在本論文中，我們成功的研製一系列具改良式通道的場效電晶體。藉由不同組成的通道，探討其對元件特性的影響，包括：漸變組成、壓縮應力、拉伸應力、耦合式平面摻雜通道。
我們首先研製通道銦組成為反向漸變與對稱漸變的砷化銦鋁/砷化銦鎵高電子移動場效電晶體。相較於銦組成為反向漸變的砷化銦鎵通道，對稱漸變通道元件具有較高二維電子氣體載子濃度與較優良的閘極調變效率，因此可以表現較佳電流驅動能力與高頻特性，但反向漸變通道中衝擊游離較不明顯，具有較高崩潰電壓較適合高功率輸出，同時也因庫侖散射距離較遠和較高的通道/緩衝層能帶不連續而不易受溫度影響其特性，所以兩元件於高頻與功率的應用上互有優勢。在閘極長度為0.65 µm的元件中，對稱漸變(反向漸變)的砷化銦鋁/砷化銦鎵高電子移動場效電晶體的轉導值為330 mS/mm (314 mS/mm)，高頻截止頻率為 50.0 GHz (46.0 GHz)，操作在5.8 GHz與汲極-源極偏壓為2伏特時最大輸出功率分別為84.9 mW/mm (95.3 mW/mm)。
為了使元件更能適用MMIC的應用並進一步提升元件特性改善上，我們進一步研製以變晶方式銦組成為0.53通道的平面摻雜砷化銦鋁/砷化銦鎵變晶結構高電子移動場效電晶體。由於變晶型技術的特性，我們嘗試以提高鋁含量蕭基層鋁含量至0.42的方式，以提高半導體能隙方式製作壓縮式應力通道元件，並與晶格匹配元件比較。實驗結果顯示，壓縮應力通道元件具較高能隙的蕭基層可以有效提升崩潰電壓，並因為較高的傳導帶不連續而有較高二維電子氣體濃度，具有壓縮應力通道的砷化銦鋁/砷化銦鎵變晶結構高電子移動場效電晶體具有較大的汲極電流與功率輸出，但因壓縮應力會降低通道電子移動率，故晶格匹配元件具有較高的轉導值與高頻特性。壓縮應力通道與晶格匹配通道的砷化銦鋁/砷化銦鎵變晶高電子移動場效電晶體的轉導值分別為318 mS/mm與341 mS/mm，汲極電流為452 mA/mm與366 mA/mm，高頻截止頻率為 48.0 GHz與53.5 GHz，操作在5.8 GHz與汲極-源極偏壓為3伏特時最大輸出功率分別為185.8 mW/mm與131.5 mW/mm。
由於較窄通道能隙會有較明顯的衝擊游離的現象，但高頻所需要的高電子移動率需由高銦組成的窄能隙提供，因而造成功率的輸出與高頻特性需要取捨。為了研究及同步改善元件的高頻與功率特性，我們進一步研製通道銦組成為對稱型漸變的拉伸應力通道之平面摻雜的砷化銦鋁/砷化銦鎵變晶結構高電子移動場效電晶體，由於拉伸應力與對稱型漸變皆可有效提升載子移動率，在降低銦含量下，可得到與高銦含量通道的元件更高的載子移動率並同時改善的衝擊游離效應，不但在直流特性上可提升，在高頻與功率應用上，也可超越傳統元件。實驗結果顯示，對稱型漸變的拉伸應力通道之δ-摻雜的砷化銦鋁/砷化銦鎵變晶結構高電子移動場效電晶體的轉導值可高達404 mS/mm，最大汲極電流可到達514 mA/mm，高頻截止頻率為58.5 GHz，操作在5.8 GHz與汲極-源極偏壓為3伏特時時最大功率輸出為260.0 mW/mm。
最後，我們針對高電子移動率場效電晶體高溫特性衰減提出改善方式-使用耦合平面摻雜變晶式場效電晶體。由於通道中央使用高銦含量的砷化銦鎵做為通道，在電子波函數耦合的效果下，仍可得到高電子移動率，且同時具有摻雜式通道電晶體的高載子濃度的優點，因此在直流與高頻表現上，轉導值可到達320 mS/mm，汲極電流可到達566 mA/mm，高頻截止頻率為45 GHz，操作在5.8 GHz時最大功率輸出為194.5 mW/mm。由於平面摻雜通道的高載子侷限能力，從室溫到420 K，轉導值與汲極電流變化量僅有+0.3%與-3.4%，非常適合於高溫使用。

In this dissertation, we have investigated InAlAs/InGaAs heterostruture field-effect transistors (HFETs) with different InxGa1-xAs channel structures, including graded, compressively-strained, tensile-strained, and coupled δ-doped channels. Through the channel engineering, we have studied the influences on comprehensive device performances, such as drain current (IDSS0), breakdown (VBK), extrinsic transconductances (gm), output conductance (gd), cut-off frequency (fT), maximum oscillation frequency (fmax), and output power (Pout).
First, we investigate the InP-based In0.52Al0.48As/In0.53Ga0.47As high-electron-mobility transistors (HEMTs) with inversely-graded and symmetrically-graded channels (IGC-HEMT and SGC-HEMT). As compared to IGC-HEMT, SGC-HEMT has higher 2DEG concentration and better gate modulation efficiency. Therefore, SGC-HEMT demonstrates better current driving capability and microwave characteristics. On the other hand, IGC-HEMT exhibits the relieved impact ionization and higher breakdown voltage, making IGC-HEMT more suitable for power applications. Furthermore, less Coulomb scattering in the inversely graded channel makes IGC-HEMT exhibit better thermal stability. For a 0.65 μm gate-length device, the measured gm, fT, fmax, and Pout at 5.8 GHz (biased at VDS = 2 V) are 330 mS/mm (314 mS/mm), 50.0 GHz (46.0 GHz) and 46.0 (55.0 GHz) GHz, 84.9 mW/mm (95.3 mW/mm) for SGC-HMET (IGC-HEMT), respectively.
To meet the requirements of MMIC fabrications and to further improve device performances, we fabricate the InxAl1-xAs/In0.53Ga0.47As/InxAl1-xAs metamorphic high-electron-mobility transistor (MHEMT) with the different indium composition of the Schottky and buffer layers. The energy gap (Eg) of the Schottky and buffer layer can be increased by reducing the indium composition of from x = 0.52 in lattice-matched device to x = 0.42. Therefore, the device with compressively-strained channel (CS-MHEMT) was fabricated and compared with lattice-matched device (LM-MHEMT). Experimental results indicate that wider Eg of Schottky layer increases the breakdown voltages and 2DEG carrier concentration at the same time, leading to higher IDSS0 and Pout, but the compressive strain degrades the channel mobility. Thus LM-MHEMT exhibits superior gm, fT, and fmax to CS-MHEMT. For a 0.65 μm gate-length device, the gm, IDSS0, fT, and Pout at 5.8 GHz (biased at VDS = 3 V) for CS-MHEMT (LM-MHEMT) are 318 mS/mm (341 mS/mm), 452 mA/mm (366 mA/mm), 48.0 GHz (53.5 GHz), 185.8 mW/mm (131.5 mW/mm), respectively.
In order to improve the microwave and power characteristics simultaneously, the MHEMT device with tensile-strained channel was fabricated (TS-MHEMT). Because both the tensile strain and symmetrically-graded structures can increase the channel mobility, higher mobility and less impact ionization than those of conventional devices can be obtained in low In-content channel. The DC, RF, and power characteristics have been improved in TS-MHEMT as compared to the conventional MHEMT. For a 0.65 μm gate-length device, gm, IDSS0, fT, and Pout at 5.8 GHz (biased at VDS = 3 V) are 404 mS/mm, 514 mA/mm, 58.5 GHz, and 260.0 mW/mm, respectively.
Finally, we proposed metamorphic δ-doped channel heterostructure field-effect transistor (MDDFET). Because the partial wave function traveling in the high-speed undoped channel in the center of channel, high channel mobility can be achieved without sacrificing the high carrier concentration. For a 0.65 μm gate-length device, the measured gm, IDSS0, fT, and Pout at 5.8 GHz are 320 mS/mm, 566 mA/mm, 45 GHz, and 194.5 mW/mm, respectively. Furthermore, the variations of gm and ISDD0 are as low as 0.3% and -3.4% from 300 K to 420 K respectively, which can be attributed to the enhanced carrier confinement and the special electron transferring mechanism.

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