增強型磷化銦鎵/砷化鋁鎵/砷化銦鎵假晶高電子遷移率電晶體之研究

Abstract

近年來無線通訊(wireless communication)科技日新月異，已為世界科技產業重要之一環。無線通訊系統已由類比無線通訊進入數位無線通訊，同時數位無線通訊系統是世界公認的無線通訊系統發展主力。本論文即在研究應用於數位無線通訊系統之增強型砷化鎵高電子遷移率電晶體，以期提高其功能的相關元件及製程技術。 在此論文中，由磷化銦鎵/砷化鋁鎵/砷化銦鎵 所組成的結構首次應用於製造增強型的高電子遷移率元件。以磷化銦鎵/砷化鋁鎵/砷化銦鎵 所組成的結構取代傳統結構可進一步提升元件的特性。此磷化銦鎵/砷化鋁鎵/砷化銦鎵 結構優於傳統的磷化銦鎵/砷化銦鎵 有下列原因：第一， 砷化鋁鎵/砷化銦鎵 的導電帶的不連續性比磷化銦鎵/砷化銦鎵 來的高，對於電子的侷限性會較佳，此現象將可以增進元件的輸出功率。第二, 砷化鋁鎵/砷化銦鎵 的介面比磷化銦鎵/砷化銦鎵 更平滑，因為在磷化銦鎵/砷化銦鎵 的介面砷原子及磷原子會有交互擴散行為產生。此結果造成磷化銦鎵/砷化鋁鎵/砷化銦鎵 的高電子遷移率電晶體的電子遷移率比磷化銦鎵/砷化銦鎵 的高電子遷移率電晶體來的高。所製造出來的0.5×160 μm2 元件展現出很低的0.3V knee voltage, 且當元件的偏壓在VDS＝2.5V時，汲極電流為 375mA/mm（閘極電壓在0.7V）和最大的轉導為550mS/mm 。此元件亦展現及優之高頻特性；截止頻率為60GHz 且 最大震盪頻率為128GHz. 此增強型的磷化銦鎵/砷化鋁鎵/砷化銦鎵元件，在2.4GHz的頻率下亦展現很高的輸出功率密度453mW/mm及極高的線性增益30.5 dB 。另外此增強型元件其最大的功率增加效率為70%。 另一方面，先進的無線數位通訊系統，例Wide-band Code-Division Multiple-Access (W-CDMA), 需要元件擁有高效率、良好的線性度及低消耗電壓等特性。因此，高效率且高線性度的增強型磷化銦鎵/砷化鋁鎵/砷化銦鎵元件極有發展的必要性。元件能夠在低電壓下操作必須有極低的knee voltage ；線性度方面的改善，則需源自雙載子摻雜濃度的最佳化。本論文所製作之元件，當元件偏壓在VDS = 2V時，最大轉導值為448 mS/mm。 在10 GHz的頻率下，其最低雜訊指數為0.86 dB 且增益為12.21 dB 。此元件的high output third order intercept point (OIP3)-P1dB 為13.2 dB ，且在WCDMA的調變訊號下有著極高的功率增加效率35%。 另外，此論文也探討利用鉑作為蕭基接觸金屬的增強型的磷化銦鎵/砷化鋁鎵/砷化銦鎵元件。在依序濺鍍鉑/鈦/鉑/金為閘極金屬後，在 325℃ 下做退火處理，使得閘極金屬擴散下沈。退火後，元件的 threshold voltage (Vth) 自0.17V 正向偏移至0.41V，以及 源極漏電流從1.56μA/mm 減低至 0.16μA/mm.這些特性上的改善源自於Schottky barrier height的增高；在退火後，因為鉑擴散下沈的製程，使得閘極至通道的距離減低。而且threshold voltage 的偏移非常均勻且具有相當高的再現性，並且在退火後，元件有著更好的RF 功率特性。 最後，鉑金屬與磷化銦鎵 的界面反應也在此論文中討論。在濺鍍鉑金屬後，會有著約 7.5nm厚的非晶系層存在。由穿透式電子顯微鏡的影像顯示，在 325℃下經過一分鐘的退火後，鉑 金屬會往磷化銦鎵層 擴散至約12.8nm 。且在 325℃下，經過十分鐘的退火後，成核現象開始於磷化銦鎵層中產生。另外，在相同溫度下，經過三小時的退火，即可發現新的相 Ga2Pt 及 GaPt3 生成在鉑及磷化銦鎵的介面中。

In recent years, digital wireless communication technology develops rapidly around the world. It is believed that the digital wireless technologies are the major trends for the future wireless communication systems. The purpose of this dissertation is to develop the Enhancement-mode high-electron-mobility transistor (HEMT) for the digital wireless communication systems with improved device structures and the related process technologies. In this dissertation, the InGaP/AlGaAs/InGaAs structure was used to fabricate the enhancement-mode high-electron-mobility transistors with the goal of further enhancement of the HEMT device performance. The attempt on using the InGaP/AlGaAs/InGaAs heterojunction instead of the InGaP/InGaAs is due to: Firstly, the conduction band discontinuity of the AlGaAs/InGaAs interface is superior to those form at the InGaP/InGaAs interface, the carrier confinement would be better. This will improve the output power performance of the InGaP PHEMTs. Secondly, the interface between the AlGaAs/InGaAs interface is smoother than the InGaP/InGaAs interface due to the interdiffusion behavior of As and P atoms in the InGaP/InGaAs interface. As a result, the electron mobility of the InGaP/AlGaAs/InGaAs PHEMTs is higher than the electron mobility of the InGaP/InGaAs PHEMTs. The fabricated InGaP/AlGaAs/InGaAs HEMT 0.5×160 μm2 device shows low knee voltage of 0.3V, a high drain-source current (IDS) of 375mA/mm and a maximum transconductance of 550mS/mm when drain-source voltage (VDS) was bias at 2.5V. High-frequency performance was also evaluated; the cut-off frequency (Ft) was 60GHz and the maximum oscillation frequency (Fmax) was 128GHz. The E-mode InGaP/AlGaAs/InGaAs PHEMT also exhibited high output power density of 453mW/mm with high linear gain of 30.5dB at 2.4GHz. The maximum power-added-efficiency (PAE) of the device was 70%, when tuned for the maximum power added efficiency. On the other hand, advanced digital wireless application systems, such as Wide-band Code-Division Multiple-Access (W-CDMA) system, has imposed stringent requirements on the devices while include high efficiency and high linearity operation with minimum DC power consumption. Thus, a high linearity and high efficiency Enhancement-mode InGaP/AlGaAs/InGaAs PHEMT has to be developed. The low voltage operation is achieved by the very low knee voltage of the device and the linearity is improved by optimizing the concentrations of the two delta-doped layers. Biased at a drain-to-source voltage VDS = 2V, the fabricated device exhibited a maximum transconductance of 448 mS/mm. The measured minimum noise figure (NFmin) was 0.86 dB with 12.21 dB associated gain at 10 GHz. The device shows a high output third order intercept point (OIP3)-P1dB of 13.2 dB and a high power efficiency of 35% when under wideband code-division multiple-access (W-CDMA) modulation signal. In addition, an Enhancement-mode InGaP/AlGaAs/InGaAs PHEMT using Platinum (Pt) as the Schottky contact metal was investigated for the first time. Following the Pt/Ti/Pt/Au gate metal deposition, the devices were thermally annealed at 325℃ for gate sinking. After the annealing, the device showed a positive threshold voltage (Vth) shift from 0.17V to 0.41V, and a very low drain leakage current of 0.16μA/mm which was reduced from 1.56μA/mm before gate sinking. These improvements are attributed to the Schottky barrier height increase and the decrease of the gate to channel distance as Pt sink into the InGaP Schottky layer during gate sinking process. The shift in the Vth was very uniform across a four inch wafer and was reproducible from wafer to wafer. The device also showed excellent RF power performance after the gate sinking process. Finally, we had investigated the interfacial reaction between platinum and InGaP in a Schottky diode structure. There was a 7.5 nm-thick amorphous layer formed at the interface between Pt and InGaP after metal deposition. After annealing at 325 ℃ for one minute, this amorphous layer increased to 12.8 nm and the reverse leakage current also decreased. The diffusion of Pt atoms and crystallization of amorphous layer took place after annealing at 325℃ for 10 minutes. Prolonging the annealing to 3 hours led to formation of Ga2Pt and GaPt3 phases in InGaP and Schottky diodes degraded after these new phases were observed.