氮化鎵系列材料擁有高載子飽和速度、高崩潰電場、及良好熱傳導等引人注目的物理特性。因為上述固有的材料特性，將氮化鎵材料應用於異質介面場效電晶體等高速、高功率、高溫電子元件上已獲得廣泛的研究並有不錯的結果。為了更進一步改善元件特性，仍有一些問題需要改進。舉例而言，降低因輸入訊號過大，經由蕭基閘極能障所產生的閘極漏電流而造成直流及高頻特性下降以及抑制電流崩潰，都將改善元件功率特性。近來，四種閘極介電層元件－金屬-氧化層-半導體異質接面場效電晶體(金氧半異質接面場效電晶體)、金屬-介電層-半導體異質接面場效電晶體(金介半異質接面場效電晶體)、金氧半雙異質接面場效電晶體、金介半雙異質接面場效電晶體，已經問世並改善異質接面場效電晶體之直流及高頻特性。
本文致力於研究應用三氧化二鋁作為閘極介電層於金屬-氧化層-半導體高電子移動率場效電晶體(金氧半高電子移動率場效電晶體)及金屬-氧化層-半導體假形高電子移動率場效電晶體(假形高電子移動率場效電晶體)。本文應用異相沉積法 來成長三氧化二鋁薄膜。因為異相沉積法物理及技術的特性適合在氮化鎵及砷化鎵系列材料上沉積一層三氧化二鋁薄膜。所沉積的三氧化二鋁其介電常數K值介於8~10之間，相較於氮化矽、二氧化矽及其他高介電常數材料有著較大的K值。三氧化二鋁可承受極大的崩潰電場，最大崩潰電場可達10 MV cm-1。三氧化二鋁能障值為9eV。許多傳統製程使用三氧化二鋁作為閘極氧化物製作金氧半高電子移動率場效電晶體。然而，利用高溫成長三氧化二鋁，在這過程當中，易因為高熱產生應力，而造成薄膜缺陷，進而導致電子元件性能下降。本文為應用異相沉積法沉積三氧化二鋁為閘極介電層於氮化鎵及砷化鎵系列元件之應用。異相沉積法是一種低成本、低溫以及容易使用的方法。利用異相沉積法可以沉積奈米等級的三氧化二鋁薄膜。所有藉由異相沉積法所形成的三氧化二鋁薄膜都是非定型形式，氧/鋁原子比例成分穩定(原子比例與藍寶石非常接近)，當熱退火加熱至750℃，三氧化二鋁與n-氮化鎵可形成良好的介面，適合作為閘極介電層。在30~35℃，每小時可以沉積30奈米薄膜。異相沉積法利用預先調配硫酸鋁與碳酸氫鈉混合液來沉積薄膜於半導體上，此外，沉積的過程當中，整個沉積系統的pH控制，對於薄膜成長有著極重要的影響。沉積好薄膜後，利用熱退火技術，可以有效改善表面粗糙度。對三氧化二鋁薄膜品質而言，利用X光光電子能譜儀(XPS)、歐傑電子能譜儀(AES)、原子力顯微鏡(AFM)及掃描式電子顯微鏡(SEM)來鑑定薄膜品質好壞。對崩潰特性而言，50奈米三氧化二鋁氧化層，於1 MV/cm電場下，其漏電流介於10-4 - 10-5 A/cm2，最大崩潰電場可達10 MV cm-1。經過30分鐘750℃熱處理後，於1 MV/cm電場下，其漏電流更可改善至10-6 to 10-7 A/cm2。對界面陷阱電荷濃度而言，剛沉積好50奈米厚的三氧化二鋁，經計算其界面陷阱電荷濃度為5.59×1011 cm-2eV-1，當經過30分鐘150℃熱處理後，其界面陷阱電荷濃度為3.89×1011 cm-2eV-1。
就元件應用而言，沉積15奈米三氧化二鋁當作n-氮化鎵/氮化鋁鎵/氮化鎵高電子移動率場效電晶體的閘極介電層並同時進行表面鈍化。閘極的長度與寬度，分別為2微米及200微米。利用異相沉積法可以有效的減少閘極漏電流及抑制電流崩潰效應。逆向崩潰電壓可達到100 V。在Vgs = 1 V時，汲極飽和電流可達到583 mA/mm，最大外質轉導可達到83 mS/mm，在Vgs = -1.5 V及Vds = 10 V時。利用傳輸線理論，得到的源極接觸電阻分別為3.5 and 3.8 ohm/mm。給定擁有閘極長度2微米寬度200微米傳統高電子移動率及金氧半高電子移動率與最大外質轉導相同偏壓條件，可得到外質截止頻率分別為3.73 GHz及2.53 GHz，最大功率增益頻率分別為5.63 GHz及4.73 GHz。本文同樣研製應用異相沉積法沉積20奈米三氧化二鋁作為閘及介電層於砷化鋁鎵/砷化銦鎵/砷化鎵高電子移動率場效電晶體。對崩潰特性而言，金氧半高電子移動率場效電晶體在Vgs = 15 V時，逆向崩潰電流為4.51×10-7 A，金氧半高電子移動率場效電晶體及傳統高電子移動率場效電晶體逆向之崩潰電壓分別為38 V 及17 V。金氧半高電子移動率場效電晶體及傳統高電子移動率場效電晶體逆向崩潰電壓之正向閘極擺幅分別為1.4V及0.8V。三氧化二鋁/砷化鋁鎵界面陷阱電荷濃度為2.5 × 1012 /cm-2 eV。
眾所皆知陷阱效應將會限制微波領域場效電晶體的功率特性，特別是碳化矽、氮化鎵系列場效電晶體。陷阱效應與氧化層、表面能障及通道下方的半導體有關。所以利用短期的汲極和閘極直流偏壓進行氧化層可靠度分析。經過20小時施加汲極偏壓，電流將會漸漸縮小，這顯然是因為熱電子注入並被緩衝層捕獲以及能障層操作在高汲極電壓下而沒有表面陷阱捕獲。金氧半高電子移動率場效電晶體同樣也完成了交流量測及電流對電壓磁滯效應的量測。 最後，我們成功的應用異相沉積法這個低成本且成長於室溫下的技術，沉積三氧化二鋁薄膜作為閘極介電層在金氧半高電子移動率場效電晶體及假形金氧半高電子移動率場效電晶體之研製，適合微波功率元件的應用。

GaN and related alloys have attractive physical properties such as high saturation velocity, high breakdown electric field and good thermal conductivity. Owing to these inherent material properties, heterojunction field-effect transistors (HEFTs) have been extensively studied as promising for high-speed, high-power and high-temperature electronic devices. For further improvements in device performance, however, there are still some issues to be solved. For example, reduction in gate leakage current through the Schottky barrier gate causing DC and RF parameter degradation under large input signals and suppression of current collapse are required to improve the power density. Recently four novel insulating gate device types—metal-oxide-semiconductor HFET (MOSHFET) metal-insulator-semiconductor HFETs (MISHFET) and MOSDHFET and MISDHFET were introduced to significantly improve HFETs DC and RF performance.
This thesis is focused on studies of metal oxide semiconductor high electron field effect transistor (MOSHEMT) and metal oxide semiconductor pseudomorphic high electron mobility transistor (PHEMT) devices where aluminum oxide (Al2O3) behaved as a gate insulator. A new liquid phase deposition (LPD) technique is developed to prepare Al2O3 films. A thin layer of Al2O3 might well fulfill the requirements on a gate material for GaN and GaAs-based devices because of its suitable physical and technological properties. The dielectric constant of Al2O3¬, ε = 8-10, is larger than that of SiN and SiO2¬ and compared with other ‘high-k’ materials. The breakdown field of Al2O3 is also large, E > 10 MV cm-1. Its bandgap is Eg = 9 eV. Many conventional methods were used to prepare MOSHEMT device with Al2O3 as a gate oxide. However, these high-temperature processes may create film defects because of large thermal stress, thereby degrading the performance of electronic devices. In this thesis, Al2O3 gate dielectric was prepared by an alternative liquid phase deposition method in both GaN and GaAs–based device application. This LPD process is cost-effective, low-temperature as well as very easy to access. The LPD is proved to be suitable for producing ultrathin alumina films of nanometer scale. All deposited Al2O3 films by LPD are amorphous, stable composition of O/Al atomic ratio (is very close to the value of sapphire) as well as good interface with n-GaN when annealed up to 750oC, which is most favorable for gate dielectric. The deposition rate of oxide film was 30 nm/hr at 30oC~35oC. Aluminum sulfate with crystallized water and sodium bicarbonet are used as precursors for film growth and the control of the system’s pH value played an important role in this experiment. Post growth annealing was used to improve the surface roughness. The good quality of the Al2O3 films in this work is supported by X-ray photoelectron spectroscopy, Auger electron spectroscopy, Atomic force microscopy, Scanning electron microscopy etc. It is found that the leakage current density of 50 nm-thin Al2O3 oxide film is between 10-4 - 10-5 A/cm2 at a negative electric field of 1 MV/cm, with the breakdown electric field being greater than 10 MV/cm. After annealing oxide at 750oC for 30 mins, the leakage current density is lowered to the value of 10-6 to 10-7 A/cm2 at the negative electric field of 1 MV/cm. The calculated interface trap densities are 3.89×1011 cm-2eV-1 for an oxide thickness of 50 nm annealed at 150oC, (30 mins) and 5.59×1011 cm-2eV-1 for the as-grown oxide film.
The 15 nm thick Al2O3 oxide films are used for gate insulator as well as surface passivation layer in case of n-GaN/AlGaN/GaN based MOSHEMT device application. The gate length and gate width are 2 μm and 200 μm respectively. The gate leakage current and current collapse effect are highly suppressed in our LPD-Al2O3 MOSHEMT device application. The reverse breakdown voltage was more than 100 V. The drain source current value in the saturation region (Idss) at Vgs = 1 V was 583 mA/mm and maximum extrinsic transconductance was 83 mS/mm obtained at Vgs = -1.5 V when Vds = 10 V. The source contact resistances Rs of 3.5 and 3.8 ohm/mm are measured from TLM method for MOSHEMTs and conventional HEMTs, respectively. The 2 µm gate length and 200 µm gate width HEMT and MOSHEMT devices biased at peak gm, yielded an extrinsic current gain cutoff frequency fT of 3.73 GHz, 2.53 GHz, and a maximum frequency of oscillation fmax of 5.63 GHz, 4.73 GHz, respectively.
AlGaAs/InGaAs/GaAs metal-oxide-semiconductor pseudomorphic high electron mobility transistors (MOS-PHEMTs) with 20nm Al2O3 as a gate dielectric oxide was prepared by liquid phase deposition (LPD) are also presented in this study. The reverse leakage current was 4.51×10-7 A at the gate voltage of 15 V for the MOS-PHEMTs and two terminal off-state gate breakdown voltages were about 38 V and 17 V for MOS-PHEMT and PHEMT, respectively. The turn-on voltage of the MOS-PHEMT was about 1.4 V, which exceeds that of the PHEMT at 0.8 V. The interface trap density (Dit) is estimated to be 2.5 × 1012 /cm-2 eV for Al2O3/AlGaAs.
It is well known that trapping effects can limit the output power performance of microwave field effect transistor, it is particularly true for wide band gap SiC and GaN based FET device. The trapping effects associated with the oxide, surface barrier and with the layers underlying the active channel. So our fabricated MOSHEMTs are subjected to short-term DC bias drain and gate stresses to investigate reliability of the oxide. The gradual reduction in current is found within a 20 hours drain bias stress, which is apparently caused by the hot electron injection and trapping in the buffer, and barrier layer at high-drain voltage operation without any surface trapping. The AC measurements and I-V hyteresis of MOSHEMT device were also performed.
Finally, we have successfully deposited the Al2O3 films by an efficient low cost alternative LPD technique near room temperature behaved as a gate dielectric for MOSHEMT and MOS-PHEMT devices application, which is very promising candidate for microwave power applications.

Chapter 1
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