在本論文中，我們成功的研製一系列砷化銦鋁/砷化銦鎵變晶結構高電子移動場效電晶體。藉由不同銦組成的通道，完整的探討其對元件特性的影響，例如：源極電流、崩潰電壓、轉導值、輸出電導、閘極電壓擺幅、高頻特性、功率特性以及雜訊特性，並藉此優異特性應用於放大器電路上。
　　我們首先研製通道銦組成為0.35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。相較於銦組成為0.53的砷化銦鎵通道，銦組成為0.35的砷化銦鎵通道具有較大的能隙，因此可以降低通道中衝擊游離的現象，改善紐結效應，同時可獲得較低的輸出電導(0.69 mS/mm)、較佳的電壓增益(442)。又由於使用寬能隙的砷化銦鋁(銦組成為0.35)為蕭基層，所以元件具有較高的崩潰電壓，使其更適合於高功率的應用。在閘極長度為0.65 µm的元件中，其閘－源極兩端的崩潰電壓為-15.2 V，三端截止崩潰電壓為14.1 V；在操作頻率為2.4 GHz時，可獲得小訊號功率增益為22.7 dB，飽和輸出功率為14.1 dBm (128.5 mW/mm)，最大功率附加效率為46.4 %。
　　為了使元件更能適用於高頻率的應用上，我們進一步研製銦組成為0.45/0.65之複合式通道的δ-摻雜砷化銦鋁/砷化銦鎵變晶結構高電子移動場效電晶體。由於銦組成為0.65的砷化銦鎵通道具有較窄的能隙，所以載子有較佳的傳輸特性，進而改善元件的轉導值及高頻特性。就閘極長度為0.65 µm的元件而言，其轉導值為321 mS/mm，單一電流增益截止頻率和最大震盪頻率分別為43.1 GH以及50 GHz。
　　但由於較窄的能隙會有較明顯的衝擊游離的現象，並使得紐結效應更為嚴重，因而造成較高的輸出電導以及降低了元件的崩潰電壓。為了研究及改善元件的紐結效應，我們研製通道銦組成為0.53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，並分別鍍上金、鎳/金、鈦/金、鉑/金等不同的閘極金屬，探討其對特性的影響。不同的閘極金屬可以獲得不同的蕭基能障，而當蕭基能障變大時，會使得空乏區増大，因此減少了通道的有效寬度，進而抑制通道中載子的數量，使得衝擊游離的效應減低，同時紐結效應亦隨之改善。
　　除了藉由不同閘極金屬之外，我們也藉由不同的通道設計來改善紐結效應。我們研製兩種不同銦組成通道之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。分別為銦組成為0.65的均勻式通道以及銦組成為漸變式對稱型通道(0.5→0.65→0.5)。由於對稱型通道有較大的平均能隙，因以可以改善衝擊游離效應和紐結效應，同時亦可增加閘極電壓的擺幅及崩潰電壓，使得元件較適合於高功率的應用。然而，另一方面，由於均勻式通道有較佳的載子傳輸特性，所以可以獲得較佳的源極電流、轉導值，使得元件較適合於高頻率及低雜訊的應用。又因為均勻式通道有較大的導帶不連續，可以增加載子的侷限能力，因此我們對其做温度變化的測試。由實驗結果顯示，均勻式通道有良好的熱穏定性。當温度從300 K變化至500 K時，源極電流下降21.9 %、轉導值下降17.7 %、閘-源極崩潰電壓下降31.6 %，而臨界電壓則僅變動4.7 %。
　　最後，我們針對均勻式通道及對稱式通道之變晶結構高電子移動場效電晶體來模擬及設計C-band (4 ~ 6 GHz)增益放大器。由於均勻式通道有較佳載子傳輸特性，因此可以獲得較高的增益放大器。但是，另一方面，由於對稱式通道有較佳的閘極電壓擺幅，所以可以獲得較寬廣的閘極電壓操作範圍。

　In this dissertation, we have successfully fabricated and investigated InAlAs/InGaAs metamorphic high-electron-mobility transistors (MHEMT’s) by employing different InxGa1-xAs channel structures with varied In compositions. Through the channel engineering, we have completely studied the influences on comprehensive device performances, such as drain current, breakdown, extrinsic transconductances, output conductance, high-frequency characteristics, power performances, and noise behaviors. Meanwhile, we also design and simulate gain amplifier by using superior characteristics of studied MHEMT.
　First, we’ve investigated the δ-doped In0.35Al0.65As/In0.35Ga0.65As MHEMT’s. Since the energy band-gap of In0.35Ga0.65As channel is larger than that of In0.53Ga0.47As channel, the impact ionization of the channel is expected to be reduced, thus improving the kink effects. Consequently, lower output conductance (0.69 mS/mm) and higher voltage gain (442) have been obtained. In addition, due to the use of wide-gap In0.35Al0.65As Schottky layer, the breakdown voltage characteristics of studied device have also been improved. For a 0.65 μm device, the gate-drain breakdown voltage and three-terminal off-state breakdown voltage are -15.2 V and 14.1 V, respectively. The measured saturated output power, small-signal power gain, and maximum power-added-efficiency, at 2.4 GHz, are 14.1 dBm (128.5 mW/mm), 22.7 dB, and 46.4 %, respectively. The studied device is promisingly suitable for high-power application
　To facilitate its high-frequency applications, we have further investigated a δ-doped In0.45Al0.55As/InGaAs MHEMT by using the In0.45Ga0.55As/In0.65Ga0.35As composite channel. Due to the narrow-gap property of the In0.65Al0.35As channel layer, better carrier transport characteristics have been obtained to improve the transconductance and high-frequency characteristics of the studied device. For a 0.65 μm device, the measured extrinsic transconductance, unity current gain cut-off frequency, and maximum oscillation frequency are 321 mS/mm, 43.1 GHz and 50 GHz, respectively.
　Nevertheless, impact ionization effects usually accompany with narrow-gap compounds, thus deteriorating the kink effects. Higher output conductances and lower breakdown voltages were observed by using In0.45Ga0.55As/In0.65Ga0.35As composite channel. To improve the above-mentioned kink effects, we have investigated the δ-doped In0.55Al0.55As/In0.53Ga0.47As MHEMT’s due to improved kink effects by evaporating alloys, including Au, Ni/Au, Ti/Au, and Pt/Au, as the Schottky contacts to discuss their respective influences on device performance. Since different gate alloys can induce different Schottky barrier heights, and higher Schottky barrier height can extend more of the depletion region, the efficient channel can then be decreased and the carriers within channel are suppressed. Then, impact ionization and the related kink effects within the channel can be reduced and improved.
　Besides using different gate alloys, we’ve further designed different channel layers to improve the kink effects. We investigated In0.425Al0.575As/InxGa1-xAs MHEMT’s with two different channel designs: one with a uniform In0.65Ga0.35As channel, and the other with a V-shaped symmetrically-graded InxGa1-xAs channel (x = 0.5 → 0.65 → 0.5). Due to larger band-gap value, in average, for the symmetrical channel, the impact ionization and kink effects can be improved. Simultaneously, the gate-voltage swing and the breakdown performance can also be enhanced. The studied device with symmetrical channel is more suitable for high-power applications. On the other hand, since higher values of drain current and extrinsic transconductance have been achieved, due to the improved carrier transport characteristics in the uniform In0.65Ga0.35As channel, the device is more suitable for high-frequency and low-noise applications. We have also investigated the temperature-dependent characteristics. The results show that the studied device provided good thermal stability, due to larger conduction-band discontinuities (ΔEC) between In0.425Al0.575As barrier and In0.65Ga0.35As channel and the improved carrier confinement. At 500 K, we’ve observed only 21.9 % decrease for the drain current densities at 300 K, 17.7 % decrease for the extrinsic transconductance, 31.6 % decrease for the gate-drain breakdown voltage, and only 4.7 % variation for the threshold voltage.
　Finally, we have simulated and have designed a gain amplifier for C-band (4 ~ 6 GHz) by using the uniform and the symmetrical channels, respectively. Due to better carrier transport characteristics, the device with uniform channel has demonstrated better gain. Besides, due to better gate-voltage swing for the device with symmetrical channel, wide range of gate-voltage operation has been achieved.

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