節省能源的議題在近年來越趨重要，一些日常電子產品，例如3G產品以及車用電子就必須降低能量散失，自80年代以來其皆為使用矽基板材料製作，但由於材料的限制，會有大量的熱消耗以及高頻損失。新興三五族材料如氮化鎵(GaN)，在高頻的功率元件上發展迅速因為其具有高功率輸出、高電壓操作以及低切換損失之優點，此論文利用氮化鋁鎵/氮化鎵高電子移導率電晶體製作於矽基板以及藍寶石基板，應用於高轉換效率、高功率應用電路。氮化鎵電晶體由於壓電效應產生二維電子氣(2DEG)，電子速度以及密度非常高，且導通電阻低並增加其切換速度。
但當此元件應用在高功率高電流情況會在通道產生熱效應，所以近幾年紛紛發展了氮化鎵成長在矽基板之技術，矽材料具有低成本以及高散熱係數之優點，熱效應使電流散失將會改善。本論文第三章使用了藍寶石以及矽基板製作閘極長度1.5 μm以及寬度1 mm功率電晶體，第四章與第五章分別將此元件應用在直流對直流升壓轉換器以及2.4 GHz功率放大器，直流升壓轉換器在車用方面將可提升效率以及高輸出功率轉換；而高效率之功率放大器將會減少能量的需求以及冷卻系統的成本，皆對節省能源有極大助益。

Saving energy is getting important in recent years because of the huge dissipation causing from the regular electric equipments. Reducing the power from the 3G and the vehicle products is the first step. Silicon is the popular material to fabricate semiconductor elements since 1980’s, but it causes much power loss in heat effect and high frequency loss. The rapid development of the RF power electronics requires a wide bandgap material as GaN device due to its high output power density, high operation voltage and low switching loss. The AlGaN/GaN HEMT devices have a lower on-resistance and a higher switching speed due to the high electron mobility and the high electron density of the 2DEG caused by polarization effect. But when the devices are used for high power and high current applications, the heating effect would be produced in the device channel. Recently, the GaN HEMT devices have been grown on Si substrates because it has some advantages about low cost, large area availability, and the acceptable thermal conductivity, the current dispersion effect in this epitaxy structure is therefore not obviously.
This thesis focuses on the high voltage characteristics of AlGaN/GaN HEMT based on silicon and sapphire substrates, and using the devices to design the high efficiency and high power performance circuits. The characteristics of the AlGaN/GaN power devices (Lg=1.5 μm,Wg=1 mm) which were fabricated on the silicon and sapphire substrates are discussed in Chapter 3. Chapter 4 and Chapter 5 present the circuit applications of the DC-DC converters and the 2.4 GHz power amplifiers using these power devices. The DC-DC converters demonstrate a performance of 10 V-to-20 V voltage conversion which can achieve the high efficiency and high output power for supplying the vehicle electronics applications. The GaN power amplifiers designed for 2.4 GHz wireless transmissions are also presented with high efficiency performance to reduce power requirements and simplifies cooling system.

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