摘要
在這個論文中，我們以AlGaN/GaN High Electron Mobility Transistors (HEMTs)的研究為基礎，並將其應用於液體pH值的量測。在論文的第一部分中，從HEMTs的基本電性及霍爾量測中，可以發現當溫度下降時，HEMTs的通道電子遷移率會提高而通道電子濃度則會降低。但是溫度對通道電子濃度的影響小於對遷移率的影響，所以相同電壓下在低溫時會得到較高的電流，因此溫度對元件特性會有相當的影響。另外我們同時發現在量測的過程中，若是照射到室內照明，都會影響量測結果。因此進行pH值液體感測實驗時必須將光以及溫度的環境影響排除。
接著我們就利用製作好的HEMT應用在pH液體感測實驗中，我們發現在定電壓的操作模式下，drain-source兩電極間的電壓會隨著 液體pH值的下降而增加。從量測結果可以判斷目前元件靈敏度可達到0.1pH以下。而感測液體的溫度上升時，我們發現感測靈敏度也會提高。HEMT的感測能力，一般推測是來自於元件表面狀態的改變。當感測液體中氫離子濃度越高時，表面會有較高的帶正電荷表面電荷濃度，使得通道電子濃度也隨之上升。相同的感測液體中氫氧根離子濃度越高，表面會有較高的帶負電荷表面電荷濃度，使得通道電子濃度也隨之下降。於是我們進一步進行元件接面位勢的量測實驗。實驗結果發現每改變1個pH值時，接面位勢改變46.77mV，更進一步分析我們可以推測出每改變一個通道電子濃度，表面電荷需改變1056個。
在了解HEMTs應用於pH值感測的操作原理後，我們緊接著提出新的設計，希望利用製作差動放大的原理來達成更高靈敏度的pH值感測元件。此外我們也設計pH感測陣列元件希望來同時且快速的量測不同位置的pH值變化。這些新的元件的製成，能更進一步提高我們對此元件的了解以及發展出更多不同領域上的應用。

Abstract
In this disseration, the fundamental properties of AlGaN/GaN high electron mobility transistors (HEMTs) and their application as pH sensors were investigated. First, the electron mobilities for the HEMTs increase as the ambient temperatures decreases. In the other hand, the sheet carrier density also decreases. The observed current between electrodes increases because the effect of the mobility increases is larger than the effect of the sheet carrier density decreases. We also found that the experitmental results is also affected by the ambient light illumination. Therefore, the effects from the ambient temperature changes or the light illumination must be suppressed for an accurate pH measurement.
The fabricated AlGaN/GaN HEMT devices were then used to measure pH value for a liquid. It was observed that the voltage drop between the drain-source electrodes increases as the pH value of the liquid decreases when constant current flow between the electrodes. The device was able to distinguish a 0.1 pH change in the liquid. It was also observed that the sensitivity increases as the temperature of the liquid increases. The pH sensing ability is believed to be originated from the change of the surface states of the device. More positively charged surface states presented in the surface area when the concentration of H+ increases in the liqud. In the other hands, more negatively charged surface states presented when the OH- concentration increase in the liquid. A further surface potential investigation reveals that the surface potential change 46.77 mV for each pH value changes. It is proposed that a change of 1056 surface charges is required for each channel carrier density change.
The previous experiments helped us understand the operations of the pH sensors and paving the way for newer device designs. A newer and high sensitivity design is proposed utilizing the techniques of differential amplifying. In addition, a design of pH sensor arrays is also proposed in order to measure the pH value at different locations simultaneously. The fabrications of these new devices will help us further understand the operations of the pH sensors and develop newer applications in different fields.

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