壓之位移。
　　在變形晶格式高電子移動率電晶體中，為了要獲得更高的載子濃度，我們利用了雙重�-摻雜層和對稱漸變式通道。另外為了改善電子遷移率，銦的組成在通道的中央設定為0.5，而朝著通道的兩邊線性減為0.3。相較於In0.53Ga0.47As通道的結構，對稱漸變式通道的結構，不但具有較高的二維電子雲密度(4.8×1012 cm-2)且仍能維持高的電子遷移率(7520 cm2/V．s)。由於在對稱漸變式通道中改善了電子移動率及具有較佳的電子侷限能力，使得此結構的最大轉導值可達276 mS/mm。對於閘極線寬為1.5 �m的元件中，我們可以得到電流增益截止頻率及最大震盪頻率分別為22.4 GHz及45.5 GHz在具有對稱漸變式通道的結構中。這些特性以1.5 �m閘極線寬而言，是相當優越的。
　　最後，我們提出一個以對稱漸變式InxGa1-xAs (x ＝ 0.5�_0.65�_0.5)為通道，以反向步階漸變緩衝層的新型變形晶格高電子移動率電晶體。由於有較低的表面散射，在室溫及77 K時，其電子遷移率分別可高達9500及30600 cm2/V．s。藉由自我一致性的理論模擬，我們發現在漸變式通道中產生三個次能階，這和我們使用Shubnikov-de Haas (SdH) 分析所得之結果相同。此外，特藉由使用對稱漸變式通道、In0.425Al0.575As蕭基層和未摻雜之磷化銦層，元件的反向閘－汲崩潰電壓高達24伏特。同時，對於閘極線寬為1.5 �m的元件中，我們可以得到電流增益截止頻率及最大震盪頻率分別為18.9 GHz及48.4 GHz。這些改善的元件特性適合應用於高功率及高頻電路中。

　　In this dissertation, a variety of compositionally graded InxGa1-xAs channels have been applied and investigated in the doped-channel field-effect transistors (DCFET’s) and the metamorphic high electron mobility transistors (MM-HEMT’s). Device performances are significantly improved due to the graded channel.
　　First, a symmetric double-side �-doped AlGaAs/InxGa1-xAs heterostructue field-effect transistor with a homogeneously Si-doped graded channel has been studied. The In composition in the graded InxGa1-xAs channel was varied linearly from 0.22 (at the bottom of channel) to 0.12 (at the upper of channel). Improved two-dimension electron gas (2DEG) density and mobility at 300 (77) K as high as 3.6 (3.2)×1012 cm-2 and 4850 (18000) cm2/V‧s are achieved, respectively. Meanwhile, we can obtain a large gate voltage swing (GVS) over 2 V. Besides, the use of a superlattice AlGaAs/GaAs buffer layer can simultaneously reduce the leakage current, output conductance, the shift of threshold voltage with increasing temperature.
　　For obtaining higher carrier concentrations, double �-doped layers were adopted in the metamorphic InAlAs/InGaAs HEMT’s. In order to improve the mobility, a higher indium composition (x = 0.5) was set at the center of channel and then decreased linearly toward both sides of the channel (x = 0.3). As compared with the metamorphic In0.42Al0.58As/In0.53Ga0.47As HEMT, the metamorphic InAlAs/InGaAs HEMT with symmetric graded InxGa1-xAs channel can obtain higher 2DEG density of 4.8×1012 cm-2 while keeping the electron mobility as high as 7520 cm2/V‧s. Due to the improved electron mobility and good carrier confinement in MM-HEMT’s with the symmetric graded InxGa1-xAs channel, the maximum transconductance of 276 mS/mm is obtained for the gate dimension of 1.5×125 �m2. Moreover, the measured current gain cutoff frequency and maximum oscillation frequency for a 1.5 �m gate length device are 22.4 GHz and 45.5 GHz, respectively. The device performances are among the best for 1.5 �m gate length.
　　Finally, we have fabricated a novel MM-HEMT with a symmetric graded InxGa1-xAs (x＝0.5�_0.65�_0.5) channel and an inverse step graded buffer layer. Due to the lower interface roughness scattering, an improved electron mobility as high as 9500 (30600) cm2/V‧s at 300 (77) K was achieved. By using the self-consistent calculation method, three subbands in the graded channel were determined, which are identical to the Shubnikov-de Haas (SdH) characterization. By using the symmetric graded channel, In0.425Al0.575As Schottky layer and the undoped InP setback layer, a high gate-drain breakdown voltage of 24 V is obtained. Meanwhile, the measured current gain cutoff frequency and maximum oscillation frequency for a 1.5 �m gate length device were found to be 18.9 GHz and 48.4 GHz, respectively. The improved characteristics are suitable for high-power and high-frequency circuit applications.

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